Water heater in return air plenum

Can anybody give me some good reasons not to install a water heater in a return plenum besides having electrical wiring that is probably not rated for the plenum. The air handler is up above. The airhandler is a heat pump unit.

James need more information as to location of system or better pic’s This is making no sense to me???

Above this water heater is the furnace. The water heater is located in the return air plenum.

James…ironically, I ran into this same situation a week or so ago and was hard pressed to define how, and even if, to write it up. I searched the IRC and local ordinances for specific reasons to not allow this installation but I couldn’t find anything. I know there is a rule somewhere against non-plenum rated materials in a R/A chase but I couldn’t find it. Even then, I don’t think there was anything in my situation that would have been prohibited. Mine looked very similar to your’s, AHU was above the WH.


I find that in condo units all the time.

Micheal, its in the Texas SOP. Or at least how I read it. Unfortunately I cant never get a strait answer for TREC on this item.

(v) Ducts, vents (including dryer vents) and flues. The inspector shall:
FONT=Times New Roman report as in need of repair deficiencies in installation, such as gas piping, sewer vents, electrical wiring or junction boxes in the plenum, returns or chases or improper sealing, where visible;


FONT=Times New Roman Heating systems. The inspector shall:

(9) report as in need of repair a return chase or plenum that are not free of improper and hazardous conditions, such as gas pipes, sewer vents, refrigerant piping or electrical wiring; and

Cant we just build every house the same. Just kidding.

By the way, how are you Micheal?


It appears to obstruct air flow. If system has, or ever gets A/C, it reduces efficiency by discharging heat into the air stream. Accumulation of dust in electrical equipmemt could create a fire hazard.

Jim King

James L, things are going well, thanks! Yes, you are correct, those are part of our SOP. I do, every now and then, write up exposed wiring, gas pipes, pvc used for ducting the freon lines, etc, things like that in the R/A chase that shouldn’t be there. In the case of the water heater though I’m not sure if anything needs to be mentioned. I’d like to read and better understand the official code, wherever that might be, that supports the specifics of our state SOP. Here’s the other thing…the local AHJ accepted it when the home was built so who am I to argue with them? :slight_smile:

Not to hi-jack this thread but this is interesting and was also present in my scenario. The homeowner had an energy audit about a year ago where the contractor sealed up many small, potential air leaks including around the AHU. The first photo shows the filter access door sealed up with mastic and the second photo is shot from below looking up into the filter (the water heater is just below the filter). Homeowner says, “oh yeah, I change the filter all the time”…sure…

Access.jpg filter.jpg


Micheal can you clarify on exposed wiring. Im little confused. The electric water heater has wiring entering the top of the unit.

I call out exposed non plenum rated NM/Romex, junction boxes, etc. The wiring in my case was protected BX cable so I wasn’t as concerned.

Micheal I agree with you the AHJ didnt say anything. Its like the freon lines in the plenum, see them all the time including newer construction. The SOP can be unclear. It doesnt say with BX cable or not. How about the wiring inside the water heater, this wiring if it overheats could introduce toxic fumes into the system. I wish the Texas SOP was more clear on these issues and others.

Thank you everyone.

How can you tell if the electrical lines are plenum rated? I would imagine it would say on the insulation. What does it look like?

James, I’ve not seen nor am I familiar with plenum rated NM so one of the resident Sparkies needs to chime in here on that. I’ve seen plenty of LV/Data cabling labeled “plenum rated” in commercial applications though. I suspect standard residential NM/Romex type wiring has to be in conduit to meet the plenum rating.