Thread: Exposed wood joist in return air

Just below the return air box in the crawl is the air handler. The HVAC guy used the space between the joists as part of his ductwork. It is my understanding that joist can not be inside the return air system at all unless they are flashed (wrapped) in metal.

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You HVAC Guru's need to help me out on this. I thinks its common sense, however the Mechanical Contractor said he's done it this way for 12 years and has never been called on it. Is there a reference that can help me out. This is Brand New Construction

2006 UMC 602.1 & 2006 IRC 1601.

John

2006 IRC M1601.1.7 Stud wall cavities and the spaces between solid floor joist to be used as air plenums shall comply with the following conditions:
7.1 The cavities or spaces shall not be used as a plenum for supply air.
7.2 These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
7.3 Stud wall cavities shall not convey air from more than one floor level.
7.4 Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fire blocking in accordance with Section R602.8.

It does not say anything about return air, only supply air. This would lead me to believe that it would be acceptable.

Jeff Euriech
Peoria, Arizona

I know in certain states it is against code to use any type of building cavity as a return duct.
Some states say nothing about it & allow it like in KY.

We personally do not do it & hard duct all returns with dampers. I know how this type of installation typically performs.

IMHO using any type of building cavity for air movement is a bad idea for many reasons.

Originally Posted by John Tullous

You HVAC Guru's

Lol.....You don't know us very well do you?

John
Please post your bio on the user area as this will help answer the question based on region and area. I live in New York and the return air in stud spaces have been used as far back as I can remember. The point about penetrating a fire wall does come into play and it is not allowed. David does make a good point as a ducted return air system is a better system as it can be balanced easier. It's just used in this area very much.

Originally Posted by Brian E Kelly

John
. I live in New York and the return air in stud spaces have been used as far back as I can remember. .

Same in Houston, Tx. I have seen some stud space returns in some $400,000 old homes in the
Memorial area in Houston it's real common here.

What's bad about using those stud spaces as ducts is nearly 99.9% of them I have tested are communicating directly with either the outdoors or some area of the unconditioned space.

The only place I want air pulling through is the grille inside the building envelope.

Originally Posted by DavidR

What's bad about using those stud spaces as ducts is nearly 99.9% of them I have tested are communicating directly with either the outdoors or some area of the unconditioned space.

The only place I want air pulling through is the grille inside the building envelope.

And those in the photo are floor joists over an unconditioned crawlspace.

Let's see ... ummmm ... the a/c is trying to remove moisture from the air, soooo ... I KNOW! ... How about if we suck some moisture laden air from the crawlspace into the return air?

I personally think the cigarette butt ads a unique character to that return myself.

John here using the joists as a return is common practice if it is inside the "envelope" (conditioned space). If the return lies outside of the envelope insulated rigid or flex ducting is mandatory. Most prefer rigid to flex since flex creates too much resistance and decreases airflow. The contractor who did this probably relies on the "beer can cold" method to charge an AC. He learned it that way years ago and it was OK back then, but times and methods have since changed.

sometimes we dont have a choice but to "pan" the return. this one definatly needs the caulking redone and a good cleaning. we do it in maybe 50 out of three hundred new houses a year. believe it or not its more time consuming for my duct men to pan than run hard duct. my guys would also have ductlined the metal. also its not mandatory here in va to run duct work in the crawl we can pan it the whole way if we wanted but we avoid it too

Originally Posted by John S.

The contractor who did this probably relies on the "beer can cold" method to charge an AC. He learned it that way years ago and it was OK back then, but times and methods have since changed.

Or maybe the 'if there is enough air flow to put a cigarette out, its good enough'.

While this practice is widespread, and conserves floor space, it doesn't perform well. During construction, the drywall is rarely caulked to these studs or joists. Since most lumber is S-dry or green nowadays, there is considerable shrinkage. The resulting gaps cause shifts in the air balancing of the ducts. What worked at settlement may develop problems down the road. I come in investigating soot claims (Black Particulate Matter) and find air shunting under wall plates, esp. near returns. The BPM gets filtered by the carpeting along the baseboard on those walls. We also see BPM stains telegraphing the 4x8 plywood subfloors when there is panning underneath of light colored carpets.

I doubt anyone would argue panning performs better than properly sized and installed hard ducting but it just isn't economical for the whole house with the price of each square foot so high. It would take a colossal chase to house one hard duct for the entire home and that just ain't happening. Besides, the code wants a return is each room (except kitchens and baths) properly sized, sealed, and balanced in most cases in lieu of one central return.

Maybe some of our HVAC friends could comment on the effects on static pressures, Delta T, etc. on panning versus hard duct returns?
TIA,
Bob

Originally Posted by Bob Harper

I doubt anyone would argue panning performs better than properly sized and installed hard ducting but it just isn't economical for the whole house with the price of each square foot so high. It would take a colossal chase to house one hard duct for the entire home and that just ain't happening. Besides, the code wants a return is each room (except kitchens and baths) properly sized, sealed, and balanced in most cases in lieu of one central return.

Maybe some of our HVAC friends could comment on the effects on static pressures, Delta T, etc. on panning versus hard duct returns?
TIA,
Bob

We do the hard duct in the chase but we charge out the wazoo for our work as well.
I have had very little complaints from homeowners giving up the space once the benefits & drawbacks for each type of install are explained.

Are there times we have to put returns in wall cavities?
Sure there are some homes that will beat you & leave you with no alternative choice.
Can I make this type of installation airtight?
Yes I can but it is very tedious.

If a panned return system is used & sized properly the effects on static pressure are the same as they are on a ducted return.
How many guys check static pressure anyway?
One of the big issues is pressure imbalances due to the panning being able to not be dampered unless other methods are utilized.

Most panning is not insulated & will murder the delta T if installed in an unconditioned space, with a ducted return it can be insulated properly to slow down the transfer of heat versus bare metal.