Plywood duct work

**Sam Morris** · 07-06-2011, 03:22 PM

House had a 1995 Armstrong heat pump with insulated flex and plywood trunk line, couldn't tell what type material was used inside. Is this unusual and is there any reason for concern, has anyone else seen this type of application?

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James Duffin · 07-06-2011, 03:51 PM

Here is what NC says about plenum material. As you can see in Exception 3 the flame spread requirements do not apply in dwellings so it seems that wood plenum's would be OK in NC.

602.2.1 Materials within plenums.

Except as required by
Sections 602.2.1.1 through 602.2.1.5, materials within plenums
shall be noncombustible or shall have a flame spread
index of not more than 25 and a smoke-developed index of
not more than 50 when tested in accordance with ASTM E
84.

Exceptions:

1. Rigid and flexible ducts and connectors shall conform
to Section 603.
2. Duct coverings, linings, tape and connectors shall
conform to Sections 603 and 604.
3. This section shall not apply to materials exposed
within plenums in dwellings.
4. This section shall not apply to smoke detectors.
5. Combustible materials enclosed in noncombustible
raceways or enclosures, approved gypsum board
assemblies or enclosed in materials listed and

labeled for such application.

James Duffin · 07-07-2011, 04:36 AM

Originally Posted by Frankyh

I'm new here in this forum and everything i read is really nice.
I learned a lot here.
Just want to say Thanks.

When reading the replies on here you should remember that there are lots of different opinions and everything is not black and white...there is a lot of gray areas where you will have to decide what is right for your situation. Plus what is right in one area may not be right in another area of the country.

**Bill Hetner** · 07-08-2011, 03:51 AM

Yes Sam that is very unusual and aside from writting it up as a fire hazard further investigation is needed as why and what is on the inside. proper ducting is cheaper and way safer. maybe they thought it would help keep the heat in but at what expense?? Thanks for sharin and ya it makes you wanna go hmmmmmmmm wut were they thinkin......

**timjimbob** · 07-08-2011, 06:00 AM

The obvious problem is lack of vapor barrier. This looks like a supply trunk and the temperature would be below the dewpoint when in AC mode resulting in sweating and mold growth.

I do not know the Mechanical Code violation, but it can be found.

Texas AC lic#21428C

**Glenn Duxbury** · 07-10-2011, 04:09 PM

Hi (ALL) &

Had the exact same thought as 'TJB' (above) - as to poss. sweating /mold-growth...

Would tear that out & return to normal ducting, myself.

CHEERS !

**Randy Haywood** · 07-11-2011, 08:54 AM

I was surprised to see the almost identical situation just a week ago in Washington State. The return air plenum was wood ! I was also curious about possible temperature differentials, mildew, dirt build up.... but also the wood was on the concrete floor of the basement and is not treated as I can tell.... It just did not seem to be a wise choice of materials..... I did not make an issue of it but the client was with me and we speculated a bit and moved on. Since I was not sure, my wording in the report stated "Non typical application / materials noted at return air duct work, further elavaluation as to proper application recommended. If anyone knows a code against the application, let me know. I do believe, that poor choices such as this is in part, what leads to new codes ! And It is our job to point out non-typical details.

Randy

**timjimbob** · 07-12-2011, 04:43 AM

Return air (located within conditioned space) is different then supply air. The temp difference for RA is zero and no mold will grow.

UBC 604.1. Details insulation.
UBC 601.4 Exception allows "combustible material" in house.

**Rod Butler** · 07-29-2011, 08:03 AM

Looks like a DIYer didn't know how to do sheet metal work.

It does look like he sealed the duct with copious amounts of great stuff and duct tape. The problem with the duct tape is that it (appears) is not UL rated. Regular duct tape, the cloth based type will degrade rapidly and is not permitted to be used as duct sealant.

The wood however is not a code violation but I agree with the comments above about the potential for condensation inside of the wood.

Darrel Hood · 07-31-2011, 06:17 AM

If an unlined wall chase is allowed to perform the function of a return air duct, why would the same materials be problematic in another section of the return air duct?

Darrel Hood
DILIGENT PROPERTY SERVICES

**Rod Butler** · 07-31-2011, 09:11 AM

Originally Posted by Darrel Hood

If an unlined wall chase is allowed to perform the function of a return air duct, why would the same materials be problematic in another section of the return air duct?

Darrel Hood
DILIGENT PROPERTY SERVICES

If the wooden duct were only on the return air it wouldn't be problematic, it's the lower supply air temps that might cause the condensation concern.