Combustion Air Availability?

Specific Inspection Topics Inspecting HVAC Systems
1

Would appreciate some opinions on an installation…

Background:
New construction, s.f. detached home, substantially complete

40,000 BTU gas-fired FVIR-type water heater & 88,000 BTU 80% efficient gas-fired furnace installed side-by-side in 110 cu. ft closet, 20" off garage floor.

Closet is located off-of and accessed-from a typically-sized 2 car garage, roll-up metal door, with no access or air communication from the closet to the home’s interior spaces beyond the firewall.

Closet is accessed by a pair of bi-fold type doors, solid type. When closed, bi-fold doors provide 23 sq. inches of visible gap at the door bottom, and 46 inches of clear, visible gap at the door top. Net openings are 69", with no other openings in the closet (either at the closet floor to the crawl, or at the closet ceiling to the attic spaces).

By definition, the closet is clearly a confined space. Questions:

1.) Is the garage also considered a confined space?
2.) The garage is clearly the only obvious source for combustion air availability (other than whatever air exists inside the 110 cu/ft utility closet). Do I consider the garage an “OUTSIDE” combustion air source?
3.) Even if the garage suffices as an “OUTSIDE” combustion air source, does the 69" of available openings at the utility closet door represent an unacceptable restriction of available combustion air, given the input capacity of the installed appliances?

Thanks in advance for your opinion(s)

2

Background:
New construction, s.f. detached home, substantially complete

40,000 BTU gas-fired FVIR-type water heater & 88,000 BTU 80% efficient gas-fired furnace installed side-by-side in 110 cu. ft closet, 20" off garage floor.
This gives you 128,000 Btu per hour combustion air requirement., Yes according the Peoples Energy of Chicago the room in which the appliances are located are what you consider the “room” in which calculations for confined spaces are made.

Closet is located off-of and accessed-from a typically-sized 2 car garage, roll-up metal door, with no access or air communication from the closet to the home’s interior spaces beyond the firewall.

Closet is accessed by a pair of bi-fold type doors, solid type. When closed, bi-fold doors provide 23 sq. inches of visible gap at the door bottom, and 46 inches of clear, visible gap at the door top. Net openings are 69", with no other openings in the closet (either at the closet floor to the crawl, or at the closet ceiling to the attic spaces).

Can you clarify this ? Are these completely solid bi-fold doors? Are they “cut” on the top and bottom to allow for “air” gaps? Remember you need 1 square inch for every 1000 Btu’s combustion air. (best practice is to have one high vent and one low vent with in 12 inches of top and bottom) The Garage is the adjacent room that will be supplying combustion air for that “closet”. Now comes the next factor : Is the garage "large enough " for supplying combustion air and can that air be exchanged (considering sealed room with little exchange of air)?
You can get an idea of "confined space by using calc. H’x W’ x L’ x 20 for confined space figure based on 132,000 Btu’s for cut off point minimum before it turns into confined space.

By definition, the closet is clearly a confined space. Questions:

1.) Is the garage also considered a confined space?
No, since closet is first consideration with respect of confined space. Yet the adjacent room is also a gray area since that “could also be to small” so a high and low vent duct from the exterior or attic is one answer for this. You can always check the MFG instructions for best practices with regard to any appliance install. That is the number 1 place to check!
2.) The garage is clearly the only obvious source for combustion air availability (other than whatever air exists inside the 110 cu/ft utility closet). Do I consider the garage an “OUTSIDE” combustion air source?
Adjacent room , Yes
3.) Even if the garage suffices as an “OUTSIDE” combustion air source, does the 69" of available openings at the utility closet door represent an unacceptable restriction of available combustion air, given the input capacity of the installed appliances?

One way you can look at this if you consider 1 sq inch for every 1000 Btu’s but another goofy thing is that you have a FVIR WH but the FAU can ignite any flammable vapors… So figure that one out?? $$$$$ is always before common sense… I also have a guide if you want it from my local utility regarding a lot of what you are asking here. Just drop me a note if you want a copy. Homeiguy@sbcglobal.net

Pat

Thanks in advance for your opinion(s)
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3

Thanks for the input, Pat. There’s no question that the crappy little closet doesn’t provide breathing air. The garage is roughly 5000 cubic feet, give or take a couple hundred c.f. The garage roll-up door is gasketed, but (of course) is not air-tight. I’m thinking the garage might be a rougly adequate source for combustion air, but yes, the bifolds on the closet are solid. They are over- and under-cut, for a (legitimate) total of 69 square inches available air space.

If one considers the garage air to be “outside” air, the combustion air requirement is 1 sq/in per 2,000BTU, in which case the closet over and under cuts (roughly located within 12" of the closet ceiling and floor, respectively) would need to provide 64 sq. inches, and this seems o.k. given that 69 sq. inches exists. If, however, the garage air is “inside” air, the only reasonable solution is the addition of floor and ceiling openings in the crappy little closet, or replacing the solid bi-folds with full louvered bi-folds.

The AHJ didn’t have a problem with the installation, and I’ve got to go on-record one way or the other. Think I’ll describe combustion air availability as marginal, and “recommend” installing floor and ceiling openings in the crappy little utility closet, or, a full louvered door set vice the solid bi-folds. Thanks again, Pat.

4

Hey Chris,

It sounds like this is “OK”… If the AHJ gives it the ok then go with it. Since the MFG. of the appliance is the first one to go to for installation instructions and after that is the AHJ for “tweaks” then you got your answer. I would be careful on any recommendations now about louvered doors or changes. I would just educate clients on safety but not on changing any configuration now.

The part if forgot to add to my last post was : A room the size of 8’H x 33’ L x 25’W is 6,600 Ft cubed. For 132,000 Btu’s it is considered large enough and has sufficient air volume for support is classified as an unconfined space. Anything larger than 132,000 for 6,600 cubic Ft is considered confined space.

Don’t you just wish we would go to High efficiency, 90 + condensing units with PVC piping to the exterior so we don’t have to “gauge” all this stuff… Would be so much easier! :wink:

5

I’m with you. Pipe it all in from outside. Kind of funny, my 18 year-old Rheem/Ruud 90+ furnace went belly-up in December. My heating guy installed a 93% Am. Std., and did a nice job, too, until the end, where I discovered he was going to let the furnace pull combustion air from the basement (which is plenty large, and may have been o.k.). I said no, guy, grab the hole saw, and, um…let me see, about 13’ of schedule 40, cause this bad boy’s going to breathe OUTSIDE air.

Thanks again for the opinion, Pat. I suggested in the report that the combustion air availability may be marginal, and that the client should double-check Carrier’s installation/operating manual for specific C.A. requirements.

Regards,