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# Supply vs. return duct sizing

## Supply vs. return duct sizing

#1
07-29-16, 10:44 AM
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Supply vs. return duct sizing

I have a room which is served by a 3.5" x 12" duct for forced warm air and central A/C. The air return from that room is an 8" x 12" duct. The room is basically a "den," so only one doorway to the room, and no real air circulation through it other than what's supplied by the HVAC.

Is this unbalanced? Should the return be made smaller for optimal performance? Or is it OK as is?

If more information is needed, let me know.

#2
07-29-16, 12:12 PM
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Normal is for the returns to be 100-150% of the supply area so this is just a little above normal. IMO, it's not a problem.

What's the problem that has you asking about this?

#3
07-29-16, 04:11 PM
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No real "problem." The return is going to be removed and disposed of as part of asbestos abatement. Since I will need a new duct, I wanted to make sure that replacing it with the same size would be ideal.

The supply duct is much newer than the return, and I wasn't sure if the sizing difference was done intentionally or because a previous homeowner just didn't want to pony up the cash to change the old return.

Thanks!

#4
08-08-16, 01:40 PM
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Okay, a follow-up, since I'm trying to think about ways to gain a bit of space.

Would 8" round be acceptable duct as a return? By my math the 3.5"x12" would have a total area of 42 sq. inches and the 8" round would give about 50 sq. inches, or around 119% of the supply area.

#5
08-08-16, 01:47 PM
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That sounds fine to me as long as we're talking rigid duct, not flex.

#6
08-08-16, 04:02 PM
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Yup, rigid. I have enough problems with the flex that's already here, I don't want to put any more in.

Thanks!

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