# plenum box



## 7echo (Aug 24, 2008)

Having all new hvac installed during a small remodel. AC contractor needs a wooden plenum box fabricated in the hvac closet to set his gear on. What do you recommend for the plywood-ext. cdx, osb, pressure treated ply, or something else? The hvac guys will line the box with duct board for sound insulation.
Comments are appreciated.


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## 1610 CUB (Jul 19, 2008)

You should get a new HVAC contractor, insist on a insulated sheet metal plenum.


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## 7echo (Aug 24, 2008)

An insulated sheet metal plenum was discussed, but for the application it was determined that a wood box, sealed, lined with duct board, sealed, would be the best option. The air handler will be inside a small closet(approx 4 x 6) that is centrally located. One of the main concerns is noise from the unit as it is located adjacent to the den, dining area, and kitchen. The entire floor area of the closet will be boxed to become a 24" to 28" high plenum, with a return on each end of the box, opening into different rooms. The sheet metal box sitting on the slab, or even on 2x sleepers, would be louder than the plywood box, or at least that is the theory. If we could figure a way for the tin benders to do their thing and keep the system quiet, as well as allow access to move around 2 sides of the air handler without buckling the sheet metal, we would consider it.


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## SD515 (Aug 17, 2008)

I'd contact the local mechanical inspector. 1) Find out if a wooden platform/plenum is allowed for your type of install. 2) They might have some good ideas to answer your question. I'd bet they would have seen something similar to your situation before. Doesn't hurt to ask.


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## Termite (Apr 13, 2008)

I don't see any reason you couldn't use duct board (thermopan) as the plenum with a wood structure serving as support. In this application, I'd want the wood structure on the outside of the plenum, not in it. 

People need to bear in mind that many HVAC runs in homes are "ducted" through wood framing. There's nothing illegal or unusual about it. 

I'd vote for 2x dimension lumber and perhaps some plywood to keep it sturdy. If it is sitting on concrete or will be contacting concrete, those parts would need to be treated. Otherwise I'd just use untreated lumber wherever possible.


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## 1610 CUB (Jul 19, 2008)

7echo said:


> An insulated sheet metal plenum was discussed, but for the application it was determined that a wood box, sealed, lined with duct board, sealed, would be the best option. The air handler will be inside a small closet(approx 4 x 6) that is centrally located. One of the main concerns is noise from the unit as it is located adjacent to the den, dining area, and kitchen. The entire floor area of the closet will be boxed to become a 24" to 28" high plenum, with a return on each end of the box, opening into different rooms. The sheet metal box sitting on the slab, or even on 2x sleepers, would be louder than the plywood box, or at least that is the theory. If we could figure a way for the tin benders to do their thing and keep the system quiet, as well as allow access to move around 2 sides of the air handler without buckling the sheet metal, we would consider it.


You said nothing of this in your first post! With that said, one thing, Air noise is going to occur when the runs are short, to small , too small grills. So IMO you are going to hear the furnace each and every time it comes on, the good news is you will get used to it. My two cents.


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