As I cut into a wall today to enlarge a doorway by 3 feet, I ran into something fun.
I cut the drywall back and noticed that the return air from the bedroom upstairs, was flowing through the wall cavity instead of ductwork. The HVAC guys had actually used ONLY the cavity. I made a call to my father and learned that by code standards, they can get away with that. What kind of crap is that?… We wonder why we are finding mold issues everywhere. I just find it funny how the inspectors around here enforce certain details while they let crap like that fly. I thought the reason for code book was to promote quality? Do we think this should be changed?
Rob Arnold
Replies
This is done all the time, and I don't know of any real compelling reason not to. SOP.
It's very common and an accepted industry practice for moderately priced jobs, even though it's not the best way to do it.
Cold air return is just that,.cold air returnig from the heated areas to be reheated again and sent out as supply air.
i
no metal duct is required to bring return air to the unit. You can use a metal trunk duct if you prefer, but Cavitys, joist space, chases, you name it do just as well. Its been done that way since the beginning of gravity type/ forced air furnaces,...approx 80 years or more ago.
They do it here as well.
Who Dares Wins.
My understanding is that as of last May (2004) the Uniform Building Code was changed to now require all return air to be via duct rather than stud or joist cavities. Per my building inspector, this is designed to prevent 'fire paths' and to reduce the growing and spreading of molds.
My permit got in just under the wire. I'm using ducts up to a point that then becomes impractical to try to run ductwork. To reduce the risk of mold formation, I've used two 18" UV lites in my cold air return.
Also, because much of my return is in ducts, there is the problem of sound transmission. To reduce this, I've used Class A fire rated accoustical ceiling tiles to line the portion of my cold air return not in duct. This seems to work fairly well.
BruceM
Per my building inspector, this is designed to prevent 'fire paths' and to reduce the growing and spreading of molds.
Id like for your inspector to explain why its an added fire path and explain or prove spreading of mold. Its a return air , not a supply. Its room conditioned air that you are breathing right this minute. We all are and the cavity is just an exention of our room air.
Tim Mooney
It's a (potential) fire path because the running fan draws air (and possibly flames) into the wall, where it can ignite the framing.Re mold, I don't really see how that could be a problem so long as the cavities used are internal (and using an exterior wall cavity would obviously displace insulation and be a no-no). No condensation is going to occur unless the surface of the cavity is somewhere much colder than the incoming air, and that shouldn't happen. In fact, using cavities for return air will provide a modicum of air circulation in the rest of the wall and help to remove any moisture that might somehow get into the wall.
First off I am not talking about studded returns where its raw framing in question.
No, Im talking about a drywalled plenum thats taped. Its a boxed affair that removes chance that out side air penetrates. It is also has a fire rating which a metal plenum does not have any such rating.
Tim Mooney
In requiring ducting to reduce mold, is the idea that with the duct work moist air would not find a home (backside of drywall, exposed studs, etc.) to deposit the moisture, and thus all air would flow back through the system to be conditioned?
Has anyone ever seen, or heard of, mold being found in un-ducted cold air returns?
Thanks,
Derek
Edited 12/19/2004 5:19 pm ET by drkru
I.m not a big fan of using framing [edit] cavities for any part of the air handling system because wood and drywall paper are two very mold-friendly products.Is it likely? Probably not.By why make your ducts out of a good mold growth medium?(Although I'd rather have that then ducts under a slab in contact with the soil.)I don't know about yours, but my church isn't a hotel for the holy, it's a hospital for sinners
Sojourners: Christians for Justice and Peace
Edited 12/20/2004 6:03 pm ET by Bob Walker
The reason not to use framing cavities as return ducts is that in doing so, you depressurize the cavity. In all likelyhood, that cavity connects with the outdoors at a rim joist or the attic. The result is that you're sucking in outside air, which generally should be done in a more controlled manner. Return air should run in sealed ducts intended for the purpose, as should supply air.
AndyAndy Engel
Senior editor, Fine Woodworking magazine
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Codes have nothing to do with quality and is not ever mentioned by an official.
As long as the return is drywalled and taped , its legal and why shouldnt it be?
Tim Mooney