Need some HVAC help

Hey all you HVAC guru’s…can someone cite the IRC section about using walls as a plenum for return vents between the basement to the 2nd floor? Had a pre-drywall inspection today on a 2500 sq.ft. new construction and found that the builder is leaving the cavities in the walls open for the return vents from the second floor. I know there have been issues/concerns about mold:shock: …not to mention the lack of any fire boundary in these spaces, but my client has the walkthrough with the builder tomorrow and I’d like to give him some additional ammo for when he asks the builder to do it, as I am sure it up to code for the area, but I feel it is in my clients best interests to have this done the right way. Thanks for any help…</IMG>

This help you James?


**602.1 General.
**Supply, return, exhaust, relief and ventilation
air plenums shall be limited to uninhabited crawl spaces, areas
above a ceiling or below the floor, attic spaces and mechanical
equipment rooms. Plenums shall be limited to one fire area.
Fuel-fired appliances shall not be installed within a plenum.

**602.2 Construction.
**Plenum enclosures shall be constructed
of materials permitted for the type of construction classification
of the building.
The use of gypsum boards to form plenums shall be limited
to systems where the air temperatures do not exceed 125ºF
(52ºC) and the building and mechanical system design conditions
are such that the gypsum board surface temperature will
be maintained above the airstream dew-point temperature. Air
plenums formed by gypsum boards shall not be incorporated in
air-handling systems utilizing evaporative coolers.

Thanks Dale. Now in sec. 602.2, it mentions “materials permitted for the type of construction classification”…This is a residential and the plenums are the raw, untreated, stud quality 2x’s on 2 sides and the gypsum on the others. They are in the interior walls, which I do not foresee hitting 125 deg., but condensation can form at much lower temps. I guess this would be where the dew point part of it would come into play though…correct? Also, if I am reading correctly, in .1 it mentions to be limited to 1 fire area…If it were running in openings from the basement, through the first floor and into the 2nd floor, wouldn’t this be in violation of breaking fire boundaries? Please correct me if I am reading wrong, as I want to make sure I am fully understanding it.

Here is the IRC section on the issue from Part V - Mechanical …

The biggest issue is the floor-to-floor wall plenum due to the spread of fire (Par. 7.3), and condensation on the outside can be an issue if the plenum walls are in any unconditioned spaces (Par. 5)

Thanks Robert. I advised my client to use the M60101.1.1 7.3 quote as well as the 602.1, concerning only 1 fire area.

Thanks to both of you for the help.

I would stay away from quoting specific model code sections, and just use things like the IRC (Dale’s quote appears to be from another code) as a general guide for providing general recommendations. It may not be the locally adopted code, and could put you on a slippery slope (e.g. does everything else comply with the IRC even though it may not be adopted or the standard used for code inspections).

Any questions about code compliance should be directed to the local building department, and ya may have a fight on your hands if it does indeed comply with the legally adopted building code and the issue gets pushed.

JMO & 2-nickels … :wink:

I did advise my client that there is the chance that it does meet the local code, but I could not say for sure. I also advised that I tend to stick to the national codes, as these stick as the minimums with some additional adjustments by local municipalaties normally, but also said that I use my own best judgement concerning safety. Not sure if I made it sound like I advised the client to fight tooth and nail for it to be done, as I did not, but I advised him to bring it up to the builder and have the additional information on-hand in case the builder wanted to know where the information came from. I also advised that it would hurt to ask for it (not demand, as I know what kind of trouble this could lead to) and see how the builder reacted to it and then take it from there once you get a feel for what he may/may not want to do…but I did write it up as a fire hazard 1st and foremost. Thanks again.