# Return Air Vents



## stubits

All-

We recently moved into a home that had been flipped... some of the work was done OK, the rest not so well. I have spent the past couple of months reworking, repairing and finishing the work the flippers did. 

One of the issues has to do with our return air vents. We have central air conditioning and forced air heat in a two story plus basement (its heated and air conditioned) row house. There are only two return air vents in the whole home (not uncommon from what I hear in older row homes where it is difficult to run ductwork), one return is on the main level of the home and the other is on the second floor. The AC/furnace are in the basement.

The problem we have, perhaps not a problem at all, that's why I am asking here, is that the return air vent on the main level of the house is very loud and sucks down a LOT of air while the vent on the second level of the house is barely able to hold a piece of paper up with the suction.

Any thoughts as to what might be causing the trouble? Is this a problem? Any suggestions for how to improve it?

Thanks!


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## key1cc

I've learned that air likes to follow the path of least resistance. Apparently with your current set-up it is easier for the the bulk of the return air to travel through the down stairs return grille.

It could be caused by many reasons including larger duct work (or return air passage) to the downstairs grill, proximity to the furnace (it could be much closer), less restrictions (less sharp turns to get to the grill), closed dampers to the upstairs grille, etc...

A simple solution which is usually not so simple is to make it easier for the air to get to the upstairs grille. Some may say that making it harder for the air to get to the down stairs grille may accomplish the same thing but I would not reccomend this as it would likely bump your static pressure.

Also if you have even temperatures throughout the home, a solution to the noise may be as simple as reducing the blower speed (less air flow = less noise).

Key1


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## stubits

Thanks!

We live in a 2 story (plus basement) row house in Washington, DC. The AC/furnace were of course retrofitted, as is necessary around here. With all brick construction there is not a lot of room for duct work. 

I think you're right, the downstairs duct is MUCH closer to the furnace, in fact it is immediately above the furnace. That being said, it isn't a very large vent, maybe just 3"x 6" or so. The upstairs return is larger, I think a 6" round duct. 

I am actually less concerned about the noise and more concerned about how well the unit is functioning. I know that return air is important and I want to make sure my system is not deprived (I think it must be!). The house isn't very large, maybe 1,200-1,4000 sq. ft. of living space. 

The heat throughout the house is pretty even, but the AC was not so great. Although I hadn't attempted to balance the system (using the louvers on the grilles) and we just insulated the attic. Airflow in some rooms is definitely much stronger than in others.

Any thoughts?


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## key1cc

stubits said:


> Thanks!
> 
> We live in a 2 story (plus basement) row house in Washington, DC. The AC/furnace were of course retrofitted, as is necessary around here. With all brick construction there is not a lot of room for duct work.
> 
> I think you're right, the downstairs duct is MUCH closer to the furnace, in fact it is immediately above the furnace. That being said, it isn't a very large vent, maybe just 3"x 6" or so. The upstairs return is larger, I think a 6" round duct.
> 
> I am actually less concerned about the noise and more concerned about how well the unit is functioning. I know that return air is important and I want to make sure my system is not deprived (I think it must be!). The house isn't very large, maybe 1,200-1,4000 sq. ft. of living space.
> 
> The heat throughout the house is pretty even, but the AC was not so great. Although I hadn't attempted to balance the system (using the louvers on the grilles) and we just insulated the attic. Airflow in some rooms is definitely much stronger than in others.
> 
> Any thoughts?


What are the model numbers of your furnace, coil, and outdoor condenser? How old are the units?
Posting Pics of how the duct work connects with the furnace may shed some light.
Once you post you model numbers also post your exact return grill sizes. Someone will likely be able to tell if you would benefit from making them larger. 
Ultimately, I think you will need to check the static pressure (or have it checked) to gain more insight to whether or not it is functioning properly.

Lot's of other things can be checked also......

How far are you willing to jump into this is a question you will have to ask yourself.

Key1


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## integlikewhoa

stubits said:


> That being said, it isn't a very large vent, maybe just 3"x 6" or so. The upstairs return is larger, I think a 6" round duct.


Yea I agree with Key, we need more info on the unit and the ducts. We need info on the unit to see what the CFM or air flow rating is. The return ducts seem to small for what I'm used to. 

Your saying you only have a 3"x6" grill and a 6" round grill or duct for your only returns. The CFM rating on that is very low. You should have more Square inches of return vents then you do of supply vents. Which will slow the air flow and keep it quiet. 

In your case you probley should do something to increase the return grills, but ideally you want to get the upstairs return to bring in more air then what it is. If you only inlarge the lower vent your only going to throw off the balance of upstairs to downstairs, but will help increase the air flow threw your system with will reduce static pressures, lower energy use of the blower and increase airflow threw the house. 

Give us some more info and someone should be able to help you.


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## biggles

the sheet of paper trick was a good test how about cutting a piece of card board the size of the 1st floor return and cover 50% of it then check the upstairs for the audible air noise,and it will at least prove the returns are common.cooling side of the system is more effected by a minimal return so keep that cardboard handy for the summer....the spaces basically balloon if there is no returns....try it with cold night and check upstairs during a long heating cycle.


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## stubits

Thanks everybody!

I will check out the specific details of my system tonight and will attempt to take some photos.

I can however offer some additional information.

I don't think the system was designed particularly well, although I think much of it was limited by the house itself. There aren't a lot of spaces to run ductwork. As far as I can ascertain, the return ductwork runs straight up from the basement to the attic. The ductwork is rectangular, not round. On the main floor of the house(where the majority of the return air is being pulled from), the return air grill is connected to the smaller side of the rectangular duct. The ductwork continues up to the attic where is connects to a 6" flexible round duct, maybe 10' in length which then connects to the upstairs grill.

We have significantly more square inches of supply vents than we do return vents, for sure.

Given the VERY simple nature of the design, and how little air is being returned from the upstairs, I am not sure how to add more return air vents. I could do so easily in the upper level, given access to the attic, but on the lower level, all I can imagine doing is expanding the current vent and making is taller, but not wider, as the supply duct that goes from the basement to the attic is immediately in front of the return duct in the heating/cooling chase.

The heat has definitely performed MUCH better than the cooling did in the summer.

I will take some photos and get some additional info tonight, but would appreciate any help at all!

Thanks!


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## integlikewhoa

stubits said:


> We have significantly more square inches of supply vents than we do return vents, for sure.


That's going to be one big problem right there. Need to see what you can to try and even things out a bit. If you find out the CFM rating on the blower in the heater unit, you can find out how big of ducts and the square inches the intake grill should be to some extent. 
After you get them up to proper size you'll lower static pressure, increase flow and use less electricity. The fan is going to waste alot of electricity trying to suck threw air threw a pin hole. You will also end up with cooling problems as the coil will freeze up, due to the lack of airflow over the coil.


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## stubits

Ok, I need to get down to the basement utility closet to figure out what equipment I have, but I thought I'd share some photos and measurements from the return ducts. It is definitely funky!

As I indicated earlier, we have only one return air "trunk" in the house, it runs straight from the basement to the attic where it branches off via flexible duct to the second floor return air vent. As you can see in the photos, the main level return is effectively a large hole in the side of the trunk (no wonder there isn't much umph left by the time it gets to the attic!

Here are the measurements:

Main Level Return(Bottom Photos):
Ductwork (trunk): 8x12
Opening: 7x10 (area beneath the metal flap) or 7x25(total opening)
Grille: 10x26

Second Level Return(Top Photo):
Round duct: 8" diameter
Grille: 16x22

I'll post photos and more info on the equipment later. Again, the house is perhaps 1,400 square feet of air conditioned/heated space. Also, the basement, where all the equipment is housed, is finished and is air conditioned and heated, but there are no return vents there.

Any thoughts?


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## stubits

Also, I just tried the suggestion of covering half the the main level return air vent with cardboard. Indeed, it increased the amount of return air being sucked in by the upstairs return air vent.


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## integlikewhoa

An 8x12 return line flows somewhere around 400 CFM at ideal conditions. And that's on the high side. I still don't know what the CFM is of your blower. With that you can figure out the size of trunk line you should have under ideal conditions.


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## stubits

OK, I think I have the necessary details...

The furnace/air handler is model number 80PS10EDR01 which seems to be a Rheem 80% Upflow/Horizontal Gas Furnace. From what I can ascertain it seems like it is 100,000 BTU and 5 ton. I think the manual and other info about the unit can be found here: http://www.allreds.net/WS2/item_detail.html?item_num=RHE+80PS10EDR01+++++

The indoor component of our air conditioner is model number RCFA-HM4821AC which seems to be Rheem 4 TON 21", 13 SEER cased coil. More info about this component can be found here: http://www.allreds.net/WS2/item_detail.html?item_num=RHE+RCFA-HM4821AC+++

Finally, the outdoor component of our air conditioner is model number 13AJA36A01 which seems to be a Rheem Corsaire 13 SEER, 3 ton condenser. More info on this unit can be found at http://www.allreds.net/WS2/item_detail.html?item_num=RHE+13AJA36A01++++++

So, I am certain of all the model numbers, but the rest of the information is based on a simple google search, so I can easily be wrong.

I am guessing, based on the little bit I've learned here that a single 8x12 return trunk is very much insuficient. At the unit itself, the return starts out as a 8x20.

So, with all this information, any suggestions? Can I add an additional return trunk? There seems to be 2 or three supply trunks, which makes a lot more sense. One thought, if multiple return trunks are possible would be to use the current trunk the return air from the second floor of the home and add a trunk to handle the first floor of the home and perhaps a third to return air from the finished and lived in basement?

Thanks!


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## biggles

that 2nd floor return is mounted in the ceiling with the furnace in the basement?you say it is 8" duct but if it isn't connected to the main level return it is useless.the 2nd floor needs to be mounted directly above the main level return on the wall,and the 8" dropped down onto the top of the main level return opening.does the 8" duct loop up into the attic and then drop down into the house someplace?that main level rectangular return does that go down into the basement and connects to the side of the furnace..if it does that is the main return the 2nd level is doing nothing doubt it is connected with the original failed that sheet of paper test.if you could open the wall just above that main level return and drop a flex duct down onto the top of the duct in there you would pull a true 2nd floor return with a little restriction on the main level return.


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## stubits

It is connected to the main level return.

Here is how it is setup.

The return air trunk goes from the furnace in the basement straight up to the attic where is terminates. As you can see from the photo on the main level of the house, they've cut a large hole, 7"x25" into the trunk itself and attached a grill. In the attic, where the trunk terminates, they've connected a piece of 8" round insulated flexible duct which then connects to a 16"x22" sheetmetal box with grille.

The upstairs return air definitely works, just not well. It will loosely hold a piece of paper. When I covered over half of the main level return air vent with cardboard as suggested earlier, the pull from the upstairs vent was stronger.


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## biggles

your real close then with the trunk up into the attic,and restricting the main level return.see the size of that attic return......cut a sheet of card board the size of the main level opening that the grill goes into.then on the sheet of cardboard cut the same size that the attic return is into that sheet going into the main level opening,so your main and 2nd floor returns are the same sizes....see how the air noise is with that.go for the SQUARE INCHES(width X lenght) when doing the main level cutting since it is more retangular and the attic is closer to a square.


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## stubits

Biggles-

Really great idea. Will definitely try. I am concerned however because it seems like I just don't have enough return air at all for the system?

What do you think?


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## integlikewhoa

A 5 ton is way over kill for you house. 
So according to your manual for a 5 ton unit depending on your DIP settings you should be pushing around 1600 cfm. Now with your return sizing your static pressure is going to be really high and your not going to be pushing that now. But you probleyt should have around a 18-20" or equilant size of return duct. Ofcourse you can split this up into several smaller ones. Make sure you have enough grill square feet to cover 1600cfm.


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## integlikewhoa

heres a calculator you can use to get some idea of what size ducts you need. http://efficientcomfort.net/jsp/ResDuct_Web.jsp

For a return to flow freely and quietly you should have the air flowing threw the ducts around 600-700 feet per min. 600 is better tho. The slower the air is going the quieter it is. So if you figure 650fpm. and your blower puts out around 1600 cfm. Then you need around 21" round duct. (that puts you at 665fpm and 22" puts you at 606fpm)
Now for a retangle duct. 18x20" puts you at 640fpm at 1600 cfm. 

Now longer lenghts 90 degress and flex duct vs. metal duct will make you flow less air so you would even need to go bigger then that.


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## integlikewhoa

stubits said:


> It will loosely hold a piece of paper. When I covered over half of the main level return air vent with cardboard as suggested earlier, the pull from the upstairs vent was stronger.


The upstairs vent sounds fine with losely holding it. It shouldnt be sucking so hard, as that will make to much noise. You can figure out from the calculator how much cfm should be going threw that vent at that size with the air traveling at 650fpm. 

You probley need to try and block off the lower vent. Cover it patch the hole and such so that truck is only going for upstairs, then try and add another duct for a new down stairs vent. might want to increase the upstairs grill and flex duct size to fit the maximum that the square trunk going to upstairs can hold. 
8x12" is equal to an 11" round metal duct. both carry 400cfm at 600fpm. So probley a 12" flex duct would do nicely (since 11" is an odd ball size round up) 
Increase your grill size. to maybe a 20x20" or a bit smaller for your 12" duct size. Check for probley grill size for 400cfm and 12" duct.
This will take car of the grill and ducts for upstairs. Then find a good place to run a new duct to downstairs and a large grill. You probley need a 18" duct downstairs to cover the 1200cfm you still need. a 20x30 or 24x30" grill size for downstairs is needed to cover an 18" or 20" duct and 1200cfm. 

I currently have a 
1) 20x30" grill with a 16" metal duct, 
1) 6" fresh air duct from outside
3) 12x6" grills with 7" ducts from each of the 3 bedrooms 
all connected to my return plenum fro my 3 ton 1200cfm unit. 

This is in a 1600 Sf. house near los angeles california. 
Hopse this helps shead some light and give you some ideas.


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## stubits

This is fantastic! Thanks so much, way above and beyond any call of duty. 

A few questions, clarifications, etc...

1) Definitely oversized, I realized that when I was pulling the info this morning. I guess it is slightly better oversized than undersized, right? And we are planning 150 sq. ft addition on the house, so at least it will be able to accommodate that.

2) I completely agree with the idea of closing off the vent to the main floor and using the current trunk to pull from the upstairs. Just a thought, but instead of having one large return grille on the upper level, could I perhaps have 2 or 3 return air vents on the upper level? It might make sense to get air moving, right? We have one room towards the back of the house that just gets hit big time by the sun in the summer, and it might help to suck some air from there, no? Or do you think it is better to just keep things simple?

3)I understand the need for a large trunk for the main level of the home. I guess I am confused because my floor joists run 16" on center, I believe, so how would, or do people, get 18" ducts through that type of space?

4) MOST IMPORTANT!! Ok, how on earth do I go about "adding" the additional trunk to the return air plenum. I know this is the million dollar question and might not be so easy to explain, but if you could even so much as point me in the right direction, it would be great!

5) Do you see any reason to add return air vents to the basement? It is heated/air conditioned and my mother-in-law lives down there (now you can see why it took me so long to get down there and pull the model numbers).

6) So, at present I am really undersized in my return air, right? Is it possible I've done any damage to the various equipment? We ran thr AC for a full season and now the heat. How urgent is it that I make these repairs? Also, will correcting this issue result in noticeable increase in performance, ie, will the house heat/cool faster, more evenly, better, or will it reduce costs at all? I guess, what exactly are the detriments to not having enough return air?


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## integlikewhoa

stubits said:


> 1) Definitely oversized, I realized that when I was pulling the info this morning. I guess it is slightly better oversized than undersized, right? And we are planning 150 sq. ft addition on the house, so at least it will be able to accommodate that.


Yes to some extent. There is problems with it being to big too. Short runs times. The unit kicks on and off to quickly, due to it meeting the demands fast. Now while that sounds good, if the unit doesnt run long enought its harder to circulate air threw the house properly, doesnt filter very long, doesnt have enough time to dehumidify the house, which leads to mositure issues. Also it just plans sucks up to much energy. It's better to have a small unit that doesnt use much electricy and can run for loneger periods of time to circulate and filter the air. The more it circulates the air the more even the house temps are going to be threwout the house, not just in one or 2 rooms. 



stubits said:


> 2) I completely agree with the idea of closing off the vent to the main floor and using the current trunk to pull from the upstairs. Just a thought, but instead of having one large return grille on the upper level, could I perhaps have 2 or 3 return air vents on the upper level? It might make sense to get air moving, right? We have one room towards the back of the house that just gets hit big time by the sun in the summer, and it might help to suck some air from there, no? Or do you think it is better to just keep things simple?


 Sure put a return in every room if you choose. That would help move the air around, and it really helps when room doors are closed. If a room door is closed the air can go into the room but has no where to come out, so it gets lost threw the windows, loseing conditioned air. Also the vent in the bed room just might not blow out much air if the door is closed and the windows are sealed good. It will build pressure in the room and cause the air from the vent to go into another room. With a return in the rooms this helps this if the doors are closed. Plus it help circulate better. Just split the 12" into a few 7" or 8" ducts going to different rooms and such. What ever fits for 400 cfm that the main truck going up there can handle. 



stubits said:


> 3)I understand the need for a large trunk for the main level of the home. I guess I am confused because my floor joists run 16" on center, I believe, so how would, or do people, get 18" ducts through that type of space?


 Well it different for different people. I'm in a single story and my unit is in my attic. So I can fit the duct, but I have to cut and frame out some joists to fit my grill or box which is 20x30 and didnt fit between joists. You can also split this up if need be into smaller ducts. For example. My plentum wasnt tall enough to fit a 16" duct in the side or end of it. So I pulled 2 12" ducts one from the side and one from the end of it and Wyed them together (with a 12"x12"x16" wye) right next to the plentum then ran 16" the rest of the way into the return grill. So you have to get creative. If you can fit one big figure out the CFM split them up. Or open the wall and frame out a big opening. 



stubits said:


> 4) MOST IMPORTANT!! Ok, how on earth do I go about "adding" the additional trunk to the return air plenum. I know this is the million dollar question and might not be so easy to explain, but if you could even so much as point me in the right direction, it would be great!


 Need to seem more pics. You can cut holes in any side of the plentum you can see and go from there. Use the example from above. If you have alot of room you can extend the plenum to a bigger size then add ducts. More pics and me might have some ideas. 



stubits said:


> 5) Do you see any reason to add return air vents to the basement? It is heated/air conditioned and my mother-in-law lives down there (now you can see why it took me so long to get down there and pull the model numbers).


 If you have supplies down there then the air needs to make its way from there back to the unit some how. If she keeps the door closed then this falls into the anwser from above. Treat it like a extra bedroom. All this is about is getting the air to flow easy to everyplace in the house and BACK too. So stand next to your return and figure how easy is the supply air in basement going to make it back to the return vent. If its a condition room and has a supply then it would probley benfit from haveing a return around the same size as the supply. 



stubits said:


> 6) So, at present I am really undersized in my return air, right? Is it possible I've done any damage to the various equipment? We ran thr AC for a full season and now the heat. How urgent is it that I make these repairs? Also, will correcting this issue result in noticeable increase in performance, ie, will the house heat/cool faster, more evenly, better, or will it reduce costs at all? I guess, what exactly are the detriments to not having enough return air?


Yes your undersized by alot. No probley no damage. Not life or death, more about when do you want the unit to work properly and save money. As far undersize as you are you will see a big diff. in the performance and it will reduce the cost of your electricy bill. 
The fan is haveing a hard time spinning when theres no air flowing threw it. Also its not flowing much air threw it so its going to take more time to heat or cool and its going to use alot more enrgy to do it in. Think of driving you truck with a trailer behind you. The fans trying to spin, but its just not going fast enough cuz its being held back. This results in you useing more gas to try and move, and even after useing more gas, you still are not going as fast as you normally would be. 
So mostly you mwould notice lower electric bills, faster cooling and heating, and less noise from your unit. 

Some one else can probley chip in on how bad it is for the unit, but altho Im sure its not good I dont think its going to fall apart.


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## stubits

This is fantastic. Thank you so much for all the information and help.

I will upload some pictures of my current setup tonight, running the duct to the main level shouldn't be difficult, but I am concerned about how to tie it all in to the plenum.

Thanks!


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## Garasaki

Are you POSITIVE there isn't an opening for the return duct in the basement somewhere? Maybe in an area you can't quite see? Because all the information so far indicates the return is so undersized I can't see how any HVAC tech/installer would do it that way.

It's not too hard at all to add duct in. As long as you have working room. If I were you, I would extend the 16x22 return box all the way back to the main trunk (on the second level). That 8" round is killin you, and never mind how bad of an idea it was to use flex duct for return in the first place.

Get yourself a pair of tin snips (there are different "directions" to tin snips, I hate to say this but research them a bit before you buy them). Mark your 16x22 cutout on the return main. Drill holes at the corners. Connect the dots using the tin snips. Now you have a cutout.

Obtain the 16x22 duct - this may mean having a sheetmetal place make it for you, or building it yourself, or maybe you get lucky and find it at Home Depot (don't bet on it). The pieces that connect into the return main and the plenum will have tabs on it - you insert those into the cutout on the return main (and I suppose the same cutout on the plenum box at the grill), bend em over, apply tape or duct sealant paste (mastic), connect all duct in between, and away you go.

I suppose you could use any duct size smaller then 16x22 - so you could find some stuff at the local borg (HD or Lowes) that would be relatively inexpensive. But anything bigger then 50 sq inches (area of an 8" roudn duct) will help. Even using a round duct that's as big as you can find will help.

Actually if you can find like a 15" round duct, including takeoffs, that would be a real easy solution.

I'm feeling rather helpful today - here's a place that sells the takeoffs - you'd need 2

http://www.alpinehomeair.com/viewcategory.cfm?categoryID=168&p=Duct_Take_Off_Collars

Then add a few lengths of 14" duct (typically sold in 5' or 6' lengths, you just slide em together).

Hopefully you can do a straight run between the 2. Get a 8" cap to seal off the existing flex duct takeoff.

Going from 8" to 14" will triple the area of the duct. Having a straight run of smooth walled duct will help out too.

It gets hot in the summer because heat rises. And your AC isn't able to effectively pull the hot air out of the 2nd story too cool it back down. So the heat just builds up.

For the same reason, in the winter your system appears to work great. But there probably is a big temperature diff between the 2nd floor and the basement.


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## stubits

Glad you're in a helping mood.

I am absolutely certain that the basement has no return air vents. The folks who "flipped" the house ended up being really shady, bad plumbing, bad electrical, and of course bad HVAC, but we are working our way through the issues, fixing stuff when and where we can. Thank you and everyone else for your help on this project!

So, a couple of follow-up questions.

Do you agree with *integlikewhoa* that the whole trunk is undersized? The trunk itself is only 8x12. Would it even be possible to connect a 16x22 duct to that? I definitely could not connect a 15" round to it. 

What I am thinking about doing is the following...

1) Connect 2 9" round ducts to return air plenum to take care of the main floor of the home, one in the dining room, other in the living room.

2) Close off the current main floor return so that the 12x8" rectangular return air duct just takes care of the second level. In the attic I'll attach 3 7" round ducts to the trunk with registers in the hall, master bedroom and the guest bedroom.

3) I'd like to add one or two small return vents in the basement as well.

I have plenty of room to work in the attic, not a problem at all. I am pretty sure I can find or make room in the basement utility closet for all of the duct work, but I am not sure where to begin at all. Given that presently there is only one return are trunk, the 12x8 basically just attaches directly to the plenum and runs straight up the whole house, from basement to attic. I'll post some photos tonight to see if you can help me figure out how to tie all this into the plenum.

Do plenums come in different sizes? Is it possible I will need to replace the plenum to pull this all off? Is there any limit to how many times you can hack into a plenum? Any thoughts or suggestions would be appreciated!


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## integlikewhoa

stubits said:


> Do plenums come in different sizes? Is it possible I will need to replace the plenum to pull this all off? Is there any limit to how many times you can hack into a plenum? Any thoughts or suggestions would be appreciated!


You can buy plenum's in all different sizes, but they uselly don't fit right to the unit or atleast in my case. I had a local sheet metal guy make one up for me to fit what I wanted to do. They can make then wider at one end, longer or what ever you need to fit the size of ducts you need. There is not really a limit to what you can hook up to it. I have all (5) sides of mine used up. Between a fresh air intake a pressure bypass, 2 12" (that wye into one big 16") and a 10" (that wyes off to smaller returns in the bedrooms). My plenum is 24" long of the unit and around like 15x20 or so what ever the unit size on the filter end. 

I think the 2 9" your talking about are going to be way under sized for the main floor return. You need around a 20" down stairs. Installing two 9" is about a 1/4 of the flow you need. each 9" will flow around 275cfm or so. So thats only around 550cfm. We said before that your current 8x12 trunk for upstairs flows around 400cfm. So with 2 9" and your exsiting you will only flow around 950cfm. You should be atleast 1600cfm and I always like to keep it on the high side for bends, flex and other things that will make acutal even less then what I'm showing.

A 20" duct will flow around 1,327 cfm depending on lenght and such. An 18" (the next size down) flows around 1100cfm. With your existing trunk handling around 400cfm of your 1600cfm total I would go with a 20" and youll be about right where you should be. 
A 12" duct will flow around 475cfm. So even 2 12" for down stairs wont be big enough. Now 2 12" and one 9" sounds about right.


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## Garasaki

stubits said:


> Do you agree with *integlikewhoa* that the whole trunk is undersized? The trunk itself is only 8x12. Would it even be possible to connect a 16x22 duct to that? I definitely could not connect a 15" round to it.


I got the impression somewhere that the trunk was bigger.

8x12 is ridiculously undersized. Like I said, it makes me suspect there's an outlet right near the furnace/ac somewhere that you mighta missed.

If you can, replace the whole trunk. Any renovations you make at this point look pretty major. And it means some extended downtime on your furnace. So you may want to considering hiring someone - whom you can give a drawing to, then then come out one day, measure everything, then come out a second day and complete all the work. It'll cost $$ but it'll get done quickly and save yourself a lot of frustrating doing the work yourself. Or at least wait till spring.



stubits said:


> 1) Connect 2 9" round ducts to return air plenum to take care of the main floor of the home, one in the dining room, other in the living room.


Beware of adding sound to areas you don't already have it. You may find it quite annoying.



stubits said:


> 2) Close off the current main floor return so that the 12x8" rectangular return air duct just takes care of the second level. In the attic I'll attach 3 7" round ducts to the trunk with registers in the hall, master bedroom and the guest bedroom.


Not a bad idea. But if your trunk is 96 sq inches, there's not a lot of point to adding more then 96 sq inches of run between the trunk and the registers. Doing the math, I see that the 7" ducts combine to be about the same area as the trunk. So that makes sense. 

Plenums come in different sizes and you may be able to buy a premade one relatively cheaply that will help. The bigger the better. THere's no real limit to how many times to add a takeoff. But when you do, arrange your takeoffs logically, trying to keep in mind that the air has to flow thru them. Try to make the airpath as smooth as possible.

I would also consider adding balancing dampers to each run while your at it. So you can try to redirect the airflow at a later date, or even turn off runs you may decide you dont want to use. Like when your wonderful mother in law moves out.


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## stubits

Awesome.

Thanks!

So, it is possible I can get by with my current plenum... just depends on how much room I need to get this all hooked up correctly, right?

Just out of curiosity, what did it cost to have your plenum done custom?

I am not sure how I came up with the 2 9"s, must have misinterpreted something. Would 2 14" round work (or maybe 2 12? Could I tie that into the plenum with a 14x14x14 wye? 

Thanks!


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## stubits

I'll definitely wait till the spring to do any of this. Hopefully can get it done before the summer hits, DC summers are hard.

I've been all over the basement, it isn't that large really, and alas, no return air vent, as crazy as it seems.

I'd rather do the work myself, not just to save $$ but because it is interesting. Replacing the whole trunk would be a lot of work, whereas I can run a couple of ducts to the first level pretty easily. 

I know it won't be perfect, but trying to fit a modern AC/Furnace into a 75 year old brick rowhouse probably never is.


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## integlikewhoa

Also something to remember another reason you have such large or so many returns is cuz you have a unit that is way oversized for your size house. A 5 ton unit is used alot in 2500sf homes. I'm slightly oversized according to a MAN J for my 1600sf house with a 3 ton unit. My unit flow less cfm since its a smaller unit. around 1200cfm. The general rule of thumb is 400cfm per ton. Your unit for a 5 ton actually flows less then normal I would say at only 1600cfm. That actually helps you out. If it was a new high efficent unit you would probley be flowing 2000cfm out of a 5 ton unit. That wound mean bigger ducts and more of them. So this might help you as your trying to understand why do these ducts seem so big. Your unit is way oversized which you already know. But Everything comes in proportion. An oversized unit for you house, means oversized ducting, oversized electric and gas bill, oversized wiring (for the a/c compressor) etc...... Bigger is not always better. And the 150sf your planning on adding is not even dent in what this sized unit is normally installed in. 2500sf houses normally. Hope this Helps, JIM


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## stubits

I think I am understanding more and more. It sucks because the furnace and AC are both brand new (6 months or so). What a shame, and waste of money, that flippers put in such oversized equipment. Well, what can I do at this point, right? Tearing it all out and starting over is just prohibitively expensive!

So, a couple more quick questions...

1) If I do all this work now and say in 10 years I end up replacing all the equipment with correctly sized equipment, am I going to be stuck with oversized return ducts? Or does it not really matter?

2) So, it is possible I can get by with my current plenum... just depends on how much room I need to get this all hooked up correctly, right?

3) Just out of curiosity, what did it cost to have your plenum done custom?

4) Would 2 14" round work (or maybe 2 12"? Could I tie that into the plenum with a 14x14x14 wye or does one arm of the wye need to be larger?

5) If I tie in a vent or two for the basement, are they going to be super load because of their proximity to the furnace? Or, given that I'll have increased overall air flow, will it all sort of balance out?


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## Garasaki

stubits said:


> 1) If I do all this work now and say in 10 years I end up replacing all the equipment with correctly sized equipment, am I going to be stuck with oversized return ducts? Or does it not really matter?


Essentially there is no such thing as an oversized return duct. You can have oversized supply (as there is a certain velocity you want the air to shoot out of the supply register at) but not really return.

2) Still would be a good idea to get a bigger plenum on there. Or at least one that's appropriately sized. Maybe your current one is, I don't recall. But remember all the air going back to your furnace has to flow thru there so it could be a major restriction

4) You'd be better off having 2 takeoffs rather then 1 takeoff then a Y. Amoungst other things, it'll probably be cheaper to have 2 takeoffs.

5) If you add balancing dampers (at about 3 bucks each...) into the ductwork you add, you will be able to balance the airflow so the vents you add near the furnace won't blow way more air then the ones that are far away.


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## stubits

Great info. Thanks!

I don't actually know the size of my plenum. I will check that out tonight. I do know that I use a 20x16 filter, and the plenum seems to fit right up to it, so I'd guess that's at least part of it.

I am actually suggesting 2, maybe three trunks coming out of the plenum, 1)the current 12x8 will suck from the second floor, 2) a 14x14x14 wye to pull from the main floor and perhaps 3) something to pull from the basement. Am I understanding you correctly?

I have balancing dampers on all the supply vents, are you suggesting balancing dampers on the return vents as well?

Thanks!


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## Garasaki

The plenum probably forms a lopsided "ell" with 20x16 being a bigger dimension (horizontally into the back of the furnace) then it likely necks down for the vertical component.

Again I'd suggest just bringing each new run as a separate takeoff from the plenum. Your calculations will probably tell you a 14" round duct can handle about 650 cfm. That same calculation applies to the Y too. Your planning on having 650 cfm thru each branch, so that means the "base" of the Y will have 1300 cfm. Way too much air for a 14" round duct. It will be an area of large restriction.

Typically in residential heating/cooling, you wouldn't see balancing dampers on the return. But you CAN put them there. And since your goal here is to even out the airflow thru your house, it seems like a logical step. Especially for the low initial investment. An installer would probably tell you to forgo them. Again, I'm saying for 15 bucks...why not? Could come in handy.


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## stubits

I'll check out the dimensions of my plenum tonight. I think that will help a lot in figuring out what to do here. Unfortunately the utility closest that houses the furnace, as well as my hot water heater and soon my Pex manifold, isn't huge, so I might need to get a little creative here. 

I completely understand what you're saying about the wye. 

The dampers you are referring to are actually built into the ductwork itself, right? Not louvers at the grille?

Do you think I can get away with 12" round or would you stick with the 14"?


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## Garasaki

stubits said:


> The dampers you are referring to are actually built into the ductwork itself, right? Not louvers at the grille?


Yep they are just a round piece of sheetmetal the same diameter as the duct, with a tab and handle stuck on the side. You drill a single hole, stick the piece in the duct, slide the tab thru the hole (from the inside of the duct outward) then attach the handle to the tab. Takes 30 seconds at most. THen you just adjust the handle to rotate the piece of sheetmetal in the duct. When the handle is perpendicular to the direction of airflow, the piece of sheetmetal is sitting across the duct filling almost the entire thing. When the handle is parellel to the direction of airflow, the piece of sheetmetal is just a thin sliver the air flows right over. 



stubits said:


> Do you think I can get away with 12" round or would you stick with the 14"?


Well right now the whole system is served off a 12x8 duct right? Well a single 12" round duct has more area then a 12x8 duct. So it will undoubtedly still be an improvement.


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## stubits

The more I talk to you guys the more I realize just how poorly designed my current system is. It's more than just a little frustrating, although not nearly as frustrating as the basement bathroom that was installed without vents and traps. That was fun!

Little by little I am figuring things out. We love the house, but hate the flippers. I am sure I'm not the only person to ever say that.

Well, it sounds like there's some hope here. I am excited to try and resolve this problem, but will wait until the temps warm up a bit. I need to open the ceiling in the utility closet and some walls on the main level to install new hot and cold water lines (getting rid of the galvanized steel pipes, the only issue I _really_ planned on having to fix).

Does my plan sound reasonable? At least so far?

How hard is it to tie this all into the plenum? Running the ducts will be easy, straight shot from the utility closet up to the living room and dining rooms, maybe 10 feet each, so no big deal there, but how hard will it be to get it all tied in?

Also, inevitably there are going to be a couple of 45's and maybe a couple of 90's. How much of an impact will that have? The runs will be really short.


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## Garasaki

Actually you could do a lot of this while it's cold out. YOu can definately run all the new return branches, and bring them back to the furnace. Then make the final connections later.

It won't be hard to tie these in. Especially since they will be larger diameter circles. The worst part might be dealing with the cramped spaces.

You'll no doubtedly have some elbows. Each elbow is sorta like adding 5 feet of straight duct. Again, compared to what you have now, it will be a major improvement. Take your time laying it out and planning it, and measure carefully when you start cutting. You'll be golden. It may help to pay attention to duct runs you see elsewhere - grocery store, local bar, free treatment clinic (lol J/K), etc. The pro's lay stuff out very neatly - try to emulate what they do.


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## stubits

And when I tie it into the plenum, all I do it cut a hole using some tin snips and then use a take off collar to connect the round duct?


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## integlikewhoa

stubits said:


> Does my plan sound reasonable? At least so far?
> 
> How hard is it to tie this all into the plenum? Running the ducts will be easy, straight shot from the utility closet up to the living room and dining rooms, maybe 10 feet each, so no big deal there, but how hard will it be to get it all tied in?
> 
> Also, inevitably there are going to be a couple of 45's and maybe a couple of 90's. How much of an impact will that have? The runs will be really short.


Everything said above sounds great. Its not hard to tie all this in to the plenum. 

Mine was made for free as a favor from a friend, but dont quote me on this but probley less then 100 for a custom plenum from some one that does sheet metal. They sell premade ones and you can see if they fit your filter opening properly, but mine didnt. 

You should go with 2 14" atleast. or like I said before 2 12" and one 9" or so. If you go with 2 14" dont wye them from (2) 14" down to (1) 14" that dont do any good. I'll see if I can find some pics of mine, but I wyed 16". so it was a wye 16x12x12. If not you get something like a 16x16x16 and get (2) 16x12" reducers.

So if you can get some sizes and pics of the plenum and a wide shot around the plenum so we can try and see how much room we have to work with and what will fit. 

Also I didnt look threw your manual again, but there should be some DIP switchs in the heater to slow down the blower. Since your unit is over sized this might be a good idea. Im also not so sure why you have a 5 ton blower/heater 4 ton coil and 3 ton a/c unit. This im sure will play a bad part into the efficiency. 

Just work with you have and try to make the best of that and you should be good.


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## stubits

Sounds good to me.

I am guessing the mismatched components are just whatever these losers could get their hands on cheap and easy. You'd think people would want to take pride in the work, even just a little, but I think for so many it's only about $$$.


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## integlikewhoa

Heres an unfiinshed pic of a return plenum. it has 2 12"s into one 16" for the grill in the hallway 6" fresh air from outside 10" for the bedrooms and 12" on the bottom as a bypass. 
















Everything Just bearly fits. The other plenum looks the same. With 2 12" and 2 9" for the house and 1 12" for the bypass. 











http://i152.photobucket.com/albums/s186/integlikewhoa/IMG_1149.jpg


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## stubits

These photos are great, really, really helpful. It’s great to see how a system is designed as I don’t really have much to compare my system with. I haven’t had a chance to get good photos of my system yet (I took some, but they don’t really show much), but I think this might just work.

What do you think of this? Can I get a wye that is 16x14x7? If so, my thought is to attach two of these to the plenum, running both the 14’s up to the main level of the house, one to the living room, one to the dining room. I’d use the remaining two 7’s to add two return vents to the basement, one in the bedroom and one in the living room area. Or, if I am doing this, could I get away with two 12’s to the main level? Of course I’d keep the 12x8 running to the second floor. What do you think? It would have to be a major improvement, but would it be sufficient in your estimation?

Also, any thoughts on oval ducts? Just looking at them, it seems like I could use the full width of my floor joists while minimizing the protrusion into the various rooms. That said, oval duct doesn’t seem quite as easy to come by and the connections might be overly complicated, but nevertheless would appreciate your thoughts.

I think I have a good idea of how to connect this all up at the plenum, but what about at the register itself? Can I run a straight line up, cap it with a 90 and tie that 90 straight into a register? Would something like that work? Assuming I am running 14’s up to the main level, what should the dimensions of the register be? What about the grille?

So, this all started because the current main level vent makes so much noise. By increasing the number of vents and ductwork, less air should be pulled from each vent and the noise all around should go down, right?

Some more logistical questions, if you don’t mind. 
1) Can I get most of these pieces stock at either a big box store of an HVAC supply house? Or am I going to have to get a lot of stuff custom made?
2) What Sorts of tools will I need? A pair of tin snips for sure, a good pair of gloves I bet, mastic and tape, but anything else? Do I need something to crimp the ductwork? Any special tools to cut the plenum holes correctly?
3) How do I go about securing the ductwork as it runs up vertically? I am sure there must be straps of some sort. How often does it need to be secured? 

Oh and at some point you mentioned something about settings on the furnace itself. I looked through the manual but either didn’t see anything or didn’t understand what I was reading. Is it possible to lower the CFM’s through some sort of setting?

Oh and I did a bit of research on the mismatch of equipment and it doesn’t seem as bad as originally thought. First, the AC compressor is only 3 tons, so not horribly oversized for my house (oversized for sure, but not as bad as 5 tons would be!). And I read a number of sources that actually indicated that you can improve performance of the system by having the AC coil a half ton or a ton larger than the compressor, apparently it makes for greater surface area. Of course it is crazy that my furnace/air handler is 5 tons, 100,000 BTU, but what can you do.

Finally, do I see PEX in your photo? If so, did you do that yourself?

Thanks for everything!


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## integlikewhoa

stubits said:


> What do you think of this? Can I get a wye that is 16x14x7? If so, my thought is to attach two of these to the plenum, running both the 14’s up to the main level of the house, one to the living room, one to the dining room. I’d use the remaining two 7’s to add two return vents to the basement, one in the bedroom and one in the living room area. Or, if I am doing this, could I get away with two 12’s to the main level? Of course I’d keep the 12x8 running to the second floor. What do you think? It would have to be a major improvement, but would it be sufficient in your estimation?


16x14x7 would work but 16x14x8 would be better. The 2 smaller should flow the same as the bigger side. Also your going to have a hard time finding weird sizes. You'll have to use reducers possibly. No you should use 12's stick with 14's. Also if you do end up going 12's you dont need a 16" on the plenum. I say that cuz a 12" and 7" don't flow enough for you to need a 16" 


stubits said:


> Also, any thoughts on oval ducts? Just looking at them, it seems like I could use the full width of my floor joists while minimizing the protrusion into the various rooms. That said, oval duct doesn’t seem quite as easy to come by and the connections might be overly complicated, but nevertheless would appreciate your thoughts.


Never delt with oval? do they even exist? You need to insulate your duct work. Leave room for that. I think you option is either round or rectangle. 


stubits said:


> I think I have a good idea of how to connect this all up at the plenum, but what about at the register itself? Can I run a straight line up, cap it with a 90 and tie that 90 straight into a register? Would something like that work? Assuming I am running 14’s up to the main level, what should the dimensions of the register be? What about the grille?


Yes should work. Where are you installing a grill? In a wall floor or celing? 
Also a grill and register is the same. You will need a what they call a boot or celing or floor box. This will connect to the duct, screw to the studs and provide and box for the register or grill. A register is used on the output side and uselly hase louvers that open and close or can point in different directions. You wont need those for returns. You need grills. You can buy grills with filter build into them, that you remove and replace when they are dirty or you can but them with no filter if you have a filter down on the unit. Need to know with or without filter and for sure what size duct your going to have to see a grill size. 


stubits said:


> So, this all started because the current main level vent makes so much noise. By increasing the number of vents and ductwork, less air should be pulled from each vent and the noise all around should go down, right?


 Yes correct. the noise comes from the speed the air is traveling. Since the duct is to small the air has to travel fast to try and get so much threw. The bigger duct size used the slower the air goes. 
Same goes for when you pinch the end of a garden hose or cover it with your thumb. The water shoots farther. Theres not more water comming out, theres actually less. But since you pinched or covered the end of the hose the water shoots out further cuz it has to speed up to fit the water threw a smaller hose or hole.


stubits said:


> Some more logistical questions, if you don’t mind.
> 1)Can I get most of these pieces stock at either a big box store of an HVAC supply house? Or am I going to have to get a lot of stuff custom made?


 Bix box stores suck at Duct supplys and they wont have any big parts. maybe small 8" pieces and small register covers. You need to find a HVAC supply store. they will have everything. 



stubits said:


> 2)What Sorts of tools will I need? A pair of tin snips for sure, a good pair of gloves I bet, mastic and tape, but anything else? Do I need something to crimp the ductwork? Any special tools to cut the plenum holes correctly?


 There is a nice tool that hooks to a drill and cuts circles in the plenum. I bought mine at home depot, but HVAC probley cheaper. Youll need soem self tapping screws to hold the metal parts together before you tape and mastic them. 


stubits said:


> 3)How do I go about securing the ductwork as it runs up vertically? I am sure there must be straps of some sort. How often does it need to be secured?


 HVAC store will have a strapping like plummers tape.( metal cut to lenght with holes in it, but its wider). 



stubits said:


> Oh and at some point you mentioned something about settings on the furnace itself. I looked through the manual but either didn’t see anything or didn’t understand what I was reading. Is it possible to lower the CFM’s through some sort of setting?


Well most do have some setting but mine is a brand new 95% dual stage unit, so maybe it has more options then yours im not sure. But inside the heater on the main control board where the wires hook up there should be some DIP switchs that can be changed depending on different settings for you unit. Mine has different blower speeds for each stage of heating and cooling. Im thinking your might have something like that. It should be in the install manual or writen inside the unit on the back of the door or somthing. I figured maybe you could turn down the CFM, to match the house and save some electricity. Im not so sure on your unit how it works. 


stubits said:


> Oh and I did a bit of research on the mismatch of equipment and it doesn’t seem as bad as originally thought. First, the AC compressor is only 3 tons, so not horribly oversized for my house (oversized for sure, but not as bad as 5 tons would be!). And I read a number of sources that actually indicated that you can improve performance of the system by having the AC coil a half ton or a ton larger than the compressor, apparently it makes for greater surface area. Of course it is crazy that my furnace/air handler is 5 tons, 100,000 BTU, but what can you do.


 OK. youll be ok. Not saying throw everything away and start over you can make this work. Also maybe you need the heat more then the A/C in your area. I need more A/C then heat here. 


stubits said:


> Finally, do I see PEX in your photo? If so, did you do that yourself?


Yes thats 1/2" pex. Yes pex myself and it is eaiser then the A/C work. That pex was run up into the attic for the Humidifier. I ran it next to the drain comming up an interior wall going down to the crawl space below.


Also you can check out this website, to see whats out there and send links of parts you want to use. http://americanhvacparts.com/duct-work.htm


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## stubits

Sounds great. I will stick with the 14's and go with 8's for the basement returns.

How far do you think a 16" 90 would stick out from the plenum? Based on your experience? Trying to figure out what would be sufficient clearance.

They do have oval ducts, but not so common, so I will avoid. Do I need to insulate the return air ducts? I know the supply ducts need to be insulated. Good to know, if that's the case!

Thanks for clearing up the confusion on the grill/register issue. All of the return I am doing will end up being on the wall I think. What size box will I need of the 14s? How about for the 8s? How deep are the boxes? Just making sure I am going to have enough room in my design. How about the grilles, what size would be appropriate?

Is your whole house PEX? I am getting ready to repipe the whole house in PEX. We currently have 75 year old galvanized steel which has caused really awful flow throughout the house. I think I'll combine the ductwork with the plumbing project as it involves some of the same walls. Any suggestions on the PEX? It seems pretty manageable to me.

Thanks!


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## integlikewhoa

stubits said:


> Sounds great. I will stick with the 14's and go with 8's for the basement returns.
> 
> How far do you think a 16" 90 would stick out from the plenum? Based on your experience? Trying to figure out what would be sufficient clearance.


Probley around 20-24" or so with the starter ring. 


stubits said:


> They do have oval ducts, but not so common, so I will avoid. Do I need to insulate the return air ducts? I know the supply ducts need to be insulated. Good to know, if that's the case!


Sure the oval is not for furnace exhaust or hot water heater exhaust like double walled? Thanks all I have seen around here.
Insulate if its not in a conditioned room. Mine is in the attic. It gets over 110 up there in the summer and the return air will be the same as in the house. So I dont want the return pipes to turn 100 degress and heat up the return air. So if your unit sits in a place thats condition and all the pipeing is inside the conditioned parts of the house then your ok. Same goes for you in the cold. If you unit sits in the basment and its 30 degrees or less down there and your running the heater the return air in the pipe should be 70 degrees. You will want to insulate it. 



stubits said:


> Thanks for clearing up the confusion on the grill/register issue. All of the return I am doing will end up being on the wall I think. What size box will I need of the 14s? How about for the 8s? How deep are the boxes? Just making sure I am going to have enough room in my design. How about the grilles, what size would be appropriate?


Different boxs are different. you can do any of these types.
http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=duct-stackhead_boot
http://americanhvacparts.com/Mercha...GY&Category_Code=duct-stackhead_perimeter_box
http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=duct-cone_boot
http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=duct-floorboxes

Just browse threw the site and youll get some ideas on what is what and what will work for you app. 


stubits said:


> Is your whole house PEX? I am getting ready to repipe the whole house in PEX. We currently have 75 year old galvanized steel which has caused really awful flow throughout the house. I think I'll combine the ductwork with the plumbing project as it involves some of the same walls. Any suggestions on the PEX? It seems pretty manageable to me.


My house isn't all in pex now. What I did is got the manifold setup off the main line and I'm slowly adding new lines to the manifold (with valves) as I remodel each room. So I'm half and half now. I redid the main line when I bought the house and that enters the house in the crawlspace and splits off to the old galvinized and the new pex. Eventually I'll cap off the part going to galvinized when everything is moved over.


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## stubits

Good deal. I'll need to see how well I can fit this in my space, I am sure I can come up with something.

Most of the ductwork will be in conditioned space, except for the couple of lines I run up in the attic, so that's good.

What size grille would you suggest for the 14's? For the 8's?

Thanks!


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## stubits

I've poked around a lot on the internet and can't seem to find a stackhead box that accepts 14" duct, seems to max out at 8". Any thoughts?


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## stubits

Ok, so after a very long hiatus, I'm back. integlikewhoam, I certainly hope you see this!I've done a bit of investigative work and it looks like I'll need to change course just a little bit based on some new measurements. Last we talked, the plan was to run 2 14" rectangular ducts to handle return air from from main level. Unfortunately, I have a plumbing stack in the way, so it just won't work. That said, I think I have a plan. Here is what I am thinking. I'd really appreciate your evaluation.

1) As previously discussed, I'll close up the current 8x12 rectangular duct so that it only services the second floor of the home. = 400 CFM

2) I will add either 1 8" round or 2 6" round returns to the basement. = 250 CFM

3) I will run a 21" x 10" rectangular duct to service the first floor of the home. = 1000 CFM

By making these changes it looks like I will go from 400 CFM of return to 1650 CFM of return, so big, big change.

Now, the problem is, I am still uncertain as to how to pull this all off. I've taken a couple of photos of the current setup in hopes it might help. To put it all in perspective, our air handler uses a 16x20x1 filter. Looking at the wider angle photo, working from the bottom up you have the plenum, the next run of ductwork from there is 20x8 and then it reduces down to 12x8.

So, is my proposal even possible based on the setup? Is the 20x8 duct part of the plenum? If so, can I tie right into that? Or, do I need to start by finding a new plenum? Also, just so I can figure out (tell my wife) how this is going to look, what size grills will be required for the various elements? 

Thanks!


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## beenthere

Your getting some good advise.

You may also want to check a few things before you do your duct alterations though.
First, check the temp of the air at the supply closest to the furnace, or the one that blows out the most air. See how hot it is, compared to the return air temp.

We can tell roughly how much air your furnace is currently moving by its temp rise.

You have a 100,000BTU input furnace with a 5 ton drive, matched to a 3 ton A/C.

You need to be able to move enough air for the furnace in heating. But, still be able to lower that to 1200 CFM for your A/C in summer, other wise, if you move too much air in cooling. You will have high humidity problems.

Running individual returns to your upstairs rooms is a good idea.

Abandoning the current first floor return, and making one some where else on the first floor is a great idea.
Making it that you can restrict that first floor return in summer will help in cooling the second floor in the summer.

Adding a return to the basement, WELL, depends if the furnace room has outside combustion air or not. If outside, good, if not, bad idea.

A study done in Florida. To prove the pit falls of oversizing went bad, when they forgot 1 little detail.

In these houses, they removed system that were oversized by ½ to 1½ tons.
They did load calcs, and installed the proper size.
Unfortunately, they did not install any way to restrict the return air. So they were not able to slow the air flow down for the new smaller A/Cs.
Few of the houses saved any money. And there were complaints that the houses were now too humid. Causing some to lower their thermostats a couple degrees lower then with the oversized units. So they used more electric with the proper size.

Point of telling you that. Is that if you increase return, and don't allow a way to decrease the air volume for the summer, you will make your house more humid, and use more electric then last year.


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## stubits

Wow, thanks so much for all the advice. I certainly couldn't take on a project like this without it.

I can figure out to measure the temp at the supply, but how do I go about measuring the temp at the return?

So, regardless of heat or cooling, the 12x8 return air duct is significantly undersized, right? Am I about on point with my proposal, so far as obtaining 1600 CFM of return?

What do you suggest for how to limit the return on the main floor? A louvered grill or do they make a damper or something I can install down in the basement?

As for installing a return air vent in the basement itself, it is actually living space, heated and cooled. I do not believe that there is any outside combustion air, but the door to the utility closet is louvered. There are actually three rooms in the basement. What do you think?

So, Now, the problem is, I am still uncertain as to how to pull this all off. I've taken a couple of photos of the current setup in hopes it might help. To put it all in perspective, our air handler uses a 16x20x1 filter. Looking at the wider angle photo, working from the bottom up you have the plenum, the next run of ductwork from there is 20x8 and then it reduces down to 12x8.

So, do you think my proposal from my last post is even possible based on the setup? Is the 20x8 duct part of the plenum? If so, can I tie right into that? Or, do I need to start by finding a new plenum? Also, just so I can figure out (tell my wife) how this is going to look, what size grills will be required for the various elements?

Thanks so much!


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## stubits

Bump...


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## beenthere

Even your 8x20 is too small.

You don’t really have a return plenum.
You have a return drop with a ell. And no vanes.

Your drop should be 12x22 or 10x28.

A sidewall return grille (non filtered) for the first floor should have roughly 380 sq in of free area, making it about a 20x20 to a 20x24 grille face.
Depends on what type of return you have installed on the main floor. Sidewall or floor.
And on how its tapped into the system.

If by a duct into the drop, then a damper can be installed in the duct that runs between the grille and duct.

Since the basement air is used for combustion. You MUST be careful not to install a return that could put the room into a negative pressure, and draw flue gasses into the basement and harm or kill someone.
You’d be surprised how much air a furnace needs for combustion.
Since basements tend to cool quickly in the summer, a small return, may be ok. But many people shut the supplies in the summer, and if the duct system, and return grilles to the other floors in not enough, it will put the basement into a negative pressure.

As for measuring the return temp. Place a thermometer at or in the return duct on your first floor.


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## stubits

Thanks so much. Looks like we're getting somewhere. This project seems fascinating, but I am still a bit intimidated. I've got a fair amount of DIY experience, but not much experience with HVAC, so it's great to learn. Thanks for your patience and guidance.

So, a couple of follow-on questions...

1) If I don't have a plenum, do I need a plenum? Or am I OK by increasing the return air drop to the sizes you indicated?
2) In order to increase the return air drop do I need to change out the ell? 
3) What I am confused about is how to connect a 12x22 or 10x28 drop to the air handler. Do I need to replace the ell? Do I need to replace the filter frame? Can I use something like this? http://www.audubonsupply.com/browse.cfm/2,154.html
4) The main level will utilize a sidewall grill. Unfortunately due to space restrictions, I am planning on using something like this, http://www.audubonsupply.com/browse.cfm/4,1122.html to cap the 20x10 run to the main level. Would this work?

I will check the temperature differential this evening and report back.

Any thoughts? Is this work remotely feasible? I am just hoping I can fit it all into my little utility closet. Ugg! It would be nice if people did things the right way the first time around!


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## beenthere

The link to the return packages, have the ell for connecting to your furnace.

I thnk those packages have teh filter slot in them, but I'm not sure.

The second link, you can use that ell.

Where there is a will, there is a way.
Do you know anybody named Will?  LOL...

Measure how much room you have in your furnace room, before proceeding.
Easier to change a plan in the beginning, then in the middle.
You may need to have a sheetmetal shop make a fitting for you, depending on the amount of install room you have.


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## stubits

Couldn't agree with you more, owning a home has taught me that the only limit is my patience, really. Thanks for all the help. I am just hoping your patience doesn't wear out!

I have been looking through the materials related to the furnace and am a little confused. You can find the specs for my air handler here, http://www.allreds.net/WS2/docs/rhe/80ps_specs.pdf. There's some contradictory information in the document, or at least based on my understanding. On page 3 it indicates that the standard filter for my unit is 19.25x25. That said, the diagrams on page for seem to state that the right/left side return air cutout is only 23x15. What do you make of this? 

The kits seems to include just about everything I'd need to make this work, 
but not so sure about the sizes. From what you said earlier, the 20x10 with 20x16 filter is too small, right? The next size up seems to be 25x10, which is too big for my air handler. Any thoughts?

I think I need to start here, figure out how much room I need and then go from there. 

Thanks!


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## stubits

Also, just a thought. In reviewing the information on the air handler, it looks as though it runs at a higher CFM when cooling. Based on your previous indication that we need to reduce CFMs during the cooling cycle, would it be possible to change this setting somehow? ** This is getting way ahead of myself, I know, but wanted to write it while thinking of it**


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## beenthere

The filter the manufacturer suggest is always smaller then the opening.

The 10x25 would be fine for your furnace.

The blower speed for cooling can be slowed down by using the blower speed taps.
Just have to selelct the correct one for your A/C. Will probably be the low speed tap.


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## stubits

Beenthere, I feel guilty not knowing your name, you've been so helpful. Mine is Adam.

Thank you, thank you, thank you for looking through all those specs.

Excellent! So, if I use the 10x25, even thought it is a bit smaller than what you suggested (22x12 or 28x10) I should be fine? Will this allow me to move 1600 CFM or so? I suppose I can just enlarge the hole in the air handler where it attaches by a few inches to get it to fit (currently I think it is 20x16), right? Do I just use a hacksaw?

With this setup, I can tie in the current 12x8 duct running up to the attic, as well as the 20x10 I am proposing to serve the main level of the home, right? Can I just tie straight into the 25x10 drop? Just cut an appropriately sized hole in the side? That simple?

Returning to the issue of the basement, the house is really not the least bit airtight, it is a 75 year old home. Does that help? I'd plan on installing the basement return in different room from where the utility closet is housed. Does that work?


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## stubits

bump...


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## beenthere

For some reason. i don't get email notification from this site. Could be my ISP.

My name is Jim.

The 10 x 25 will be ok for 1600CFM, it will run a slightly higher static pressure, but with it being short, not very high.

Both the duct to the attic, and the new one for the first floor can be cut into the 10 x 25.
It takes a little practice to cut them in smoothly and correctly.

You may want to install the 10 x 20 first, with the duct from the attic tapping into it, right above where the 10 x 20 sits over the 10 x 25.

The hole in the furnace, can be enlarged by either tin snips, shears, sawzall, jigsaw, or, the hard way, hack saw.

I prefer, electric shears.


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## stubits

Jim-

It's been a while. Thanks for all the help. It's been a long winter and I've had too much time to think. I am getting ready to start some major projects on the house over the next little bit, adding a new laundry room on the main floor of the house, repiping the place with pex and the big one, a small, albeit three story addition on the back of the house to add a full bath, powder room, mudroom and some storage. It should be fun, the architect is working up the plans now. But somewhere in there, I am going to get to this return air problem. Figure if I am opening up one can of worms, why not open them all. 

Anyway, I feel great about the plan we setup to improve the return air but I am wondering, while I am at it, how hard would it be to convert the system over to a multizone? I think ultimately I'd like to have 3 zones, one for each floor of the home, does that make sense? I've done a bit of research and overall it doesn't seem too complicated, but was hoping you could talk me through it a bit. Anything to watch out for? Any special considerations? Any suggested materials?

Thanks!


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## integlikewhoa

A multi zone system is going to need to have some room to split everything up. You need to seperate things a bit more then what you have. Your returns should also be seperated to each area that you have a zone. So each zone Idealy would have a seperate trunk line off the end of the coil. Also idealy the returns would be the same. A seperate return for each zone. So any air that gets pumped into a room (or zone) doesnt have to leave that room (zone) to return to a return grill, if that makes sense. Zoneing I found for me wasnt to expensive. The dampners are 100-150 each per zone. The key pads are 100-150 per zone. The panel was few hundered. You will need a bypass dampner from the plentum to the return. Youll need a few other sensors too (outdoor & discharge). You will find if you take this project on (which is nothing compaired to what you have planned already) you will probley remove and replace both plentums you already have on the unit in order to make more room and seperate the zones. You will most likley end up haveing to reduct the majority (probley all) of the house. So if you prepaired to spend 1500-2k worth of equipment and Reduct the house I say go for it. I have done it and love it. I have a much smaller house then you (1500 sf) and have & zoned 3 bedrooms and the rest of the house seperatly (much diff. then yours). At night I run only the bedroom heat or a/c and have the rest of the house on a 7 day timer to turn on in the Am. Guest bedroom/office I turn on when Im going to use those rooms. And If I have them all on. Its evens out all the temps threwout. The side of the house that the sun hits and those rooms always seem hotter now dont. 

Heres the equipment I have. Honeywell Networked Zoning. 
http://customer.honeywell.com/Honeywell/UI/Pages/Catalog/SystemCategory.aspx?Catalog=Homes&Category=Networked+Zoning_131&ChannelID={2EB2F178-20ED-44E0-97FB-CCFB4218DD64}

Panel:W8835A1004
Outdoor Temp sensor: 50022037-001
Discharge Air temp sensor: C7835A1009
Thermostats: TH9421C1004 (4 of them)
Dampners: ARD12's (2) and ARD8's (2) ARD9 (1) 
Bypass Dampner: SPRD12
120 to 12volt transformer: AT140D 

Dont go by the list prices on the Honeywell sight. I payed around half those price for everything. Search google you will find atleast 3 good, big websites that have everything. Heres one.
http://www.iaqsource.com/category.php/honeywell-zone-controls/?category=656
They have part number for complete kits. 

Heres a PDf that might help your too. 
http://customer.honeywell.com/techlit/pdf/50-0000s/50-9420.pdf

There is also a telephone access modual I have that's very nice: *W8735D1009*
You could look into that too. Everything uses 3 wire IAQ wireing to each componet back to the main zoning panel. 

Feel free to ask any questions if you start this project. Thanks JIM


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## beenthere

Thats a good zoning system.


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## stubits

Makes perfect sense. I think I am actually in better shape for this than it appears.

First, I need to improve the return air regardless and it will effectively be divided up by the 3 zones, 2nd floor, main floor and basement. 

As for the supply, currently there are two branches from the air handler, one that supplies the 2nd floor and one that supplies the main floor and the basement. My thinking is that I will simply close off all the current supplies to the basement and run a third branch for all new ductwork in the basement, while leaving the 2nd floor and main floor ductwork the way it currently is. Will this not work? The way I see it, there will be a separate return air and supply air branch for each zone. 

Do I need to install motorized dampers on the return air as well, or just the supply? 

I understand the need for the bypass damper, but what do the two sensors do?

So, any thoughts?


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## beenthere

Outdoor sensor, is for freeze protection when using a humidifier.
Also displays outdoor temp on thermostats.
Discharge sensor, protects the furnace from over heating, and the A/C from freezing the coil.
When only one zone is calling.
No need for dampers on the return.


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## stubits

OK, great. So, assuming I have 3 distinct branches for the supply, this should be a pretty straightforward process? That is to say, no need to worry about reducting the whole house? Or do I?


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## beenthere

Don't need to reduct the whole house for 99% time.


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## stubits

So, effectively I need to...
1) Add dampers to all three of the supply branches
2) Place a thermostat in each zone
3) Add a multizone panel to manage everything
4) Install the bypass damper between the return and the supply
5) Install a discharge sensor

Is that the project in a nutshell?


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## beenthere

Pretty much.

Of course, you need to size the damper to move enough air for the smallest zone to call by itself.

If you use the panel suggested earlier. Seldom will the basement zone have to call.
It uses auto master technology.
Helps prevent needless short cycling.


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## stubits

Hmm. So, they damper isn't just the same size as the duct? Can you explain this a bit more? I am confused. Or are you just talking about the bypass damper?

As for the basement, that's actually why we are doing this. My mother-in-law lives with us in an apartment in the basement. Ultimately we'd like a way to heat/cool the basement without necessarily heating and cooling the rest of the house. We'd like her to be comfortable without having insanely high energy bills. So, am I going about this the wrong way?


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## beenthere

I was talking about the bypass damper.

Zoning is a good way.

The 8835 panel will auto master.
Means the zone that calls the most will be the master zone.
So if the basement were to call for heat more then the other zones. The basement will become the master, and the other zones become slave zones.
So that when teh basement calls for heat. The zone panel looks at teh temp of the other zones, and will give them some heat also. Depending how far away from set point temp they are.

This prevents short cycling because it eliminates those zones from calling for heat 5 minutes after teh basement zone is done calling.

Same in summer. The zone that calls for cooling the most, will become the master zone.

It saves a lot of wear and tear on your equipment, and can save money on heating and cooling.


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## stubits

Ok, starting to make sense.

So, how do I go about properly sizing the bypass damper? All of the zones are actually the same size, each is one floor of the home, approximately 650 sq. ft. per zone. Any suggestions?

The auto master system is interesting to know. Are there other sorts of systems? Is the auto master system the best? Are there systems that are more programable? For example, we'd like the basement zone to be the master during the day, but one of the other zones to be the master during the evening. Is that too complicated? Are are the options?

Thanks!


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## beenthere

The system will determine which to use as the master, by determining the zone of greatest demand.

You can't change it yourself. 
Its a comfort feature. 
Having the basement as the master zone, when one of the other zones calls twice as much. Would mean the other zone wouldn't be able to maintain temp.

Their are other zone systems out there.

But, not as well set up. They won't have auto zone mastering. So they will allow your system to satisfy one zone. And 3 to 5 minutes later start up for another zone. That could have used some heating or cooling while the first zone was running.
But because its not a communicating system. It can't know if another zone will need conditioning if a few minutes.

Your basement zone won't need as much A/C as the first floor zone, which may not need as much A/C as the second floor zone.

Suppies are suppose to be sized to deliver X amount of CFM.

So you need to find out which zone has the least amount of CFM delivered to it.
Then subtract that from how many CFM your equipment must move.
That tells you how many CFM your bypass must handle.


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## stubits

This is great information! Thanks! 

Ok, so from what I understand about the auto master system, it won’t really affect how we as users experience the system, right? What I mean is, does it give preference to the master zone? Let’s say during the summer I want my bedroom (zone 3) cooler than the basement (zone1 – master). Does the basement win out, or, within reason, can the two zones have different temperatures? Does this make sense? Is the auto master mostly to make sure that the HVAC system is running most efficiently?

Ok, now some questions about sizing. As you know from all my previous posts, I don’t think this system was designed with a whole lot of thought and care (not enough return air, oversized air handler, etc). I’d rather not rip out all the ductwork and the whole system, but I want to make sure I’m not making any big mistakes here.

So, each floor is approximately 650 sq. ft. of living space and I think I’d like to make each floor its own zone. Can you give me a VERY rough estimate of how many CFM I _should_ have per floor? 

Currently the supply air plenum is 22 ¼ x 20 x 20 and there are three branches coming off of it, there are two 10x8 rectangular ducts that service both the basement and first floor. There is also a supply stack that runs from the plenum up to the attic where it branches off to service the 2nd level. I haven’t been able to figure out what size it is yet, I need to get up into the attic to figure it out. 

In order to make this all work, I will seal off the existing supply to the basement so that both of the 10x8 supply ducts will only service the main floor of the home. I will add new ductwork just for the basement. I will keep everything the same with 2nd level.

Ok, so here are my questions…

1) Can one zone have more than one damper? That is to say can I put a damper on both of the 10x8s and have them operate together? I assume the answer is yes, but just checking.
2) What size ductwork would you recommend to service the basement? It is 650 sq. ft and three rooms (bathroom, bedroom and living room/kitchenette).
3) I think it would make sense just to replace the supply plenum, no? If so, is this one sized correctly? Should I get one that is bigger? Smaller?
4) In order to make this work I think I need to move the whole HVAC setup forward a few inches. Will that be hard to do? I will already by disconnecting the return and the supply. I know I’ll have to rework the gas line a little bit, as well as the vent, but otherwise, should it be manageable?
5) As for the bypass damper, thanks for the explanation on sizing it. That makes sense. Where would it be located? Can I connect it from the supply plenum to the return plenum/drop?

Thanks!!


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## beenthere

> 1)Can one zone have more than one damper? That is to say can I put a damper on both of the 10x8s and have them operate together? I assume the answer is yes, but just checking.
> 2)What size ductwork would you recommend to service the basement? It is 650 sq. ft and three rooms (bathroom, bedroom and living room/kitchenette).
> 3)I think it would make sense just to replace the supply plenum, no? If so, is this one sized correctly? Should I get one that is bigger? Smaller?
> 4)In order to make this work I think I need to move the whole HVAC setup forward a few inches. Will that be hard to do? I will already by disconnecting the return and the supply. I know I’ll have to rework the gas line a little bit, as well as the vent, but otherwise, should it be manageable?
> 5)As for the bypass damper, thanks for the explanation on sizing it. That makes sense. Where would it be located? Can I connect it from the supply plenum to the return plenum/drop?
> 
> Thanks!!


1. Yes. You just daisy chain the wiring from one damper to the next.
2. Do a load calc, and you will know how many CFM needed for each floor.
Also makes it easier to size the bypass damper.
HVAC CALC that advertises here, is a good program to use.
3. Depends. If you have room to install the additional supply trunk. And to install the bypass damper. No need to replace the plenum. If you don't have enough room on the plenum. Then making it longer will help.
4. Might have trouble with the A/C lines.
5. If you have a central return. Connecting it to the return box and supply works good(This contradicts Arzel instructions, but we ain't talking about an Arzel system).
If you don't have a single central return. Then connectin the bypass to the return that can handle both the bypass volume, and the retrn air for the area it serves works.

Last place, is direcly to the return plenum.

1-*Honeywell Y8835A1028 System saver kit.*

Comes with:

1) W8835A1004 Envirazone Zone Panel (3H/2C)
(3)TH9421C1004 VisionPro IAQ Thermostats
(1) AT140A Transformer
(1) C7835A1009 Communicating Discharge Air Sensor
You may want to get the optional outdoor temp sensor. Comes in handy if you have a humidifier.
Bypass damper is not included, since no way for them to know what size you need.
You can set the thermostat for any zone at any temp you want.
You can even use auto change over. 
Which would make it that one zone can call for cooling. And later, that zone could call for heating if it were to get to warm.


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## stubits

Excellent. Thanks!

This all makes sense. I will look into doing the load calculation, although given that the system is already installed, there isn't much I can do to change things now. I have heard that a fair estimate is between .8 and 1.5 CFM's per sq. ft. Is that even remotely accurate?

As for the return, I think there are photos on page 4 of this thread. Effectively all I have a a straight drop into an ell. Based on the photo, do you think I can go right from the return air drop to the supply plenum?

Thanks for the suggestion of the honeywell unit. It looks great! I'd just need to add the bypass damper and the regular dampers, right?

I will probably add a humidifier in at the same time. Do you have any thoughts on whole house dehumidifiers as well? DC has horrible humidity!


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## beenthere

Each floor will need a different amount of air flow.

Those rules of thumb are good for causing trouble.

The basement will need less then the third floor.


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## stubits

Makes sense. I will run the calculation. I wish I was installing the system from scratch, but what can you do, right?

Also, if I need to move the unit and the AC lines don't have any give, how hard is it for a pro to extend them? Is it even possible?


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## beenthere

Its not hard to extend the lines.


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## stubits

Good news. Is extending the lines DIYable? Or best left to a pro?


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## beenthere

Need a tech to pump down, vacuum, and recharge, or check charge.
Pkus to braze them if you don't have a tourch set up.


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## stubits

I am sorry to revive this old thread, but I thought it made sense to do so as my questions are really just a continuation of the issues raised previously. Hopefully this makes things easier and less confusing. 

Short story is that with the help of some of the forum members here, especially beenthere, I learned that the ductwork in my house was poorly designed. We will be adding a small addition over the course of the next few months and I'd like to at least start to address some of these issues. The air handler/furnace as well as the ac coil/condenser were both installed just two years ago, right before we purchased the home and while they're not the best and are a little oversized for the home, but we plan on sticking with them. The details for the equipment can be found here.

Right now I just have some basic duct design questions, but I am sure more specific ones are not too far away.

1) I downloaded the demo for HVAC Calc and have been entering the specifics for my house. If anyone has any experience with this software, how do you handle and open floor plan? Our kitchen and dining room are only divided by a half wall. Should I treat this as just one room?

2) I have heard that HVAC Calc does not do a great job with duct design. Given that I do not need help choosing my equipment, but only with duct design, is HVAC Calc even going to be helpful? Is it worth purchasing the two month license? I am guess that the heat loss/gain info it will give me will help to make the right decisions, right?

3) I know that there are some best practices for duct design/placement, like installing supply registers in the ceiling on exterior walls, usually above windows and doors. Are these registers meant to "wash" the wall or blow air into the room? Given that I am retrofitting a 75 year old brick rowhouse, these locations aren't always possible. How big of a deal/problem is this? Such as ducts being located in the floor along interior walls? 

4) Our basement is finished (and my mother-in-law lives there), so it is important that it be as comfortable as the rest of the house. How effective is it to install a circular register in the center of the ceiling? Assuming it is properly sized, is this effective?

5) As this thread determined, our return is significantly undersized. Currently we just have a return air drop into an ell through a filter and into the side of the air handler. There are photos here. Our furnace room is tight on space and I am wondering if there is a more efficient setup that might gain me some extra space to allow me to up the size of the return air drop. For example, I have seen right angle return air shoe or this support box with filter or any of the options found here. Do these options use less space than a standard cold air shoe/ell?

Any thoughts or advice would be very much appreciated!!


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## beenthere

stubits said:


> 1) I downloaded the demo for HVAC Calc and have been entering the specifics for my house. If anyone has any experience with this software, how do you handle and open floor plan? Our kitchen and dining room are only divided by a half wall. Should I treat this as just one room?
> 
> Yes, list it as one room, since the air can travel relatively freely between the 2.
> 
> 2) I have heard that HVAC Calc does not do a great job with duct design. Given that I do not need help choosing my equipment, but only with duct design, is HVAC Calc even going to be helpful? Is it worth purchasing the two month license? I am guess that the heat loss/gain info it will give me will help to make the right decisions, right?
> 
> It isn't good for designing the actual physical size of the duct. But it is good for finding out how many CFM's an area needs. And then using standard methods to determine duct size from there.
> 
> 3) I know that there are some best practices for duct design/placement, like installing supply registers in the ceiling on exterior walls, usually above windows and doors. Are these registers meant to "wash" the wall or blow air into the room? Given that I am retrofitting a 75 year old brick rowhouse, these locations aren't always possible. How big of a deal/problem is this? Such as ducts being located in the floor along interior walls?
> 
> A register in the ceiling for heat, should throw the air down. It should have a spread that washes an area of the wall also.
> 
> A floor supply on an inside wall, won't get you as much comfort as if it was on an outside wall. But as long as it has good through to miz teh 2 airs. it will heat or cool the room.
> 
> 4) Our basement is finished (and my mother-in-law lives there), so it is important that it be as comfortable as the rest of the house. How effective is it to install a circular register in the center of the ceiling? Assuming it is properly sized, is this effective?
> 
> Round ceiling registers are great for cooling, not so good for heating.
> 
> Anyway to position it off center so its toward an outside wall, and then install a rectangular one instead.
> 
> 5) As this thread determined, our return is significantly undersized. Currently we just have a return air drop into an ell through a filter and into the side of the air handler. There are photos here. Our furnace room is tight on space and I am wondering if there is a more efficient setup that might gain me some extra space to allow me to up the size of the return air drop. For example, I have seen right angle return air shoe or this support box with filter or any of the options found here. Do these options use less space than a standard cold air shoe/ell?
> 
> Any thoughts or advice would be very much appreciated!!


The "support" type you linked to. Are very very restrictive to air flow(I'd have to censor myself if I told you what I think of them).
The right angle is better.


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## stubits

Jim-

Thanks so much, this is tremendously helpful. I will go ahead and purchase the two month license of HVAC Calc and start to figure out the ductwork from there.

When you say determining duct sizes using "standard methods," do you mean using a ductolator or some of the rules of thumb, like a 6" round supply line puts out about 110 cfm? Can you point me in the right direction for the standard methods? I have read some interesting material here and even better here. Do either of those resources look good/usable?

I am located in DC and my experience is that we have equal parts heating and cooling demands (although the heat loss/gain calculations will tell the truth, right?). We have cold dry winters, 30's and below for about 3-4 months and hot, very humid(90%) summers, 90's and above for about 3-4 months. Our supply registers on the upper level are in the ceiling on outside walls and I think these are sized more or less correectly. On the home's main level, the registers are all in the floor and are located wherever they could be placed conveniently, but not on exterior walls. We are in a rowhouse though, so we only have 2 exterior walls, the other 2 outer walls are shared with our neighbors. Based on "rule of thumb" the vents seem to be properly sized, if not perfectly located. The problem however is that while the ductwork for the main level is sized perhaps close to correctly, the previous owners simply tapped into it to supply the finished basement, effectively cutting it in half. I guess I will have a better idea of what is going on after I get the data from HVAC Calc, but any thoughts or impressions? 

I am hoping to be able to keep the ductwork for the upper level with minor modifications and am hoping that the ductwork for the main level will be workable if I close off the vents to the basement, but that means I will need to add new runs for the basement. Thanks for the info regarding the round registers, I will be sure to use the rectangular. What is the best place for the register in a room that does not have any exterior walls at all? Does placement mattter in that case?

Does the right angle return take up less space than the standard cold air shoe/ell with a filter frame? I am guess it is a better option, but what has your experience been? I will definitely avoid the support box. Thanks!

Thanks!


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## beenthere

The right angle box and a return ell will take up just about the same amount of space, if the ell is made for a 4" air filter.

That PDF uses some real bad rules of thumb.

Udarrels guide line are based a bit on rules of thumb. So if you use it. You end up with having to put in ALL new duct work.

This is where a manometer, and or vane anometer would help a lot.
They would allow you to determine existing CFM to each room, and to measure current static pressure. And from there determine what course of action to take.

Lacking those.

Figure. You are NOT getting more then if even 100CFM through a 6" sheet metal supply(undersized trunk line prevents it).

A larger trunk line will help reduce static loss before the supply runs, and increase the amount of air from all of your supplies.

Can you draw and post a diagram of your duct work. And give lengths and sizes of it.


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## stubits

As always, thanks so much. Really helpful. So, I will stay away from the PDFs for fear that they are just going to cause more trouble than they are worth.

If buying a manometer/vane anometer would make this easier and better, I am happy to do so. Which is preferred, any recommendations? Since I'll only be using it this one time, I'd like to go with the bare minimum necessary to get the data I need. Any thoughts? This looks pretty cheap, but would it work? Or how about this? Or am I cheaping out too much and need something with more features like this?

I will draw up a diagram of my ductwork tonight, its not too complicated, so it shouldn't be all together difficult. As a preview, I have three trunks connected into the supply plenum, two 8x10s, each serving half of the basement and half of the main floor - (4) 6" ducts + (1) 4" duct on one and (4) 6" ducts on the other, each 8x10 extends about 20' from the furnace - and a 12x10 that runs ~ 20' straight up to the attic where tees off into two 8x8 each servicing half of the upper level - (1) 6" duct + (2) 4" ducts on one and (2) 6" ducts on the other, each 8x8 extends about where it tees off, or a total of about 40' from the furnace.

I am willing to do pretty much whatever is necessary with the ductwork to get this to work right, but as I said, I am pretty much stuck with the equipment. I am curious to see, after I finish with HVAC Calc just how oversized my system really is. The house is three finished floors, each about 625 sq. ft, or about 1,875 sq. ft total, with limited insulation in the attic (r-14) and no insulation in the exterior walls (double wythe brick with plaster applied directly on top). I am hopeful I can reuse as much as possible of the existing ductwork, and especially the holes already cut for the registers in my hardwood floors!

I need to figure out return air as well, but I think our previous conversations helped a lot, I just need to figure out how to fit it all in my small furnace room!!

Thanks!


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## beenthere

The first one doesn't seem to display FPM.

Both of the other 2 do. So either of the those 2 will work.

Using them will allow you to figure out the CFM being delivered to each room.

Don't really need to get the most expensive.


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## stubits

Excellent, I will order the lesser expensive of the two ASAP. 

I know that my return air is WAY undersized, which means current CFMs of supply is also low, so testing it will only show so much. If I open the blower door while the system is running, should that compensate for the undersized return?

Does it make sense to figure out the ductwork in a "perfect world" and then figure out how to make that work with what I've got?

What is the best place for the register in a room that does not have any exterior walls at all? Does placement mattter in that case? This is related to one of the rooms in our basement.

Any thoughts on the current supply ductwork I described in a previous post? What does your gut say?

Thanks!!


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## beenthere

You want to check supply and return CFM with the blower door on. This will show you if your return has more air leakage then the supply.
You'd be surprised how many homes have 10 to 20% more supply air then return. Which means its pulling in air from some place other then the conditioned areas.

Generally works best to do the duct design for ideal standards. Then see what can really be done.

If a room has no outside walls. Placement of a register is based on where its both convenient, and will not be blocked by furniture, or interfere with people.

Gut feeling on the supply duct? Um, shouldn't have to redo all of it.


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## stubits

Awesome. Thanks so much!

So, I guess the next step in this process is to finish entering my info into HVAC Calc and get the heat loss/gain calculations, right?

Once I have that, how do I go about designing the ducts for ideal standards? Are there any good resources you can point me to? Is this ductolator a good resource? 

Once I get the anemometer, what should I do with it? Obviously I want to measure the fpm coming out of/into each register, both supply and return, but any method to the madness? Should I open each register completely? Right now I have some of them dampered a bit to improve air flow throughout the house. Or leave it as is?

I would like to add zones to the system, either 3 or 4, but the duct design I am considering will accommodate it well, I think, but I have a couple of questions. 1) Does zoning change the duct design much, as in the sizing? 2) In an attempt to spread out costs, can I install the dampers as I upgrade the ductwork, run the wires, but not install control panel? 3) How does one go about sizing the bypass damper? 4) Is this guide any good?

We would also like to add a humidifier, is there a particular type that you like best? Steam? By Pass? Flow Through?

This whole thread started because of major problems with my return air and I want to make sure to correct that as well. I have a couple of questions to start out with: 1)Are return registers better located in the ceiling, the upper part of the wall or close to the floor? Does it depend on each floor? In the basement and in the main level it will work out so that there is just one return on each level, but can install them in any location, high, low or in the ceiling, what's the best option? On the upper level, I will install returns in each bedroom, but can only really install them in the ceilings, 2) they should be on interior walls, right? In the basement, I want to be careful not to pull air that is necessary for combustion for the water heater and the furnace. The basement has 3 rooms, a bathroom, a living room (where the utility closet is located) and a bedroom. There are doors between each of these rooms, but the bottoms are cut to allow airflow. I would like to install the return in the bedroom, is this a bad idea? Is it possible to use outside air for combustion? If so, how?

Thank you so much all the time you've taken to answer my questions. I have no idea where I'd be without you. I am excited to embark on this project and excited for the outcome... a more comfortable home!!! 

Thanks!!


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## beenthere

Yes, do the load calc, and then you'll have the size equipment that you should have, and CFM each room should get.

For duct design. Need to know how much air to move first.
To know what frictin rate to use, you need to jknow the total equivalent length of the longest run.

Open all registers full. Then take several readings on each register to get an average reading. Then you can use the free area of the register, and multiply it by the FPM to find the CFM from the register.

That guide for zoning is ok. And its nypas formula is the correct one.

You can install the dampers as you go, as long as they are power close, spring open dampers.

As for a humidifier. Which type depends on how much moisture you need to add.
I like both Aprilaire flow through bypass and power, and EWC Autoflo steam humidifiers.

Return position, weather high or low is seldom of importance.
In a basement, I prefer to get them low. But if you use the correct supply registe, it doesn't matter.

Pretty much the same for any other floor as far as, if you have registers with good throw, high or low doesn't matter.

Outside air for combustion is semi easy. If you have a wall you can put a hole in that will except a 6" round pipe.
Make the hole insert pipe. use an intake hood on the outside. Rune the pipe toward the equipment, and run it down and into a 5 gallon bucket. The bucket helps prevent teh area from being flooded with cold outdoor air when the furnace or water heater isn't on.


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## stubits

As always, thank you so much for all of your help. 

I spent last night collecting all the measurements for HVAC Calc and tonight I will get everything entered in. I am curious to see how it comes out!

Given the other work I will be doing, adding a 6" pipe will be easy. Is PVC OK? If I do this, is that sufficient for both the furnace AND the water heater? Does it mean I can put a solid door on the furnace room, or do I need to keep the louvers? And how much cold/hot air is going to get in/out? My concern is that the furnace room is in the living room of my mother-in-law's apartment. She complains enough as it is, I don't need to add draftyness to her list!!


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## beenthere

You'll still need the louvered doors. A 6" pipe isn't enough for both the furnace and water heater.

Look at the BTU input rating of both the furnace and water hater, and add them together.

You need 50 cubic foot of volume for every 1,000 BTUs of input in order to not need a combustion air intake.

So if your furnace is 80,000 BTUs input, and your water heater is 32,000 BTUs input, thats 112,000 BTUs total.
So 112X50=6,700 cubic foot of volume that your basement needs to not require a combustion air intake.

Is the living room that big, if thats the size of your equipment?

PS: Direct vent furnaces aren't included in that since they already get their combustion air from outside.


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## stubits

As always, thanks!

I got all my info entered into HVAC Calc last night and just got my registration code this morning, so hopefully this evening I will learn the heat gain/loss details for my house.

So, I am really glad I asked about combustion air because I think I am in big trouble! My furnace is 100,000 btu and my hot water heater is 40,000 btu, neither of them are direct vent. So, 140,000 but means I need 7,000 cubic feet of combustion air, right? The problem is, at best, the entire basement is only 4,500 cubic feet (650 sq. ft. x 7 ft. ceiling), and worse yet, the living room is only about 1,500 cubic feet. That said, it is an old house and it is not "tight" at all, but how serious of a problem is this? I would assume it is dangerous. Will it be enough to add a 6" pipe, or do I need more?


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## beenthere

If all of the space in the basement is open, or has the ability for the living room to draw air from.
Then a 6" is enough.

This includes the bedroom area if its door is under cut.


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## stubits

Yep, the door is undercut, as is the door from the basement to the rest of the house. That said adding the 6" is necessary, right?

What happens with not enough combustion air? Is it dangerous?

Also, what does this mean for installing a return air vent in the basement? Can I? Or is it a problem given the amount of combustion air needed?


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## beenthere

Unless your leaving the door from the first floor to the basement open. You need the 6".
Or you need to make the door a louvered door.

As for a return in the basement.

You can get a way with a small one, that removes less then the supply puts in. Other wise, the return could pull from the chimney.


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## stubits

Sounds great! I would never have thought of adding combustible air, really, but it seems like it needs it. I'll add it to the "to do" list. Fortunately, with the addition we're planning, it shouldn't be too difficult to add in.

OK, so I finally finished entering all my info into HVAC Calc and I am a bit worried. Either, I really screwed up somewhere OR my system is way, way, way oversized, worse than I had imagined. Would you mind taking a look at the attached reports? 

Basically, it is showing a total heat gain of 16,151 BTUH and a total heat loss of 38,922 BTUH. The report recommends a 1.5 ton AC and I would guess a 60k BTU furnace would be appropriate based on the heat loss, right? 

My current equipment includes:

*Furnace:* Rheem 80% upflow/horizontal gas furnace, model number 80PS10EDR01, 5 ton, 100,000 BTU(input), 80,000 BTU(output). Depending on static pressure, the low setting on the blower has CFMs of between 1,425 and 1,645. The specs/manual can be found here.

*AC Coil:* Rheem 4 TON 21", 13 SEER cased coil, model number RCFA-HM4821AC. The manual/specs can be found here.

*AC Condenser:* Rheem Corsaire 13 SEER, 3 ton condenser, model number 13AJA36A01. The manual/specs can be found here.

So, based on the HVAC Calc analysis, I am WAY oversized, right? Do you see anything funky on the report that might account for this? Did I forget anything big? I had a couple of issues when inputing my house data:
1)Do I need to enter hallways, closets, stairwells? I didn’t which is why the report sq. ft. is off from actual (approx. 1,800 sq. ft.).
2)My house is constructed entirely out of brick, the walls are two bricks thick, or about 8”. HVAC Calc had only 2 masonry options, 1) brick and block and 2) 8-12” block. I selected option 2. Was that right?
3)We have a walk-out basement, meaning that in the front of the house, we are below grade, but at the back of the house, we are at grade. I selected the option “Basement floor, 2' or more below grade” for the whole basement, but how should I have handled this?

Assuming the report isn’t totally incorrect, is there anyway that my current equipment can work in this application? Does it make sense to adjust the ductwork to work with an oversized system?

As for the duct design, assuming I don’t have to change out all of my equipment, I ran the CFM report based on my current unit, using 1,600 CFMs. That’s correct right? So, what do I do now? Is the information contained here close to correct?
1)You had indicated previously that I need to find the friction rate by determining the equivalent length of the longest duct run. Does this mean from where the trunk branches off from the plenum until it reaches the furthest register, basically? Once I have this distance, how do I figure out the rate of friction?
2)Once I have the rate of friction, I assume I use a _ductolator_ to figure out the duct sizes, right? Is this one any good? Online Residential Ductolator
3)I have read, perhaps in the HVAC Calc manual, that the average 4x10 register only delivers 90 CFM. Is this accurate, or do you use a different rule of thumb? Are there different/other types of registers that put out more? Do you generally use a 6” duct to feed a standard register?
4) Do the CFMs on the report seem to make sense? Obviously, at 1600 CFMs, it is a lot more than I need.

I know this is a lot, on a few different topics. Thanks so much for your help!


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## beenthere

Yes, you include the rooms that are not being heated. As they add volume for the infiltration calculation.

Also, you need to add 1200 to 1500 BTUs to the misc. This is to cover cooking, showering, normal amount of electronics, etc.

A 1.5 ton will struggle to meet your sensible at design temps. So a 2 ton would be better.

Yes, your furnace is way over sized.

Brick has a higher thermal resistance then block. But using 8 to 12" will be close to 2-4" bricks.

Your basement floor selection will work fine since most of the floor will be more then 2' below grade.

A 4X10 floor grille seldom has more then 80CFM going through it, if its quiet. 100 CFM through one is tolerable noise. But over that, you'll be turning up the TV. A 6X10, or 4X14 is better for CFM's in excess of 100 CFM.

A 6" supply is fine for 80 CFM.

Supply size is determined by CFM needed, and TEL.

TEL is figured by the length of the longest return and supply added together. Along with the pressure drop of all fittings in the duct system.

EG: A register boot is equivalent to 15' of duct.
A take off is equal to 15', unless there are others beside it, then its equal to 35'(air turbulence of other take offs adds to it).
A reducing transition adds 5' to duct length besides its physical length.

At 1600 CFM for your air flows. It looks ok.

Now do that CFM report over. using 960CFM.


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## stubits

Jim-

Thanks so much! First, just noticed you are from Lancaster. My wife and I went up there a couple of weeks ago just to get away for a long weekend. We stayed at the Inn at Loyola Village, went to the farmer's market downtown on Saturday morning and pretty much just bummed around for a couple of days. We absolutely loved it. You're lucky to live up that way. 

Thank you, as always for all the info, this is really coming together! When I get home tonight I will re-run all of the reports. I need to add in the closet space and hallways, probably an additional 200 sq. ft. Also, the HVAC Calc manual suggests no including window coverings because more often than not, they are not drawn, so I will correct that too. I imagine the numbers will change a little bit, but not much.

So, regarding the equipment, both the AC and the furnace, are they just too big to work with? Or, do you think I can get decent enough performance if I set things up correctly? If I hired you, would you insist I get a new furnace and AC? Or could you make it work?

Is there a resource that has the distance value of the various fittings? I need to think through it a bit, but I know I have some 90s, some Ts, some wyes, etc. I read that it is sometimes acceptable to assume a friction rate of between .05 and .08 for residential work. Is that accurate or should I just go ahead and do the math?

*I am sure I am just slow on catching on, but can you explain why I should run the CFM report at 960 CFM rather than 1600CFM? What am I not understanding here?*

Once I have the rate of friction and a correct CFM report, do I use a _ductolator_ to figure out the duct sizes? Is this one any good? Online Residential Ductolator


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## beenthere

Trying to make oversized systems work right. Is generally an exercise in futility.
So I don't bother trying it. Do it right once. Beats the heck out of try try try again.

960, is about the CFM your house should need with the proper sized equipment.

Your one room. has a CFM of 270 with using 1600 CFM. That would be 2=7" supplies min, if not 2-7's and 1-6".

Does that room have those supplies. Or does it have 2-6" for about 160CFM(which would be about right for a 960CFM through 60,000BTU input 95% furnace for heat).


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## stubits

OK, so it seems like it would be a huge error to update the ductwork, install zones, etc. based on a seriously oversized furnace/air handler. Do you think I need to replace the AC condenser (3 ton) and/or the AC coil (3.5 ton)? It doesn't seem like either are as grossly over-sized as the furnace. Are they workable? 

As for a new furnace, I've been looking at 95% efficient, 60,000 BTU furnaces and the cost isn't prohibitive. A couple of questions though:

1) Is a two stage burner worth the additional cost? How about a variable speed blower?
2) If I am able to keep my existing condenser and coil, do I need to look for anything specific to make sure the new furnaces will work well with them?
3) Am I able to replace the furnace myself? The venting for the 95% efficient furnaces seems easy enough, as does the condensation (I have a condensate pump for my AC I would tie into, and of course the gas line is already there). Is there anything that would specifically complicate me doing this myself?
4) Is there a brand of furnace or any specific features you'd recommend?

This looks interesting, as does this. Am I looking in the right direction? Are these prices reasonable? 

I updated my heat gain/loss info based on your answers above. Now I have a total heat gain of 22,111 (suggested 2 ton) and a total heat loss if 42,872. 

I have also run CFM report for 960 CFM. I have to say, it seems a little off to me. For example the basement is always chilly in the winter, although comfortable in the summer, so 152 CFM for the whole 675 sq. ft. basement seems insufficient, but maybe I'm just not understanding how this all works. That said, I think my current system might be skewing my understanding. Even though it is a 5 ton, 100,000 BTU, 1600 CFM furnace, all of the return air comes through a 12x10 duct, so just about 400-500 cfm, right? So, right now, in spite of the huge, high power equipment, we're only pushing a small amount of air, which explains the lack of comfort. Yes? No?

I am attaching the updated HVAC Calc reports in case they help!

Thanks for everything!


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## beenthere

First link, its a good furnace. Second link, says product no longer exists.

Weather a home owner/DIYer should install his own furnace. Depends on how mechanically inclined they are, and how well they can understand install instructions when they read they(there are installers that shouldn't install).

How well has your current 3 ton A/C been cooling your house? If its not too humid in the summer, you may be able to leave it in until it dies. If you have high humidity, then its probably best to change it out at the same time.

For you basement, 152CFM at a 55° difference between supply and return temp. It comes out to 9,325BTUs being delivered to the basement. So it exceeds what you need.

When you get strange CFMs like that. You have to install dampers in the supplies to help balance the system out.

A single 10X12 return duct doesn't move a lot of air. How much yours is moving, is hard to say from here. If your getting 900CFM through it. Your probably running close to a .6" static in the return alone.
Thats something that needs corrected even with a new furnace.

A VS blower is is a nice thing to have. Since it means the furnace is also 2 stage. Gives you a long run time in first stage, with a low air flow. So you tend to get a more even temp through out the house. Plus, in low heat, with the lower air flow. it is a real quiet system.

With your oversized furnace. your air is being forced out to areas that don't really need as much heat as they are getting. And the duct work prohibits the furnace from being able to push enough air to supply the rooms that need more heat. Typical bad design.


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## stubits

Thanks so much as always, this thread is becoming a bit of a book, a manual on how to fix a totally screwed up HVAC system.

OK, so right now it seems like there are two issues, 1) replacing the furnace/air handler and possibly the AC equipment and 2) designing the ductwork for the home. I have, of course, additional questions on both topics.

*Replacing Furnace/Air Handler/ AC Equipment*
I am convinced, thanks to you, that I need to replace the furnace/air handler… and I managed to convince my wife as well, which is no small task. 

1)There seems to be a big difference in price between Dual Stage Variable Speed furnaces and Dual Stage Multi-Speed Furnaces. What is the difference? Will the variable speed be that much of an improvement? I understand that the dual stage allows for lower heat air.


2)I think I am comfortable with the install, especially since I will just be replacing one unit with another, so the gas lines, electrical, etc. are all there. I will plan on “direct venting” the furnace, using a concentric vent setup, but I don’t see this being a problem, the distances are fine. Our furnace room doesn’t have a convenient place to tie in the condensation drain from the furnace, but we do have a condensate pump installed for our AC which pumps the condensate outside. Can I connect the furnace condensate drain to this as well?


3)I am having a hard time figuring out if I should replace the AC condenser and coil. They are brand new and so I feel bad just ripping them out, also AC is a different beast for DIY because of the refrigerant AND the need to braze the lines. Our home is actually more comfortable in the winter than in the summer and the AC has seemed to work well enough. But past performance is a poor indicator, I think because the return duct is so minimal (12x8 for a 1600 cfm system) AND because the addition with are adding will make the house much more efficient as it will replace a lot of the un-insulated exterior walls. I know it is oversized, but can I go with it for now and replace it later, if necessary, or when it dies? 

*Duct Design*

Ok, I think I am starting to understand things here a bit more. 

1)Do I need to select an air handler before I can start to move forward on this, or is 960 CFM a safe assumption for a 60-70,000 BTU, 95% efficiency, 3 ton (?), dual stage, variable speed furnace? 


2)I am not quite sure what to do with the friction rate. I have added up the value of all of the fittings I can envision in the system, along with the longest supply run and the longest return run. Do I divide that by total CFM? Or the other way around? 


3)I understand you’re point now about the basement. I think my understanding of HVAC has been confused by my current, poorly function system. I am anxious to get my anemometer, because I serious doubt I have 200 cfm coming from any of my registers. Based on the results of HVAC Calc CFM report, I have started to work through the duct sizes a bit, although I know I am lacking the friction rate. I am attaching it here. I am completely off or is this starting to come together? Is this and OK way to look at things, or should I be drawing a diagram? We have a small home, so none of the runs are particularly long, certainly nothing more than even 50’.


4)I have just been using a ductolator to figure out the above information. Is there a better or more precise way to go about it? I know that if velocity is too high it causes a lot of noise, but I would imagine if velocity is too low, there would not be enough throw, or is that not correct? Is there a “sweet spot” on the velocity?


5)Round duct is a better choice than square duct, right? It also seems moderately less expensive. That said, I guess it uses space a little less efficiently. I should also avoid any form of flex duct, right?


5) Are there registers and diffusers that are better than others, or are all pretty much the same, assuming they have built-in dampers?

Thanks so much! Next time I am up in Lancaster, and we hop that will be soon, I hope you'll let me buy you a beer, a coke or whatever your preferred drink it. You've gone way beyond the call of duty here.


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## beenthere

The VS blower is similar to a VFD. It monitors its torque, and adjusts its speed based on what CFM its set to. So it maintains a constant air flow. Its an expensive control set up, along with the motor costing much more. So thats what makes the big price difference.

The VS will generally deliver better air flow then a PSC/multi speed motor. Plus, furnaces with VS blowers have a setting to slow their ramp up. So that they remove more moisture when in cooling mode.

You may want to reroute the condensate pumps drain to another drain area. In the winter, the condensate pumps drain line can freeze if it goes outside.

Figure out your longest run. Add all fittings, Then subtract all other devices that cause a pressure drop. Such as the A/C coil itself, your supply and return register. Use .06 for the return and supply register combined.

A drawing makes it much easier to figure TEL.

So you would use .5" for your furnaces ASP(Available Static Pressure).
Then subtract .03 for the supply and return grille. And you would have .44" left.
Then subtract your A/C coils PD. Say .19". That leaves you with .25" ASP.

So lets say your supply and return come out to 295' TEL

Take (.25X100)/295=.084" FR. So you would use a .08"FR on your ductulator.

For supply runs, round pipe is better.

I use Honeywell mostly. And then select the type that has the throw and or spread I want.


Then


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## beenthere

Also. You don't want to use anything smaller then a 4". And only for a bathroom or mudroom when you do.

Sometimes you give up velocity, to keep static pressure low.
Duct velocity, and register velocity aren't the same thing. You can have 200 FPM in a duct, and 700FPM from the register.

You may want to get a copy of Manual D. It will have all the equivalent lengths of your duct fittings, and go over how to figure FR.


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## richiemoe

What you need is a duct calculator like this.http://www.datalizer.com/


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## stubits

Jim-

Thanks! Did something get cut off from your first post? It ends with "Then..."

Do you have any samples of a multifloor diagram? I am not sure how to draw that up without it being totally confusing?

Thanks!


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## beenthere

Not sure how that "then" got typed there . :laughing:


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## beenthere

Don't have any handy.

But, what I usually do if the second floor is just branch supplies(just oval or round pipe runs).
Is just draw a line for it, put a supply reg symbol at its end. And mark how long it is.

Don't need to be 3D, fortunately for me. Or I'd be in trouble with some of my drawings. :thumbsup:


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## stubits

Excellent. I am working on that diagram now. I will check to see if I can find a copy of the Manual D somewhere around here too. 

Have you ever seen or used this? It seems pretty neat, but maybe too easy to be true. What do you think?

How do you go about getting higher velocity out of the register than there is in the duct? Is it a matter of closing the dampers? Or using a special register? So, for example if I only need 40 cfm out of the duct in the dining room, I should use a 6", even though that is better sized for 80 cfm, right? And then I will just use the right register to make sure I have the right velocity and throw?

So, am I crazy for thinking about keeping my current condenser and coil? They are brand new and so I feel bad just ripping them out, also AC is a different beast for DIY because of the refrigerant AND the need to braze the lines. Our home is actually more comfortable in the winter than in the summer and the AC has seemed to work well enough. But past performance is a poor indicator, I think because the return duct is so minimal (12x8 for the whole 1600 cfm system) AND because the addition with are adding will make the house much more efficient as it will replace a lot of the un-insulated exterior walls. DC has extremely high humidity... we used to be a swamp! I know it is oversized, but can I go with it for now and replace it later, if necessary, or when it dies? Or is it just foolish?

What do you think of ordering HVAC equipment online? Are there any good companies? Are the prices better? I think it can be hard to get a lot of this stuff locally, as they prefer to sell to contractors/pros.

Thanks!!


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## beenthere

Velocity is a function of free area to volume of air.

A 6" pipe has an area of 28.26 sq in, or .2 sq ft.

A 4X10 register may have the same area, or you can get one with more or less, depending if you want a higher or lower velocity.

EG: A Hart&Cooley 4X10 #210 has a free area of .2 sq ft. So at 60CFM it has a velocity of 300FPM the same as a 6" pipe.
A 4X10 #411 has a free area of .17 sq ft, so it has a velocity of 352FPM moving 60 CFM.

Both registers are 4X10, but the vanes are different, making the area the air passes through smaller on the 411.

A #411 2¼X10 has a free area of .09 sq ft, and would have a velocity of 700FPM at 60 CFM. Even though it was being feed by a 6" at only 300 FPM.

Velocity in your supplies is important. But, maintaining CFM is more important.

I would probably use 5" instead of 6" on some runs that a 4" could handle. Only because once you drop below 300 FPM in a supply. You are at risk of loosing or gaining too much heat from the are that the pipe runs through. And not supply or removing enough BTUs to that area.

Supplies that are run inside of outside walls. Are supplies that you want to keep the velocity up on.
An inside wall is not as critical.


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## stubits

OK, I think this is making sense now. It is kind of like partially covering the end of a hose with your finger, you get the same amount of water, same pressure, you just change the flow, right? The idea behind CFMs is that you need to pump in and remove a certain amount of air from each room to make it comfortable right? It is about changing over the air in the room. Velocity effects how far and how fast the air travels, but you still end up with X CFM, regardless of velocity, right? I am not a stupid person, but I cannot believe it has taken me this long to come to this "a ha" moment! _Hopefully I am right._

So, what is the optimal amount of FPM that you want? Some of what I have read says you don't want it to go above 600-800 depending on supply or return, and you say that less than 300 is bad, but is anything in between good or is there a best case?

So, what do you think about my AC condenser and coil? Am I stupid for keeping them?

Thanks!


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## beenthere

You got a fairly good handle on velocity now.

What confuses most people is that the term "velocity" is applied to both ducts and registers. Instead of the terms "duct velocity" and "register velocity" 

Velocity and static pressure MUST be a compromise, with static pressure getting the bigger half of the deal(kinda like that Jerry Reed song goes. They split it down the middle, and gave her the bigger half).

300FPM would be something used for a closet, or small bathroom/powder room.

800FPM in a 6" pipe is gonna be loud. And have a high static pressure(that would be moving 160CFM). At a FR of .195".
Drop that FPM to 407, and you have a quiet supply moving 80CFM. At FR of .052".

800FPM in a 8X10 trunk line is semi ok, at 450 CFM and a FR of .11".

So, larger ducts, can have a higher FPM and be quiet, and have a low FR. Smaller ducts, need a lower FPM to have a low FR and be quiet.

You want your main trunk line to be 800FPM and less as it reduces. And your supplies maxed at about 600 FPM for short runs, 400 FPM for long runs, such as second floor runs.

Often. Installers will try to stay at those "recommended" velocities. And thats when you start getting second floors that won't cool right. Because the static pressure becomes too high to force the required CFM through the long pipe to the second floor, instead of the easy short path to the first floor.


Long way of saying there is no ideal velocity.

Give more credence to FR to keep static low, and you'll have lots of air flow(CFMs).
Look at velocity too much. And you'll lack air flow, and kick yourself in the butt.


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## stubits

I am glad I am finally getting it!

I also just got my ananometer today, so I will get a feel for what I am getting out of my system currently... I am sure it is not enough!

Also, I hope to draw up some diagrams of my proposed duct system tonight which should help me figure out the friction rate. Just to confirm, I need to figure out the length of the longest duct run, so, starting from the air handler, through the trunk, through the branch and onto the register for both the supply AND the return, right? And then add to that distance the equivalent distance of all the fittings, take offs, angles, register boots, etc? Do I have it?

I am also going to start looking at new furnaces... I will definitely go with a dual stage, variable speed, 95% efficient unit. Do you have a preferred brand, or even a favorite unit? 60-70,000 BTUs is the proper size, right? Most of these seem well paired with 3 ton AC, so, what do you think about my AC condenser and coil? Am I stupid for keeping them?

Thanks!


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## beenthere

Yes, you have the length calc right.
Minor exception. i think I typed it a bit misleading.

You add up the pressure drops for the registers, grilles, air filter, evap coil. And then subtract them from your furnaces ESP rating(can probably use .06 for the supply and return regs/grilles).

Most manufacturers will have the rating for their registers and grilles. But sometimes its not easy to find, or tell exactly which model you have.

But other then that, you got it right.

Don't forget to do your CFM checks by averaging the air flow from the registers and grilles.


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## stubits

Ok, so I just calculated the friction rate and I ended up with a rate of .08. Here's how I got there, does this seem reasonable?

First, total effective length:

*Supply*
Take off - 15 ft.
Vertical Duct - 30 ft.
90 - 10 ft.
Horizontal Duct - 10 ft.
90 - 10 ft
Horizontal Duct - 22 ft.
Register Boot - 15 ft.
*TEL SUPPLY - 112*

*Return*
Take off - 15 ft.
Vertical Duct - 30 ft.
90 - 10 ft.
Horizontal Duct - 10 ft.
90 - 10 ft
Horizontal Duct - 14 ft.
Register Boot - 15 ft.
*TEL Return- 104*

Total Effective Length (TEL) - 216 ft.

Blower ESP: .5
AC Coil: .22 ( this is at 1,200 CFM, the minimum for my current coil)
Filter: .05 (cheap pleated filter)
Supply Register: .03
Return Grill: .03

Total device pressure losses (DPL) - 0.33
Available Static Pressure (ASP) - 0.17

End result is a friction rate of .08.

So, does this make sense? If so, where do I go from here as far as figuring out the duct design? How do I figure out what to do now?

Thanks!


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## beenthere

On your return drop. Does it ell into the furnace with a 90 that has radius corners, or are the corners sharp 90 right angles?
If sharp 90° angles you need to add a min of 65' for those angles.

Add 65' to supply if the trunk goes into/attaches to the side of the supply plenum.

That will bring your TEL to 346'. Not an unusually long system.
And at an ASP of .17 will reduce your FR to .05".

Its not uncommon to end up with a FR that makes your duct system larger then what you have room to install, or ability to change to.

Often. The furnace fan chart has to be looked at to see if using a .6, or .7" ESP to get an ASP of .27 or .37". So that an more acceptable duct size can be used.

The fan chart will have CFM listings for different fan speeds at different ESP's.

With the FR you use a ductolator to check your duct work sizes to all rooms. Since you have the CFM each room needs. You can now see if the size run the rooms have is sized right for the CFM it needs.

You also do that for your trunk line.

EG: If your trunk line needs to carry 1200 CFM. The first piece of duct should be 8X26 or equivalent. For a system that is using .08" FR. Or 8X32(reason why some systems need to run at a high then standard ESP rating) for a system using a .05" FR. 

If it is sized right. You then check the size of the supply runs coming off of that section, to see if they are sized right by the ductolator.

Then you subtract the CFM of those supplies from 1200, and get your new CFM for your next piece of duct work. And check if it is sized right. Along with the supply runs to the rooms. And continue until you have checked each trunk piece and supply run. 

This now tells you what duct alterations you need to make.

From there though. You need to check to see if you can make those duct alterations. As in can you install the larger ducts you need both trunk line and supply runs.

Often the trunk line isn't a problem to increase size. But, some of the supply runs are, such as runs to the second floor.

In the case of second floor supplies. Instead of ripping open walls. Generally, you add additional supply runs to the rooms that need it. And then chaise the new run, so the wife doesn't shoot you.


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## stubits

Excellent, thanks, I really feel like I can see the end from here. I knew this was going to be more complicated than I could comprehend, thanks for walking me through it.

So, I currently have a return air ell that looks like this:









But I have been considering something more like this if it saves me some room:









Is one better to use than the other? I need to replace my current one because my current drop is only 20x8, so too small, even for the smaller furnace. You say I need to add 65' for a right angle boot, what about for the more curved one? Are there other options with even less friction?

As for the supply run, the run I used for the TEL calculation, the physically longest run, comes right up out of the top of the supply plenum. Do I need to add anything in this case? There are at least 2 trunks that will come out of the sides. Does that matter?


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## beenthere

The shoe ell is better, if it doesn't have an air filter in it.
The filter box is better if the other also has a filter in it.

A radioused ed ell is the best thing to use to keep TEL down.

The run that is off the plenum may be the longest physical run. But, it may not be the longest TEL run.

The supply that you used, would have a TEL of 30, tapping into the plenum.
But your ducts that tap into the side each have 65' of TEL. You need to check each supply for its TEL to determine which is the longest.

Its like determining who's coffee taste best. You must sample each one, to know. Color and aroma don't tell the who story.


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## beenthere

Oh, and I would probably try it with keeping the current A/C, and setting the air flow as low as possible for it.


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## stubits

Hmm. Is it an option to not have a filter? I thought I read in the furnace manual that it was necessary?

I cannot seem to find anything online about a radiused ell, do you have a picture or a link or something, just so I know what it looks like? Does it maybe go by another name?

So, my best bet if I need to use a filter is a radius ell and if not that, then the right angle/box I posted earlier.

Thanks!


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## stubits

OK, so I redid the friction rate calculation including the info you shared, here it goes again, what do you think now? Would you be willing to double check what I've included? Does it seem like I have too many fittings? Too few? If I end up with .05 friction rate, how bad is it? Do I need to increase of decrease the size of the ducts to compensate? Are there any settings on the furnace that might help? Thanks!

*Supply*
Connect to Plenum Side - 65 ft.
Take off - 15 ft. (Connects plenum to horizontal duct)
Horizontal Duct - 20 ft. 
Take off - 15 ft. (connects round duct to square duct, changing directions 90 degrees)
Horizontal Duct - 7 ft.
Take off - 15 ft. (Connects round duct to register boot)
Register Boot - 15 ft.
*TEL SUPPLY - 152 ft.*

*Return*
Return Air Plenum - 65'
Take off - 15 ft. (Connects vertical square duct to plenum)
Vertical Duct - 30 ft.
90 - 10 ft. (Turns square duct from vertical to horizontal)
Horizontal Duct - 10 ft.
90 - 10 ft (Changes direction of square horizontal duct)
Horizontal Duct - 14 ft. 
Register Boot - 15 ft. (connect register boot to horizontal duct)
*TEL Return- 169*

Total Effective Length (TEL) - 326ft.

Blower ESP: .5
AC Coil: .22 ( this is at 1,200 CFM, the minimum for my current coil)
Filter: .05 (cheap pleated filter)
Supply Register: .03
Return Grill: .03

Total device pressure losses (DPL) - 0.33
Available Static Pressure (ASP) - 0.17

End result is a friction rate of .05.


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## beenthere

Your furnace install manual should have a fan data chart(if you don't have one for your current furnace, the manufacturer might send you one if you email them).

It will give different CFM's at different static pressures. Knowing what your TEL is for your duct system, along with PD of devices. You can calc what static your system is operating at then. But you need the fan data. To know what CFM it can move at what static pressure(of course the anemometer will tell us what CFM is being moved also).

Check your ducts current size to the size it should be for your TEL and ASP. See what the difference is. On the FR chart.

EG: If your first piece should be 8X26, but is only 8X20. Then you know your system is running at a static higher then .50". Probably around 1.1"wc(based on both return and supply being equally undersizzed).

My guess is your running at a static pressure of over .35" in your current supply alone if your only moving 1200 CFM. If your moving more, then it is much higher.

For a new furnace install. If your current duct system isn't sized to .17" FR.
Then see what FR it is sized to for 1200 CFM. And then reverse calc to see what ASP you need. Then check the fan data chart for the new furnace to see if using a ESP of .6 or .7 will give you the 1200 CFM.

Or, if you have to alter the first several foot of trunk line(as in remove it and install a larger section). Sometimes, you can get away with relocating one or 2 supplies from the trunk line, and moving them to the plenum.

Or, just putting in 8 or so foot of new trunk line for the first few supplies.


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## beenthere

A few quick suggestions that may or may not work for your system.


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## stubits

Wow, all I can say is I now know why good HVAC techs charge so much... and why the bad ones use "rules of thumb." This stuff is complicated!! Thanks for walking me through it, sorry I am so dense!

So, first of all, I am definitely going to get a new furnace. I really like the Goodman GMVC950704CX - 70,000 BTU 95% Efficiency, Two Stage Burner, Variable Speed Blower, Upflow/Horizontal Application Furnace and the price here seems quite good. *Does this seem like a good choice? Do you have any issues with Goodman? Is this a good size for my situation? 
*
I have prepared the CFM report based on the furnace putting out 960 CFM, but based on the manual, located here, depending on the furnace settings we can have between 756-1,188 CFM on the low stage heating setting, 1,089-1,711 cfm on the high stage heating setting, or 540-1,572 CFM on high stage cooling setting and 378-1,022. *As I start to do the duct design, how do I know that total CFM to use? How do I calculate the optimal CFM for my home?

*As far as the duct design goes, I would prefer to use as much of the existing ductwork as possible, BUT I am more concerned with having a well performing system and a comfortable home, even if I need to rip all of the existing ductwork out! *So, pretending I have no ductwork at all, where do I start?* 

Tied into the support plenum I would like to have 4 trunks (is that the right word?), one for the basement, one for the upper level and two for the main level. 

*1) Once I figure out the optimal CFM for the home, I figure I will use a ductolator to figure out what size duct I need based on a FR of .05, right? 

2) Can you suggest a ductolator? The one I have been using doesn't allow for a friction rate of .05. The lowest it allows is .06.

3)Also, after each supply line branches off from the trunk, the trunk should reduce in size, right? *

*4) How big of a problem is a .05 friction rate? It seems like there isn't much I can do about that specifically.

5) You mentioned static pressure in your last post, that hasn't come up before. What is its importance and what do I need to do?*


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## beenthere

The price looks ok for the furnace.

CFM for duct sizing is based on the equipment/mode that needs the most air. Heating or cooling.
A 70,000BTU input 95% furnace needs to move 1120CFM to have a 55°F temp rise through it(That Goodman is probably rated for a temp rise between 40 and 70, best to pick middle or lower end, allows for air filter to get dirty that way).
I believe that model's lowest CFM for high heat is 1210, minus 10% for comfort, for a lowest setting of 1080CFM in high heat. So you know that your duct system needs to be able to move that as a min for heating. 

The A/C, at 3 tons as rule of thumb would be 400 CFM per ton, or 1200 CFM.
In reality. The A/C CFM should be based on the SHR(sensible heat ratio) that your house needs. Meaning it might only need 1100, or a 1000CFM of air flow.

I believe your homes SHR was .79, so your A/C(if the unit is the size the house needs) CFM can be set to as high as 435CFM per ton to achieve that.
Since its a 3 ton though. You would want to adjust that for that oversize. And reduce it to 365CFM per ton. Giving you a total A/C CFM requirement of 1095CFM. Making your heating CFM your designing CFM. 

Starting from a no duct work view.

You start with drawing out your trunk lines, and supply branches. Just as simple line drawings. With the linear length written at them. And then after all trinks and branches are drawn and listed. You go back and figure out their TEL to determine which one is the longest run. And then apply the ASP math formula from the previous post. And then you would look up the duct sizes you need at that FR.

I get my ductulators from the supply houses. You can order one from several different sites on the net. ACCA is one of the sites. I don't know the rest cause I don't use them.

The trunk line doesn't need to reduce at all. If its less then 28' long.

On a reducing trunk. You reduce when your velocity decreases/would decrease too much. Too much is the part that can be hard to determine. Or you can reduce to maintain velocity to a preferred velocity(preferred just means that is what you want, even though the system doesn't need to maintain that velocity). 

EG: 17"(10X24) round at 1200 CFM is a velocity of 762FPM, when you have 2 take offs using up 200 CFM of that, you reduce to 15.5"(10X20) to have 763FPM.
The small/short distance between the 2 supplies is often only 1 to 3 foot, and you don't want to reduce every foot or so since that creates additional turbulence in the duct that interferes with air flow.


Static pressure is part of ESP. External Static Pressure. In the case of a gas/oil furnace. its all the devices we have been talking about, including the A/C coil, because the A/C is not pert of the furnace. The evap coil is External to the furnace.

Static pressure that you are probably familiar with. Blow up a balloon. The amount of force it takes you to do it, is the force you must exert against the static pressure of the balloon material.

Static pressure is what the blower works against to move air. Its the force exerted in all directions in the duct, when the blower is running.
So we want to keep that low, so the blower can move the amount of air we need/want.
And on a VS blower. If that gets high, the blower uses more electric to move the air.

.05" FR is more in line with making your duct system as easy as possible for the blower to move the air through it.
How ever, if you don't want to buy a ductulator. Using .06" won't kill the system.

You can use 4 trunk lines if you have the room. Using 4 trunk lines may or may not reduce TEL and allow you to use a higher FR and still maintain a low ESP. You would need to recalc after you make a drawing of your duct system.


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## stubits

Jim-

Sorry it has taken me so long to say thank you. Your last post was incredibly instructive. I've learned so much, it really is great. I am attaching a copy of the latest CFM report based on 1080 cfm based on what you indicated in your last post.

I drew the ductwork out this weekend, just to double check my TEL. Unfortunately it was correct, so I am stuck with the .05 FR. *Is this OK?*

I have a couple of hopefully basic questions now as I am figuring out duct sizes.

1) Is it helpful to place a supply register in a hallway?

2) The ductolator I am using doesn't allow for a FR under .06. By using this, will it end up oversizing or undersizing the ducts? Is there anything I can do to account for that? Is there enough of a difference for me to try and pickup a ductolator?

3) I am trying to figure out how much of my existing ductwork I can use. The ductolator Currently I have a 12x10 and a 12x8 running to the attic to service the second floor, the 12x10 as a supply, the 12x8 as a return. The CFM report shows that the second floor needs 575 CFM. According to the ductolator, the 12x10 will move 575 CFM at 690 FPM. Is that OK? The 12x8 will move 575 CFM at 863FPM? I know that is high, but will that work? If the existing 12x8 and 12x8 can work, which is better for return and which is better for supply?

4) On the other side of the same question, is 398 fpm or 328 fpm too low for a trunk? We need to move 221 cfm and 182 cfm and currently there are 8x10 trunks for both. 

5) You said 4" supply lines are OK for bathrooms, but that all other rooms, regardless of their demand, should have 5" or more. Did I understand you correctly? So, even if a room only needs 14 cfm, I should still use a 5" line and then just use the register dampers, right?

Thanks as always!!


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## beenthere

Your welcome.

The new furnace will be able to operate fine even if its working against a starting ESP of .7"(starting FR referring to a new clean air filter). So using .06 (or .07" max FR) will work ok( above .07", you'll end up with a starting ESP of over .82", which will mean a VS blower will start to use just as much electric as a standard blower after only a week or 2).

Registers generally aren't needed in a hallway, as long as it walls are all inside walls, and the rooms it serves use it as a return air path.

Although the velocity for 575CFM through the 12X10, and 12X8 would be ok. What will happen, is that the static pressure in those 2 different sizes will prevent them from having the same amount of air flow.

EG: 575CFM through a 12X10 duct is a FR of .072", 575 CFM through a 12X8 duct is a FR of .13"wc.
So in this case. The return is too restrictive to return the 575CFM from the second floor.
So the solution would be to add another return some where to make up the additional 160 CFM that the 12X8 won't move at the same FR as the 12X10 supply.

If the 10X8 is a long run, or goes through unconditioned space, then 330FPM is a bit too low/slow.
If its a short trunk line run, then its no problem.

Your report shows 11CFM for the guest bedroom. While it might be tempting to use a 4" for it. I think you'll find that under operating conditions, that room will need double what the report says.
Reason, is it doesn't allow for 2 people to be in the room with that 11 CFM(2 people are a sensible cooling load of 400 BTU's). So you need to be able to deliver at least twice as much air, if you have company staying over for more then one night during the summer( sometimes 3 times as much, if they are "active" people, :thumbup. Along with the possibility that the room could become a computer, or office room some where down the road. So I wouldn't run less then a 5" to a bedroom.


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## stubits

Excellent information. This is really coming together... at least in my mind. I managed to sell my wife on the new furnace, but tearing up the basement is going to be a harder sell (her mom lives there)!! Oh well, all for the good of the order, as they say.

I have a couple of follow on questions...

1) The CFM report calls for about 30 CFM in the upper level hallway. All of the walls are interior, but the ceiling is somewhat under insulated. Should I just divide that amount up amongst the other rooms?

2)Currently I have a single return air register in the upstairs hallway and all the doors are undercut, but I have been considering adding return air vents to each room. Is this worth the effort? 

3) So, it seems like I can keep my existing 12x10 and 12x8 if I add an additional duct. This shouldn't be too difficult. It looks like the 12x10 @ 575CFM has a FPM of 690 and the 12x8 @ 460CFM has a FPM of 690, so I need to account for an additional 115 CFM, does that sound right? How did you come up with 160CFM? The ductolator shows I can use a 5.5" round (that doesn't exist, does it?) with 697 FPM or a 5x5(662) or a 8x3 (690 FPM). So, would something like this work?

4) With the distance of the 10x8, are you referring to its actual length, or its effective length? The actual length is approximately 20', but it connects into the side of the plenum, so with the take off, it is 100'. Can this work, or do I need to change it? The other 10x8 serves the other half of the main floor, is 13' actual or 93' effective. Would be great if I could make this work, but I want it to work correctly more than anything else.

5) Just FYI, the guest bedroom is a bit misleading. I am attaching a copy of the floor plan. The addition we are gearing up for will add a small annex to the bedroom which will connect to the bedroom via a "doorway," but there will not be a door. The CFM report calls for 80 CFM in the "annex," largely because it has 2 exterior walls, and just 11 CFM in the bedroom itself. How do you think I should break this up? Did the CFM report get it right, or should I make some changes to it?

6) Reasonably, what is the max amount of CFM you want coming out of a single register (floor or ceiling) before you break it up into 2 registers?

7) Also, I am a little confused on the CFM report. Take for example the upper level of the home, the CFM indicates total CFM of 575, but when you had up the totals for each room it comes out to 654 CFM. Which should I size the trunk for 575 or 654?

8)This is more of a logistical question, but how big of a round duct can you tie into the 6" side of a 6x8 rectangular duct? Can you tie in a 6" round? Or just a 5"?

Thanks!!!


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## beenthere

stubits said:


> Excellent information. This is really coming together... at least in my mind. I managed to sell my wife on the new furnace, but tearing up the basement is going to be a harder sell (her mom lives there)!! Oh well, all for the good of the order, as they say.
> 
> I have a couple of follow on questions...
> 
> 1) The CFM report calls for about 30 CFM in the upper level hallway. All of the walls are interior, but the ceiling is somewhat under insulated. Should I just divide that amount up amongst the other rooms?
> 
> Yes, if the hallway is part of the return path the air from those rooms will take.
> 
> 2)Currently I have a single return air register in the upstairs hallway and all the doors are undercut, but I have been considering adding return air vents to each room. Is this worth the effort?
> 
> It can be, if those rooms don't maintain temp when the bedrooms doors are shut at night. If they do maintain temp with the doors shut. It probably isn't worth while.
> 
> 3) So, it seems like I can keep my existing 12x10 and 12x8 if I add an additional duct. This shouldn't be too difficult. It looks like the 12x10 @ 575CFM has a FPM of 690 and the 12x8 @ 460CFM has a FPM of 690, so I need to account for an additional 115 CFM, does that sound right? How did you come up with 160CFM? The ductolator shows I can use a 5.5" round (that doesn't exist, does it?) with 697 FPM or a 5x5(662) or a 8x3 (690 FPM). So, would something like this work?
> 
> Your still concentrating on velocity too much. Look at the friction rate of a 8X12 moving 460CFM. Its better to match FR, then velocity.
> You'll get about 415CFM at the saame FR (.072")as the 10X12. So its better to use the lower FR that the 12X10 is at, then to increase the FR of the 12X8.
> 
> The return system has to be looked at as a whole. if you increase resistance to air flow in one return, the other returns will now draw in more air then you planed. Because that return is more resistance to air flow.
> 
> 4) With the distance of the 10x8, are you referring to its actual length, or its effective length? The actual length is approximately 20', but it connects into the side of the plenum, so with the take off, it is 100'. Can this work, or do I need to change it? The other 10x8 serves the other half of the main floor, is 13' actual or 93' effective. Would be great if I could make this work, but I want it to work correctly more than anything else.
> 
> The actual distance. A 7' difference shouldn't be a problem though.
> 
> 5) Just FYI, the guest bedroom is a bit misleading. I am attaching a copy of the floor plan. The addition we are gearing up for will add a small annex to the bedroom which will connect to the bedroom via a "doorway," but there will not be a door. The CFM report calls for 80 CFM in the "annex," largely because it has 2 exterior walls, and just 11 CFM in the bedroom itself. How do you think I should break this up? Did the CFM report get it right, or should I make some changes to it?
> 
> If the return for the annex has to go through the bedroom. Then having more supply in it works fine. If the annex will have a return, then the bedroom should have the most supply of the 2.
> 
> 6) Reasonably, what is the max amount of CFM you want coming out of a single register (floor or ceiling) before you break it up into 2 registers?
> 
> That depends on where the register is. A high sidewall can have mofre air flow then a floor register. Since it won't be blowing air on anyone. On a floor register, 120 is about the max. With the exception being. that sometimes its now practical to split up a 140 CFM supply. So in that case, a 4X14 at 140CFM is ok. Anything above that is too much air to be able to stand anywhere near it, and be comfortable.
> 
> 7) Also, I am a little confused on the CFM report. Take for example the upper level of the home, the CFM indicates total CFM of 575, but when you had up the totals for each room it comes out to 654 CFM. Which should I size the trunk for 575 or 654?
> 
> I believe its a flaw in the program. Its not using the higher CFM of the master bedroom in heating, as the both CFM calculation.
> 
> 8)This is more of a logistical question, but how big of a round duct can you tie into the 6" side of a 6x8 rectangular duct? Can you tie in a 6" round? Or just a 5"?
> 
> 5".
> 
> Thanks!!!


Think I ansswered all of them.


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## stubits

As always, thanks so much... I am sorry for all the questions, but without your help I'd be totally lost!! Thanks so much!

-Maybe I will try the central return on the upper level and if I don't get the results I want, I will change it for individual returns. All the doors are undercut AND we rarely shut the doors, basically only when we have guests, so not often.

-OK, so I have been ignoring FR because the ductulator I've been using doesn't give that, just CFM and velocity. I am guessing your ductulator also lists this information. I should head over to the supply house and pick one up, right? Or is there a way to figure out FR for each run of duct based on the info I have (CFM, velocity, distance, etc.)? Is FR also equal to "head loss"? I found a new ductulator here which seems a lot more complete. 

- Regarding my previous question #4, I just want to confirm, given the relatively short distances, 20' and 13', it will be OK to use the two existing 8x10 trunks even though the velocities are 398 fpm for 221 cfm and 328 fpm for 182 cfm. Will this work?

- Referring to the two trunks above, which each serve half of the main level, and will share a single return vent, do I need to balance them in some way?

- Currently one of our floor vents is just a hole cut in the duct with a register placed over it. There is no take off and no register boot. Does this work at all or is it totally wrong?

- Regarding the bedroom annex, yes, the return air will have to pass from the annex through the bedroom to get to the return. That said, will it be more comfortable to have more supply in the bedroom? This will be new ductwork, so I can do whatever I want here.

-OK, so no more than 120, maybe 140 out of a floor register... how about a ceiling register? What is the max comfortable there?

Thanks... and I am serious about owing you a beer or your drink of choice, or heck, even a steak the next time I am up in Lancaster!


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## beenthere

Yes, they are using the term head, in place of FR.
Thats a good program.

On the 2-8X10 trunks. You will probably have to balance them out. Some dampers in the supply runs themselves, near the take offs will take care of that.

As long as the annex doesn't have a door. The annex should get the air that is listed in the report.
Since the annex returns through the bedroom, there should be no problem heating or cooling the bedroom.

Should be a sealed path for the air to travel to that floor register. With just a hole in the floor and duct. Your probably getting a fair amount of air, blowing out around the register.

Ceiling register can have more. But it varies with the height of the ceiling for noise considerations.
But on the average house. If you follow the same limits as a floor register, you have a quiet system. With no harsh air currents.

Good chance I'll take you up on that beer. :thumbsup:


And yes, slide ductulators show FR, velocity, round and squared/rectangular duct sizes on them. 

But that Mcquay one is a good one to use.


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## stubits

You've certainly earned _at least_ a beer. Thanks for everything!

I will work today on figuring out the optimal duct sizes vs. what I have, along with velocity AND FR/head loss. It feels like we're getting close!!

Can you help me to understand FR a bit more? 1) Is it important to have a uniform FR throughout the entire system? 2) Or, should the FR just be uniform on each floor? In a "perfect" system, what should I be aiming for?

Oh and in the new McQuay ductulator, what should I use as the "air" setting? Not sure what else to call it, but it is the first drop down box. Is 68 degrees STP the best choice?


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## beenthere

Friction rate, is the amount of pressure that will be lost in the duct moving that amount of air.
Its the resistance to air flow that the duct material's surface places against air moving in it. 

So its the rate/amount of pressure that will be lost to the friction of the duct surface area. No matter what the starting pressure is.

If you start out with .4" pressure, you will lose .1"wc pressure per 100 foot of TEL of 6" pipe per 110 CFM moving through that pipe.

If you start out with .9" pressure, you will lose .1"wc pressure per 100  foot of TEL of 6" pipe per 110 CFM moving through that pipe.

The pressure loss/friction rate is always determined by the amount of air moving, not the starting/entering pressure. 

The higher the FR, the more pressure it takes to move the air that is touching the inside surface area of the duct.

So it requires more pressure from the blower. But blowers aren't made to work against a high pressure. So as the pressure in the duct increases. The amount of air the blower can move decreases. Thats why its important to keep the Total External Static Pressure low.

Take a 5/8"ID garden hose and a 3/4"ID garden hose. Which one will flow more water at a entering pressure of 40PSIG?
The larger one, since the area inside of it is bigger, its less resistive to water flow.

How much more pressure would you need to get the same amount of water from the smaller hose(I didn't bother to look that one up)?

Air is the same way. Larger duct has less resistance, so it takes less pressure to move air through it. 

Its best to maintain, or to try to maintain the same FR(not always possible though).

In a perfect world, on the perfect system. Each supply and return would have its FR set for that individual run's TEL.

Use the 68STD setting. The difference between that and the air that your furnace will be moving in either cooling or heating mode is too small to worry about. Usually less then .005"wc. Five one thousandths of an inch is not worth making the correction for on a system that will vary its air flow from heating to cooling mode.


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## stubits

Ok, I have run the calculations for all the rooms and for all of the various trunks and branches. Would you mind taking a look at this and giving me your opinion. Am I close? Does it make sense?

Also,

1) How do I go about sizing the actual registers. If it matters, all of the registers in the basement and on the upper level are ceiling and all of the vents on the main level are floor.

2) I am pretty sure I am going to need a new return air drop. How do I figure out what size is necessary? It seems like a 20x10 could work well for 1080 CFM.

Thanks!


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## stubits

Bump.


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## beenthere

stubits said:


> Ok, I have run the calculations for all the rooms and for all of the various trunks and branches. Would you mind taking a look at this and giving me your opinion. Am I close? Does it make sense?
> 
> Also,
> 
> 1) How do I go about sizing the actual registers. If it matters, all of the registers in the basement and on the upper level are ceiling and all of the vents on the main level are floor.
> 
> Registers are sized/selected for the throw and or spread you need for the room/area their serving.
> 
> 2) I am pretty sure I am going to need a new return air drop. How do I figure out what size is necessary? It seems like a 20x10 could work well for 1080 CFM.
> 
> It will work. But, if you intend to add a media air filter now or down the road. A 12X20 will work much better.
> 
> Thanks!


Presuming the left side is the intended new duct size. You want to stay with standard height duct. 8" high is standard.

So the 20X6 would be more like 14X8. 
6X6 would be 8X8.


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## stubits

Thanks so much... as always.

So, do they have catalogs for the registers? How do you know what the specs are?

I would love to add a media filter, but I simply don't have the room for it. It is a small-ish old home. I need the smallest return air drop that will work for my system. 20x10 is about the smallest I can get away with, isn't it?

In some cases I went with odd sized ducts because of space issues. For example, in the basement, 2" makes a lot of difference. The 20x6 is an attempt to not totally overwhelm my dining room with the hvac chase. Is it that these sizes are not readily available? Are they super expensive? Tough to work with?


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## beenthere

Sometimes odd sized duct has to be used.

Hart and Cooley has all their data on their website. You can download their catalogs and choose what you need.


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## stubits

So for the odd sized stuff, is it usually made up custom, or just special ordered?


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## beenthere

You'll need to go to a sheet metal shop. And have them make up what you need.


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## stubits

Thanks so much. I need to see what my options are. Would much prefer to order off the shelf, even online, than go custom, but extra headroom, or a smaller chase would be great. I need to do a bit more investigative work to figure out home much free space I have in the current chase... perhaps more than I think.

I am trying to figure out what size registers I have now... simple, right? Do I measure the outside edges? The register boot it connects to? The size of the louvered area?

Also, the Hart and Cooley catalog looks very interesting. What is the significance of the throw and spread numbers, higher means more, right? But how do I know what I want?


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## beenthere

The register boot size.

Floor register throw should be close to if not equal to the height of your ceiling(unless its a cathedral ceiling). Then 8 foot will work.

Ceiling registers. You want the throw to reach your outside wall, if the register is in the center of the room, or on an inside wall.

If its on the outside wall. Then you want is to have a throw to reach your floor(cathedral exception again).

Spread. Based some what on the wall area your covering. A register covering a 8 foot long wall needs less spread the one on a 12 long wall.
So spread is best picked based on the registers position to walls when parallel to a wall. or to room length/width when in the floor but not at the wall.


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## stubits

Great information, for sure, makes total sense. Most of my registers are well enough located, but I've got a couple of odd situations.

What do you suggest for floor registers located on inside walls. What's the goal?

I have a couple of registers located effectively in a corner. In one case it is a corner between an interior wall and an exterior wall and in another case at the corner of two interior walls. What is the best situation here?

How do I go about sizing the register for the return air? 

Also, previously we had talked about having a return air vent in my basement. You mentioned that because of the hot water heater, I needed to be careful as to how much return air I took from down there. I know normally the goal is to balance supply and return, or even a bit more return, but what should I do with the basement?

Thanks!


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## beenthere

Inside walls are some what the same as outside. As far as, your looking for throw. Spread is often reduced because your going to hit a perpendicular wall. So spread can be selected on a tighter basis.

Return filter grilles ideally are sized for 300 FPM velocity through them, so the filter is more effective.
A return grille only can be 600 FPM is ok. 

For a basement with a water heater, or furnace that uses inside combustion air. Its better to pull a little less return then you have supply. makes the basement slightly positive pressure when the blower is running. But no worry about drawing CO into the house.


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## stubits

At least this project is moving forward. Thanks for all your help!

So, is it OK to upgrade the ductwork, and the furnace itself, in stages? We'll be constructing an addition this summer, so would it be OK to upgrade the trunk that serves the addition now, based on the specs for the new furnace, and then upgrade the rest of the ductwork and the furnace itself in 6-9 months?

Assuming that's OK, how does this diagram look. I think it should be clear, but let me know if you have any questions. The required CFM are based on the HVAC Calc CFM report. 

Not sure if all of it is as clear as I had hoped, the text on the green indicates that the duct is 6x10 - 134 [email protected] and is approximately 14' long. The text on the brown indicates that it is 8x10, [email protected] and approximately 11.5' in length. Everything else seems clearer, but let me know if not! 

Any thoughts?


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## beenthere

As long as the current furnace is within its temp rise with the duct addition/rework. You can upgrade like that. Without any problem.

Your drawing shows the blue 6X10 tapping in as a tee.
If you can get a 15° tapered tap, it will work better yet.


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## stubits

That blue one is actually interesting, the brown runs along the ceiling but when it reaches that wall, it turns up into the joist bay. It will then branch off to the blue and the green from there.


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## beenthere

Ah.
Might want to use turning vanes then. To keep the air flowing better.
2 direction changes that close are very restrictive. vanes will decrease the amount of restriction by about 80 to 90%.


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