# What size return duct and vent for basement?



## user_12345a (Nov 23, 2014)

One 6" sounds pretty small for a whole basement, you may need two. You can always close down if you get too much heat. 

Don't split a small 4,5 or 6" branch line. If a ceiling vent gets too little air it won't be forced down and mix properly, it will just rise to the ceiling.



> Since I went with a 6” supply duct, will a 6” return duct be big enough or should I go with an 8” duct?


I would go for 8", near the floor. If you will have interior partition walls with one side exposed, in furnace room or unfinished closet, can use stud space as a duct, pan off the non-drywall side and run a short 8" round to it. (would put a 8" takeoff into the panned part)

It's important to have good return in a basement, pull the cooler air off the floor to the supply air can replace it. 

A combo of bad mixing, ceiling vents and insufficient return near floor can make a basement cold despite having enough supply.

Do you know how the second floor of houses with one a/c - basement air handler/furnace doesn't cool well without high returns?

Same idea, you're working against gravity.


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## Logan176 (Feb 18, 2016)

12345,

Thanks for the helpful info. I had planned on panning of the bay in the wall and running the return to the floor, so I'm glad to hear I was on the right track. I'll upgrade the 6" round to an 8" round as you suggested.

I just went downstairs to take another look at the supply. For some reason, the guy that installed the new basement ducting started with a 6" collar on the plenum. Then, he used 8" flex to connect with an 8"x8" run across the basement (see pics). 

If I change the 6" starting collar to an 8", do you think that will move enough hot air to allow for two registers in the 8x8 run? If not, what other advice can you give to properly run ducting for two registers through this tight soffit space?


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## user_12345a (Nov 23, 2014)

Use of a 8" takeoff would help.

I would stay away from long runs of flex, less flow for same size and the sags, etc can cut airflow. 

Flex may be okay for very short runs. Like connecting a panned off trunk to a return trunk near by.

If this was my project I would run two small metal round pipes. Or put a separate proper trunk for the basement, but that would be overkill for two registers.

There are calculations that can be done to estimate required heat, roughly required cfm. I think two 6" would be plenty and like I said before, close one if it gets too hot.

-------
Was the air duct system redone when the house was or re-used?

I hope that 8x8 is only feeding a smaller portion of the house.


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## Logan176 (Feb 18, 2016)

12345,

The 8x8" trunk in the pictures is a separate, dedicated run for the basement. It does not send any air upstairs. The flex is about 6 feet long. I can easily swap out the 6" collar on the plenum with an 8" collar and replace the flex with a solid duct. That should increase airflow a bit.

Two separate 6" ducts (one for each register) would be ideal, but I don't have enough room to run them side-by-side through the soffit. The only way I think I can make something like this happen would be if I installed one 6" round through the soffit (red arrow) and run it down to the far end of the room. Then, I could install the second 6" duct to the wall (purple arrow). To make sure the hot air reaches the floor, I could transition into a 3.25x10 and run it down the wall or I can keep the register closer to the ceiling and use one of those clear plastic diverters that attaches to the metal grill with magnets. I have them in other rooms in the house and they work well.

Do you still think the two smaller runs is a better option in my situation? I would love to keep the 8x8 since it's already there, but I want to make sure this is all done right before the sheetrocking begins.


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## user_12345a (Nov 23, 2014)

Soffit?

Normally the pipes are run in the spaces between the floor joists, parallel to the joists. The trunk would run perpendicular to the joists. I don't know which way the joists are running.

With a dedicated trunk, I most certainly would run separate branch lines directly from it and get rid of the 8" flex.

Ceiling vents are fine if you have return low.


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## Logan176 (Feb 18, 2016)

The main trunks are perpendicular to the joists and run right down the center of the basement ceiling. The ceiling is only 7' tall and the soffit that will enclose the trunks will drop down another foot. This effectively splits the basement conditioned space in half.

I know the correct way to run the ducts would be to branch off the 8x8 trunk. If I was going to run branches, I would want a register in the TV area and my daughter's play area, especially since this half of the basement has the windows. But then I would still need a register on the other side. That means that I would need three registers from the 8x8 and that might decrease the air flow and not let the warm air reach the floor.

I could be wrong, but if I only put a ceiling register in the TV space or the play space, I'm not sure how much heat would make its way under the soffit and to the other side of the room. This is why I had planned on attaching two registers to the bottom of the trunk, one to the left and one to the right of the stairs.

Based on the layout of my basement, where would you place the registers?


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## user_12345a (Nov 23, 2014)

> I know the correct way to run the ducts would be to branch off the 8x8 trunk. If I was going to run branches, I would want a register in the TV area and my daughter's play area, especially since this half of the basement has the windows. But then I would still need a register on the other side. That means that I would need three registers from the 8x8 and that might decrease the air flow and not let the warm air reach the floor.


What size is the main trunk for the house, how many registers and what size furnace do you have?

I'm guessing your gmv is either 45k btu or 70 btu input.

If more than two vents are needed for the basement to get the distribution right, simply make them 5" instead to not overheat the basement. 

Use 3x10 for supply vent size, one return at floor level and it will be okay.

The 8x8 can probably proportionally support two 6" pipes and one 5".

If your furnace is capable of moving the air and it has ample return, probably won't have issues given it's a dedicated trunk.


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## Logan176 (Feb 18, 2016)

The furnace is a 70k unit. The main trunk for the rest of the house is 16x10. I have 7 supply vents and 6 return vents.

To play it safe, it sounds like I should go with two or three 5" branches. Based on the layout above, where do you think I should place them?


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## SeniorSitizen (Sep 10, 2012)

My basement doesn't have any return ducts being it has an open stairwell and that's a dam good size duct in itself. It's so big I can feel air flow at a specified elevation as I traverse the steps.


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## user_12345a (Nov 23, 2014)

Logan176 said:


> The furnace is a 70k unit. The main trunk for the rest of the house is 16x10. I have 7 supply vents and 6 return vents.
> 
> To play it safe, it sounds like I should go with two or three 5" branches. Based on the layout above, where do you think I should place them?


What size are the supplies?

If the rest of what you have is small for the furnace it may make sense to do 3 6" pipes to basement just to make it easier on the furnace. 

Even at 6" metal each and typical, flow like 100 cfm per run, not really enough for that furnace -> likely needs 1100+ cfm on high, 800 on low or something. Factory settings may be for 900 cfm low/1400 high, I know that particular model and the 70k came from the factory set pretty high.

It's hard on the motor's module having high duct pressure.


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## Logan176 (Feb 18, 2016)

I have a few different size supplies branching off the main trunk.

(3) 3.25x10 ducts with 10x6 grills
(1) 3.25x10 duct with 10x2 grill
(1) 3.25x12 duct with 14x7 grill
(1) 6" round duct with 10x6 grill
(1) 6" round duct with a 12x2 grill

Note: The last 6" duct will have to be converted to a 4" because it's in the way of where I need to run one of the new basement branches. I think I will still have enough supply for that room because the damper is always keyed halfway closed. This is because I have two supplies in the room and it gets too hot with the smaller vent next to our bed opened all the way.


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## user_12345a (Nov 23, 2014)

I think your system could use more supply, the 3.25x10 is basically the same as a 6" on a practical level, same area, etc.

So probably 3 6" pipes in the basement.

It seems like parts of the old duct system were re-used, odds grill sizes and use of rectangle branch lines.


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## Logan176 (Feb 18, 2016)

Three more 6” branches sounds like a plan. I’ve done some more brainstorming and it looks like I’ll be able to get two of the three registers at the bottom of a wall. So, that should also help distribute the heat more evenly. 12345, thank you so much for your help. I truly appreciate the time you’ve spent answering my questions.


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## user_12345a (Nov 23, 2014)

you're welcome. remember to put dampers before the register boots so you can adjust as needed. the metal plate damper inside is better than just using the damper on the grill.


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## Logan176 (Feb 18, 2016)

12345,

I already have a metal damper where the 8x8 comes off the plenum, but I will make sure I have dampers in each of the branches, too.

Speaking of branches, I have one more question for you. As I mentioned above, I figured out how to get two of the registers at the bottom of a wall with only a very slight modification to my framing. However, I'm still trying to figure out the best placement of the third so that people sitting on the couches are comfortable... especially when on the couch right under the window.

In the drawing below, registers 1 and 2 are the ones located at the bottom of a wall. I have a few options for register 3. 

Location 3a and 3d has the register at the bottom of the wall aiming towards the couches. My concern here is that the only heat sources for the right side of the room are coming from the center. Although, 3d seems better than 3a because the registers are more spread out.
Location 3b would be located in the soffit, which is about 6 feet from the floor. I can aim this air towards the center of the couch area. I can also install this one almost anywhere between the lally column and 2' from the right foundation wall.
Location 3c is in the ceiling, which is 7 feet off the floor. I like that it's in the middle of the space.

What would you suggest?


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## Logan176 (Feb 18, 2016)

Forget what I said about 3d. That won't fit so close to the bottom of the steps. The base molding and stringer don't leave me with enough room.


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## user_12345a (Nov 23, 2014)

i would try to get it as close to the window as possible provided that it's not being blocked by the couch and not blowing on people.

If you end up with a ceiling register, middle of the room so it doesn't blow on people sitting on the couch.

Like 3C.


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## Logan176 (Feb 18, 2016)

So, I started taking apart some of the work the hired HVAC guy installed and it really looks like a hack job. As I mentioned earlier, the guy installed the new 8x8" trunk using an 8" collar on the trunk, an 8" flex duct, and a 6" collar on the plenum. But... the hole in the plenum was only 5"! I opened up the hole and properly installed an 8" collar. Unfortunately, this wasn't the only hackery I found.

I thought it was strange that he was able to connect an 8" round to an 8x8" trunk. I just figured he must have bought or fabricated a proper connection.

Silly me.

The guy just bent the ends of an 8" collar around the side of the trunk and slapped on foil tape. And if everything I've found so far has just been thrown together, I'm sure the cap on the other end of the trunk is nothing more than cardboard and tape.

So, here's my question... *Should I just take my time and seal up the 8" round connection to the 8x8" trunk the best I can, using foil tape and mastic, or should I pull out the the trunk and replace it with an 8" round?* I'd like to keep the 8x8" trunk if possible, but I want this done right.

If I go with the 8" round as a trunk, I assume the air would flow ever better and there will be a proper connection to the plenum. I think my local HVAC supply store carries a 6" take-off that will attach to the new 8" round "trunk" so I can branch off to the registers. Thoughts?


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## user_12345a (Nov 23, 2014)

Use the proper connection to start with regardless of whether you want to use round or rectangular.

8" round would need a bigger bulkhead in the basement by a bit. 8x8 is equivelent to closer to a 9" round based on friction rate.

The 8x8 should be off the main plenum and not a trunk.


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## Logan176 (Feb 18, 2016)

I understand what you're saying about 8x8 trunk and an 8" round having different friction rates. I don't have the room to attach the 8x8 trunk directly to the plenum and I also don't have the room to go with a 9x9 trunk. I have the main support beam for the house, another supply trunk, and the hot water tank in the way. 

So, if the only proper connection to the plenum I can fit is an 8" round (which I connected this morning), does that mean I should pull down the 8x8 trunk and install an 8" round trunk in its place?


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## user_12345a (Nov 23, 2014)

Don't leave it badly done, if you can't get a proper fitting to convert 8" round to 8x8, better to come up with an alternative.

What size is the main trunk? maybe it has capacity to spare and you can just tap off of it?


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## Logan176 (Feb 18, 2016)

I want to keep the basement trunk separate from the trunk that feeds the rest of the house. Since the main rectangular trunk reduces a few times before it reaches the far end of the basement, I won't be able to make proper connections for two of the registers.

If I stick with the 8x8 trunk, I feel I can make a proper connection using something like what you see in the attached picture. But, do you think I will have enough air flow if I go off the plenum with an 8" round to an 8x8 trunk (20 feet) and then branch off the 8x8 with 6" round to the registers? Two of the three registers will be at the floor, but the final register at the end of the run will be at the ceiling (7' high) and I want to make sure there is enough air flow for heat to reach the floor.


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## Logan176 (Feb 18, 2016)

You know what? I just realized this is crazy. It's like I'm trying to convince myself to go with the route that is so, so instead of what we know will definitely work. I have to pull down the 8x8 anyway, so I am just going to replace it with the 8" round. 12345, thanks for your help as always.


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## user_12345a (Nov 23, 2014)

No problem.

I think an 8" round will be okay two 6" metal or 3 5" metal.

why do you have to take down the 8x8?


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## Logan176 (Feb 18, 2016)

user_12345a said:


> why do you have to take down the 8x8?


I was going to take down the 8x8 because you mentioned the two different friction rates of the 8" round from the plenum going to the 8x8 trunk for the basement registers. If I go with the three 5" branches as you suggested, I want make sure I have enough air flow for the register at the end of the run to reach the floor from the ceiling. 

If you're suggesting I scrap the whole idea of a basement-only trunk, I guess I can do that, too. It would mean I wouldn't be able to shut down air flow to the entire basement during the summer using one damper near the furnace. As long as I can get that ceiling vent to kick air all the way to the floor at the end of the run, I don't have a whistling noise from the vents when they're shut down during the summer, and I don't completely throw off the supply flow to the two upstairs floors, I'm open to suggestions.


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## Logan176 (Feb 18, 2016)

By the way, the original trunk is 16x10 as it leaves the plenum. It is reduced a few times before it reaches the end of the run where it becomes 3.25x10. I went back and reread this entire thread. You mentioned earlier that having a dedicated trunk just for the basement is the proper way to go, so I would like to stick with that if possible.


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## user_12345a (Nov 23, 2014)

I think what to do should depend on the size of the trunk line, if you can add more vents or if the cfm max of trunk at a given friction rate is equal the ducts connected to it.

if it's at capacity on paper, better to run dedicated line.

At 16x10 though, it's probably a bit small for the furnace, that furnace needs like 1100-1200+ cfm on high fire so even at a highest friction rate recommended, like 0.1"/100 ft it's a bit small.

So yes, better to have separate trunk.

In all likelihood the furnace is oversized and should barely need high fire.

Took me a while to remember the original details of the thread.


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## Logan176 (Feb 18, 2016)

I've read that branches should not be attached to an end cap because it throws off the pressure. I've also read that the last perpendicular branch coming off a rectangular trunk should come off no closer than 12-18" from the end. Since I will be branching off the end cap of the 8x8 trunk with a round duct, should I still install the branch 18" away from the end like you see on the right side of the attached picture?


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## user_12345a (Nov 23, 2014)

I read that too. it may make sense to tap off the last of the 8x8 and extend it to avoid that issue if you're concerned.

or just run a new 8" round metal as a trunk, get rid of the 8x8 and flex 8".

i wouldn't try to reduce 8x8 down to 6" round and tap off the 6" like in the pic, doesn't make sense.


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## Logan176 (Feb 18, 2016)

I'm going to look into using an 8" round as the trunk, but I might run into a space issue. I don't think the HVAC shop carries a tee or take-off that goes from an 8" round to a 5" branch without requiring a reducer or two, and I just don't have the room on top for that.

Instead of reducing to a 6", would it make sense to do the same with a 5" since the branch will be a 5" anyway? If not, I know I can extend the 8x8 like you suggested and use a top take-off. 

Thanks.


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