# HVAC - Airflow Issue



## AndrewF (Dec 29, 2008)

There are 26 6" round take-offs from the main supply trunk line. The A/H sits almost in the center of the supply feed, but near the end of the return. I am open to taking the main trunks down and putting larger up....perhaps 24x10 for the supply (since it only needs to push 1000 CFM in each direction) and 26x10 for return, since the system sits at one end of the return duct.

Each room has a return in it, most rooms have two supply (except for bathrooms). All returns upstairs are up high on the wall, all supply are in the floors (1st and 2nd floor).

The reason I think I have an air-flow problem is that the highest CFM I have seen with a detector in the return is 1300 CFM. Changing the blower speed from med to high doesnt impact that number much at all. Cutting an additional hole in the return box in the basement did increase airflow a little.


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## beenthere (Oct 11, 2008)

26 x 10 is not nearly big enough for a 5 ton system.

Your return trunk is too small, and your return drop is too small(increasing to 30 x 12 will help).
But, if you don't increase the return trunk, you won't get much benefit from it.

You'll probably need to add more return to the house, and then increase the size of the return trunk and drop.

Do both supply trunks supply the same amount of air. Not just the same number of supplies tapped into them.

With you air handler dead heading into the supply the way it does. It creates a high resistance to air flow.

Might want to make the return box only 12" high, and bring 2 drops into it, to get better air flow.


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## hvaclover (Oct 2, 2008)

That has got to be the least well planned out install i have ever seen.

Beenthere has stated the reason why with the exception you are probably over sized on the HP also.

Kinda ironic.


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## AndrewF (Dec 29, 2008)

beenthere said:


> 26 x 10 is not nearly big enough for a 5 ton system.
> 
> Your return trunk is too small, and your return drop is too small(increasing to 30 x 12 will help).
> But, if you don't increase the return trunk, you won't get much benefit from it.
> ...


Thanks for the feedback. I don't want to lose anymore headroom in the basement, so the deepest I can go on the trunks is about 10", however, I can go as wide as I need to.

I also figure one can't go too big on the trunks either.

So my thought was to upgrade both the supply and return trunk to 10x24 (1320 CFM each direction), make the return box much smaller so that there is more room above the top as you suggested and have the two return drop-downs made to incorporate larger filters. Making the return vents in the walls bigger, isnt a problem for the 1st floor, the 2nd floor I have the option to add a central return vent by utilizing the space that the exhaust pipe for the old furnace was in.


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## AndrewF (Dec 29, 2008)

hvaclover said:


> That has got to be the least well planned out install i have ever seen.
> 
> Beenthere has stated the reason why with the exception you are probably over sized on the HP also.
> 
> Kinda ironic.


I am not sure if you are poking me or the original designer. I had little choice at the time to do the upgrade. With a newborn (weeks old) and another young one with outside temps around 100, I had to push up the timeline to put the new system in. I knew I would have to redo the ductwork in the winter, which is where I am today. Yes, it is more work now, but keeping my family cool was a priority to me.

The house is a mansared style house, so the upstairs is subject to heat through the rafters due to the roof design (see photo)

It also bothers me that no less than *TWO* separate HVAC "professionals" have come out and all said, nah the duct sizing is fine, just reduce the blower speed to low if anything....well I can't run the blower on low with 20kw heat strips according to the manufacture. 

I am curious why you think the 5 ton system is oversized. The current sq ft of conditioned space is 3,586. When we finish the basement, that will jump to between 4500 - 5000 (depending how much we finish)


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## beenthere (Oct 11, 2008)

Don't get fixed on a 1320 CFM split.

How many CFMs does each side need to carry.


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## hvaclover (Oct 2, 2008)

AndrewF said:


> I am not sure if you are poking me or the original designer. I had little choice at the time to do the upgrade. With a newborn (weeks old) and another young one with outside temps around 100, I had to push up the timeline to put the new system in. I knew I would have to redo the ductwork in the winter, which is where I am today. Yes, it is more work now, but keeping my family cool was a priority to me.
> 
> The house is a mansared style house, so the upstairs is subject to heat through the rafters due to the roof design (see photo)
> 
> ...



Because I have the same design temps where I live and the basement is not considered a full load as it is below grade and subject to little to no heat gain.


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## AndrewF (Dec 29, 2008)

beenthere said:


> Don't get fixed on a 1320 CFM split.
> 
> How many CFMs does each side need to carry.


I left the sketch at home that shows me how much airflow is on each side of the A/H. (# of 6" vents). I'll post back up once I have the sheet.

Thank you.


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## beenthere (Oct 11, 2008)

A 6" supply to a bathroom, doesn't actually have teh same amount of air going through it as a 6" to a kitchen(at least it shouldn't).

You chould make a drawing and also determine how much air each room really needs.

Not unusual for one side of a house to need 20% more air then the other side.


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## AndrewF (Dec 29, 2008)

Thanks. I'll be creating a new drawing tonight as apparently my other one is gone from the cleaning we did before having family over for the holidays.

There are no dampers on any of the ducts in the basement.

Is it worth paying a local guy to do a Load J? calculation or utilize an online service to do it?


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## beenthere (Oct 11, 2008)

You can do your own.

http://hvaccomputer.com/talkref.asp


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## AndrewF (Dec 29, 2008)

I had seen that one before and was curious if it was reputable. Thanks for the recommendation. I have purchased it and will proceed with it and report back here what the results are.


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## AndrewF (Dec 29, 2008)

I completed the Manual J calculation this morning. I did it twice to make sure I did it right. I did have to make some guesstimates as to what type of insulation board is behind the brick.

Attached is the completed report.

In summary:

Total Heat Gain: 37,917 BTUH
Total Heat Loss: 61,166 BTUH
Sensible Gain: 34.694.

So while the system (5 ton) is plenty large for A/C, I think it is sized ok for heating, which is my main area of concern.


I could easily add the garage or the workshop portion of the garage into the A/C factors for summer if I needed to get longer runtimes to remove more humidity.

I'll be working on the sketch of the existing ductwork this evening...time to head out and work on the barn.


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## beenthere (Oct 11, 2008)

At least at first glance, it looks ok.
5000 sq ft.

Do you actually set your stat to 68 in the summer.

Its a code violation to have the garage on the same forsed or gravity air system that serves an occupied area.
CO poisoning potential.


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## AndrewF (Dec 29, 2008)

beenthere said:


> Do you actually set your stat to 68 in the summer.
> 
> Its a code violation to have the garage on the same forsed or gravity air system that serves an occupied area.
> CO poisoning potential.


Yes, we like to have a cool house, so we run between 68-70 in the summer. In the winter we run 73-75.

Good to know on the code issue if I was to put a vent into the garage, I didn't know about that, but it makes complete sense.


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## beenthere (Oct 11, 2008)

Would you by chance, happen to know what humidity, your house was at last summer when you had it 68 in the house.

I keep my place about 72°F and 48%RH.
Which feels about the same as 68 and 54% RH.


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## AndrewF (Dec 29, 2008)

I dont as that was a different house and I didn't have monitoring setup. We never had a problem with moisture on windows, the tile on slab floor, or other areas either.


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## AndrewF (Dec 29, 2008)

Ok, I spent some time mapping out the duct paths as they are today and scanned them in.

It looks like to the one side of the A/H there are 12 6" ducts. So 12x85 cfm = 1025.
On the other side there are 15 6"" ducts. So 15x85 cfm = 1275.

If an 8"x24" metal duct is rated at 930 CFM, that would explain part of my AirFlow problem.

I think I should replace the supply and return with 10"x24" which is rated at 1320 CFM and not worry about stepping down sizes from the A/H unless the cost justifies doing so. The airflow would be close on the side that has 15 6" take offs, but still within specs I believe.

I'll also have a new box built for the airhandler to sit on to allow more headroom at the top and reduce turbulance. I would also have a 32"x12" drop down and at least a 20x25 filter slot made.

I further think my return system needs a total revamp. While each room has its own return in it, they are limited by the size of the interior walls. My walls are 2x4 on 16" centers. I need at a minimum 180 CFM per room for return, right?

For the 1st floor, that will be an "easy" fix, for the 2nd floor I think I'll have to utilize the chase way that had the fuel oil furnace doubleinsulated exhaust stack. If I remove that (already planned to) I'll have a clear 20"x20" chase up to the 2nd floor where I could put a return vent on it....this would be in the central hallway upstairs and would provide plenty of additional return capacity for the 2nd floor. There is no attic to utilize for running additional vents to each room.

Does my logic/math seem correct?


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## hvaclover (Oct 2, 2008)

Can't read it....not dark enough.


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## beenthere (Oct 11, 2008)

You should use a reducing trunk line. By reducing you maintain relatively the same velocity for the air that is coming out the vents closest to the the furnace as the ones the furtherest from the furnace. 

Using the former chimney space for additional return, will be a big help.
A room that has a 1-6" supply, doesn't ned 185 CFM of return.


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## AndrewF (Dec 29, 2008)

Ok Thanks.

The only rooms that have a single vent, are the bathrooms, kitchen and utility room. These rooms dont have returns in them already. The kitchen is pretty open to the living room.

Since you suggest I do the reduction, based on the flow, I think I can do it at the same point as the existing ductwork. SO I'll go from a 10x24 to a 10x16.


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## AndrewF (Dec 29, 2008)

I picked up the duct from the supplier today. I ended up with 10x24 and 10x20 (for the reduced sizes)

They didnt have any drop downs that would work, so now to get a sheet metal shop to fabricate me something up.


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## AndrewF (Dec 29, 2008)

What is the best way to seal each take off from the trunk line? Most of them are 6"x10" rectangle to 6" round duct.

Do I just use a bead of silicone on each joint? On the old ductwork, it wasn't sealed and I can see dust around each takeoff to show it was leaking pretty good.


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## beenthere (Oct 11, 2008)

You can use silicone caulk.
Mastic can do a better job.


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## hvaclover (Oct 2, 2008)

CONFLICT Of INTEREST......CONFLICT OF INTEREST!!!!:laughing::laughing::laughing::jester:


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## AndrewF (Dec 29, 2008)

hvaclover said:


> CONFLICT Of INTEREST......CONFLICT OF INTEREST!!!!:laughing::laughing::laughing::jester:


Must be an "inside" joke.


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## JohnH1 (Jan 9, 2009)

Re: duct design here is a very good step by step 4 CD set from ACCA.
kinda expencive maby for a homeowner 1 time use. But I have it and refer back to it all the time for proper duct design. http://www.acca.org/store/product.php?pid=212


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## AndrewF (Dec 29, 2008)

Thanks. I am only replacing the trunk lines. According to the manual J I did, the takeoffs for each room are acceptable. If after I replace the trunk lines, I am still not "happy" with the flow after new tests, I can easily upgrade/replace the first floor service as it is accessible in the basement. The 2nd floor as far as I am concerned, isn't changing.


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## beenthere (Oct 11, 2008)

AndrewF said:


> Must be an "inside" joke.


On another site I'm a member of.
Its a NO DIY site.
I'm a new mod there. And would close this thread, because it would violate the rules of that site.

So thats the joke.


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## AndrewF (Dec 29, 2008)

I am making progress. I had to reroute the water supply today as they had it going next to the old supply trunk and then across the top of it. The water line was wedged between the floor trusses and the duct. I shut the water off at the street and replaced about 25' of copper and installed a new ball valve shutoff.

I also have about 1/3 of the supply trunk and about 8 feeder lines done.

Next up is to lift up the AH and put in the new return box for it to sit on, drop the AH down on it and then continue the supply duct above the AH and then finish up with the return duct.

I have a question. When I make the filter rack in the *return* drop down, should I set two filters up in an A setup to maximize the SQ in of filter area?

For example, instead lf a single 20x25 filter laying flat in the drop down, what if I put two 20x25 or 16x20 filters in an upside down V, any reason not to do this?

My drop down from the return trunk is 20" x 25" x 75", so I have plenty of room to do what I want.


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## beenthere (Oct 11, 2008)

I'm hoping you mean return drop.
You don't filter the supply. 

V shaped filter rack can actually cause more restriction then using a single.

If you want good filtration, and low pressure drop across the filter. Use a 4 or 5" media filter.

Yes, replacement media cost more then those 1" filters do. But, they last 6 to 9 months. And allow better air flow.

Only use OEM replacement media. 
The third party media is not made the same, and has a higher restriction/pressure drop when new, compared to OEM media.

They are well worth the money.

Look up Key1cc. he did test on different air filters, for pressure drop.
He has either a 4 or 5". But, I believe he tested 1" filters also.


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## AndrewF (Dec 29, 2008)

I fixed my post. yes, I meant return drop down. I'll look up the info you said.

I am not against spending more on a good filter. I refuse to buy the cheap filters now as they simply don't work well and let the A coil build up with dust and clog.

So using a deep media is better than my idea of a V media shape then. I figured since the A coil is V shaped in my unit and the fan sucts the air through it, using filtration media in a V would work too.


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## AndrewF (Dec 29, 2008)

I have completed all of the supply upgrade. Wow! What a chore, definately not something I would want to do again.

We do notice an increase in airflow to most of the rooms. Previously one had to have their hand about 6" from a register to "feel" the heat or airflow. Now you can be about 18" away.

We havent installed all of the return yet. Hope to finish that tomorrow. Right now we have filters in front of the box the air handler sits on in the basement....which means it is sucking cooler (65 degree) air from the basement and heating it up to about 82 for the rest of the house.

I think that "helped" contribute to the extended run-time lastnight...along with the 16 degree temperature outside.


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## beenthere (Oct 11, 2008)

Yea, using the baement for a return air cavity, does increase teh load on it.


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## AndrewF (Dec 29, 2008)

Here are a few photos, including the cheap $30 nibblers I bought to cut all the holes for the take-offs.

We noticed a considerable drop in air noise with the extra space between the AH and the supply trunk. Instead of it being only 8" before, there is now approximately 15".

With the larger box the AH sits on, the access panel is not sucked shut like it used to be, an indication the system was starving for return air.


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## AndrewF (Dec 29, 2008)

It is done! WHEW!

Ok, well maybe I am still figuring out a better way to do the filtering as the 20x25x1 filter restricts the airflow too much. I am toying with a 20x25x4" filter or some other filter rack setup to allow the most CFM through without affecting the velocity of the air.

I am open to suggestions on a solution for that.

As for the install, how did I do?

Note: The masking tape is temporary until I am sure I don't need in the bottom box anymore. I didn't want to waste any of the seal tape.


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## hvaclover (Oct 2, 2008)

DUDE, YOU GOT A BULL HEAD PLENUM AND A DEAD END REDUCER IN THE TRUNK.

YOU'RE GONNA HAVE MORE TURBULENCE THAN HURRICANE KATRINA.

Wood blocks are not too attractive...there are vibration absorber blocks made for that.

Way too much alum tape...use silicone to seal open areas.


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## beenthere (Oct 11, 2008)

Should put vanes in the return drop at the plenum, and in the top connection. 
Your currrent configuration has about a 260' total equivalent length.
The vanes will reduce that to less then 100'.
Also, the supply should have a baffle/deflector to help guide the air instead of just adeadhead.

The deadhead reducers are very turbulant. A normal 15° or less transition reducer only adds about 5' of equivalent length. A deadhead adds about 35'.


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## AndrewF (Dec 29, 2008)

hvaclover said:


> DUDE, YOU GOT A BULL HEAD PLENUM AND A DEAD END REDUCER IN THE TRUNK.
> 
> YOU'RE GONNA HAVE MORE TURBULENCE THAN HURRICANE KATRINA.
> 
> ...



That 2x4 is just laying there, not supporting anything. The box is sitting on patio stones.

The alum tape is covering up some of my ugly sealed areas  I went through 4-5 tubes of silicone based caulk on the whole setup.


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## AndrewF (Dec 29, 2008)

beenthere said:


> Should put vanes in the ret, urn drop at the plenum, and in the top connection.
> Your currrent configuration has about a 260' total equivalent length.
> The vanes will reduce that to less then 100'.
> Also, the supply should have a baffle/deflector to help guide the air instead of just adeadhead.
> ...


When you say vanes, you mean long narrow strips of metal? Hmmh.....I can still get into the drop box to make any modifications.

The deadhead reducers...yeah I thought about fashioning up a better transition, I just had so much time in it that I needed to get it operational. These little tweaks I can still do as you both recommend.


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## hvaclover (Oct 2, 2008)

AndrewF said:


> That 2x4 is just laying there, not supporting anything. The box is sitting on patio stones.
> 
> The alum tape is covering up some of my ugly sealed areas  I went through 4-5 tubes of silicone based caulk on the whole setup.



It'll be primo once you get those bull head changed out.

Silver tape is a sure bust by an inspector around here, the think you are hiding big gaps. Stuff is really only supposed to be used on duct board, but it has it's uses in limited amounts. 

Other wise, not bad for a Greek Boy.

I wish i could tell you how to set the cfm, but HPs are different than ac in that regard. Don't know that much about them.


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## beenthere (Oct 11, 2008)

http://www.ductmate.com/products/turningvaneandprorail.asp


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## AndrewF (Dec 29, 2008)

hvaclover said:


> It'll be primo once you get those bull head changed out.
> 
> Silver tape is a sure bust by an inspector around here, the think you are hiding big gaps. quote]
> 
> ...


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## beenthere (Oct 11, 2008)

Makes a real BIG difference in the return.

I usualy use a$$ checks in the supply if I have to dead head like that.

Its basically a simple v.

You have to position it, for the air flow you need in each duct.

So you need to knw which side needs more air.


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## AndrewF (Dec 29, 2008)

beenthere said:


> Makes a real BIG difference in the return.
> 
> I usualy use a$$ checks in the supply if I have to dead head like that.
> 
> ...


I believe you, I am just trying to grasp why it makes such a big difference in the return. I understand the dead head of coming out of the AH...although it is WAY quieter than it used to be. I could just make a V and insert in there too, right?

To increase airflow through the filter rack, should I do a 4" deep filter instead of a V design?


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## AndrewF (Dec 29, 2008)

AndrewF said:


> In summary:
> 
> Total Heat Gain: 37,917 BTUH
> Total Heat Loss: 61,166 BTUH
> Sensible Gain: 34.694.


Referencing back to the load J I did.

Is my problem that my house is losing 61,000 BTU/HR? and at 17 degrees the heat pump can only generate 36,000?


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## beenthere (Oct 11, 2008)

That does require your strip heaters to make up the difference.
And the strip heaters cost a lot more to run, then the heat pump.

The U turn, the air makes at the base, is very restrictive. You don't want an air filter right there. The air is too turbulant.

With your current set up. Air is not moving at the outer areas of the duct. 

So your not using the full area of the duct.
Vanes will help the air to be able to use the full area of the duct.


Be carefull, not to make the V too deep, or it can restrict air flow.


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## AndrewF (Dec 29, 2008)

beenthere said:


> With your current set up. Air is not moving at the outer areas of the duct.
> 
> So your not using the full area of the duct.
> Vanes will help the air to be able to use the full area of the duct.
> ...


I understand now. As for the location of the air filter, did I pick a good spot to put it in? Near the middle of the return trunk?


I thought about taking a 2"x2" 90 degree piece of sheet metal and putting in the supply duct right about the AH....would that be an ok size?

I figure I can notch a V in the side of the supply trunk and slide it in rather easily.


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## beenthere (Oct 11, 2008)

The V can be 2 to 4" deep depending on how far it must be moved toward one side or the other, for air flow.
And the wide for eaxh sde should be 1.5 times its depth.

You want to put a radius on it, similar to this drawing.


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## beenthere (Oct 11, 2008)

The air filter in the horizontal duct is a good location.


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## AndrewF (Dec 29, 2008)

beenthere said:


> The air filter in the horizontal duct is a good location.


I am still puzzled why a 4" thick filter is better than a 1 filter in a V design....wouldnt a v design offer my sq in for filtration?


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## beenthere (Oct 11, 2008)

Because of the way the air is forced through the filter.
At the sides.

A 4" media air filter will have several times the surface area of the V coniguration. Much less restrictive.


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## derryank (Jan 31, 2009)

beenthere said:


> Makes a real BIG difference in the return.
> 
> I usualy use a$$ checks in the supply if I have to dead head like that.
> 
> ...




hi guys sorry to jump on your project here but i have a very similar problem with air flow. on two registers in my house i have a 20 degree differnce in air temp from any other register in the house. both of these ducts are 6inch rounds into the same side of the vertical plenum. they also have the same air flow problem from the rest of the registers in the house. do you think if i installed the 'V' at the top of the plenum it would help the air movement and heat differential to both of the affected registers??? thanks so much for your knowledge.:thumbsup:


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## beenthere (Oct 11, 2008)

Not enough info to say for sure.

Is it a heat pump.
Do those supplies run through an unconditioned area.
Are those runs a lot longer then the ther runs.


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## derryank (Jan 31, 2009)

Thanks for your reply.All runs are through a "comfoftable Basement" 50 degrees or so.
I have taken temps on each run.Each sample was taken 12" from the plenum and at the registers.
The run in question is 25 degrees cooler than the other runs(12"from the plenum and at the registers on the first floor). This is a gas forced hot air furnace.
Thanks in advance.


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## beenthere (Oct 11, 2008)

Changing their take offs to scooped take offs, may help.

Pull your air filter, and then see if they blow out more air. It could be a lack of return air problem.


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