Enough combustion air?

I’ve looked through the forum and despite the plethora of information I’m still a bit fuzzy. I have a gas water heater in a laundry room that is 9"x9"x8" giving me 648 cu ft. The water heater is a '01 A.O. Smith running at 32,000 BTUs. At 50 cu ft/ 1k BTUs I’m figuring I need 1,600 cu ft. There is one floor register feeding air in when the HVAC is on. There is an exterior door and an interior door to the laundry room. I figure that with either of the doors open then there’s enough combustion air, but if both doors are closed then there’s not, is this correct? I figure that if vents are added to the interior door then everything

will be ok. Thoughts anyone?

Thanks for the knowledge and expertise!

Did you test for backdrafting while it was running? Report what you see.

And, Anthony, report what you see and let the referred qualified professional determine the method of remedy. What happens if your vents added to the door don’t work?

Happy inspecting! :smile:

Hey Larry,

I did check for backdrafting, but it occurred to me later that the interior door to the room was open which would have allowed for proper draft. I’ll write it up as a potential problem with the recommendation it be evaluated by a professional.


Good idea, Anthony! :smile:

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… for future reference:

  1. Your local Gas Codes will require meeting their combustion and relief air openings as well. For example, the local Gas Codes where I am require the following for installations with less than 400,000 Btuh Input total:

" When an appliance is located in a confined space and the air supply is from the outdoors, the space shall be supplied with two permanent openings that communicate directly to the outdoors by means of openings or ducts. The openings or ducts shall conform to:

(i) Lower Opening - 1 sq. inch per 2000 btuh Total Input rating of the appliances in the space when the means of communicating air to the outdoors is by means of a horizontal duct, or

(ii) Lower Opening - 1 sq. in. per 4000 Btuh of the total input rating of the appliances in the space when communication to the outdoors is directly by an opening, or by means of a vertical duct, and

(iii) the upper opening shall be located as near the ceiling as is practical, but not lower than any relief opening of a Draft Hood or Draft Regulator, and shall have a free area of not less than the total area of all vents and chimneys from the appliances contained therein…"

Basically you have to have a low level combustion air inlet to the room as well as a high level relief vent. The Code requirements are somewhat based on the basic combustion formulas, but with saftey factors. Check your local Codes for what they require first."

Try reading this:

Yep. you do the math with the door closed. If there is an exhaust fan, induced draft appliance or clothes dryer in the room, the condition is exacerbated. Report the deficiency, let someone else design the solution.

Kudos for knowing the math and taking the time to do it.

Here’s how I used to document it. Feel free to use or adapt as suits you. I cite MODEL codes as an authoritative source. If you don’t want to or are not permitted to you can strip them out. The manufacturer’s installation standards will say the same thing. The last paragraph is specific to laundry areas.

Inadequate provision for combustion air for gas fired unit installed in confined space as required by current industry standard.

Room / enclosure is too small to provide combustion air by the standard method (Ref: IRC Section G2407.5.1 Standard method) The minimum required volume shall be 50 cubic feet per 1,000 Btu/h .

No adequate alternative combustion air provision has been provided (Ref: IRC Section G2407 Combustion, Ventilation and dilution air; G2407.1 General) Air for combustion, ventilation and dilution of flue gases for appliances installed in buildings shall be provided by application of one of the methods prescribed in Sections G2407.5 through G2407.9. Where the requirements of Section G2407.5 are not met, outdoor air shall be introduced in accordance with one of the methods prescribed in Sections G2407.6 through G2407.9. Direct-vent appliances, gas appliances of other than natural draft design and vented gas appliances other than Category I shall be provided with combustion, ventilation and dilution air in accordance with the appliance manufacturer’s instructions.

Laundry area with dryer hookup in utility closet with water heater. When the dryer is in use, it will significantly reduce available combustion air for the water heater and may cause back drafting. (Ref: IRC Section G2407.2 Appliance location) Appliances shall be located so as not to interfere with proper circulation of combustion, ventilation and dilution air.


Larry, Michael, and Chuck, Thanks for all the information. I’ll be looking closer at the local codes just so I know what’s expected. The water heater manual was very informative. I never thought about looking at that. Something I’ll be doing in the future. I also appreciate the verbiage for use in the report. Everyone in my area shies away from using code since we’re not code inspectors, but I can talk about “current standards” all day long.

Thanks again!

Chuck brings up an important point. The dryer is often overlooked, but can be a serious drag on draft. Case written up a couple years ago of a woman who did all her laundry on Wednesday. That’s the day she suffered from re-occurring headaches. That’s also the only day the back-drafting occurred from heavy dryer use.

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I encourage everyone to know and use the code just remember that a code violation doesn’t mean a defect. IMHO knowing the difference between a Home Inspection and a CODE inspection makes you a better Home Inspector.

Shut up! :sunglasses:


Unless you are a licensed plumbing contractor nobody here is authorized or has the jurisdiction to do the calculations for combustible air. I recommend if you have any questions to refer those calculations to the proper licensed contractor. As a holder of a gas license, plumbing license, licensed contractor you guys are just guessing.

I noticed there’s no seismic strapping, appliance connector, where does that TP valve discharge to? Does it need a seismic strap? Are we 100% we know the local codes? It’s all about the wheelhouse guys, stay in the swim lane.

From my perspective, no matter what the topic is around here it is all guesswork most of the time, so one more won’t matter. :rofl: :wink:

and how about a sediment trap… :wink:

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Ssshhh… don’t kill the deal, and stop guessing!

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Absolutely it may be in the basement or crawl space but yes it should be as close as possible to the fixture.

The seismic straps aren’t required here in Alabama. At least not yet. The TP pipe discharges to the exterior near the HVAC unit. If the TP exit can’t be observed at the water heater I always note where it exits, put it in the report, and throw in a blurb that if they ever see water dripping from the pipe to call a plumber.

I don’t put the calculations in my reports, just like I don’t quote the code in my reports, however, both are necessary to me in spotting possible deficiencies and alerting the client there may be a potential problem.

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You have no symptoms related to insufficient combustion air…
Did you calculate the air from the HVAC register and the cracks around two doors? I don’t think so.
What are you “Observing”? Nothing!
What is your qualification to calculate combustion air? None!
Does the buyer or REA know how to interpret this narrative? No.
What is the result? A big hassle and pissed off people with a negative opinion of Home Inspectors overall when a bunch of professional people (who install it this way every day) give their “Opinion” on the alleged issue (regardless who is right or wrong).
Do you know how to actually measure the source of available combustion air? I assure you the person they call in does not either! If you don’t know how to do it, how can you recommend remediation?
Why not just point out what you see, based upon things like back drafting, observing a smoke test of the draft being pulled from the register and around doors and leave opinion to everyone else?

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Hey David, so, since I didn’t see any evidence of backdrafting, and since I’m not a professional, I should say…? Nothing? If I have the knowledge to discern a “potential” problem why would I not share that with my client, especially in a case where there is potential for bodily harm? I understand not opening myself up to liability by only reporting what I see, but my clients hire me for my knowledge as well as my eyesight. Perhaps I’m not clearly understanding your post, in which case I’ll stand corrected.
I’m not qualified to calculate combustion air, that’s why I don’t report combustion air calculations.
My client and their REA may not know how to interpret my narrative, but that’s why I maintain a report with my client and explain my findings.
The result, a bunch of people who are aware that I have a vested interest in their investment and I’m not there just to make a buck.
No, I don’t have the skills or equipment to actually measure the available combustion air. That’s why I recommend a professional.
I don’t recommend remediation, again, I refer them to a professional.
Again, I may have misunderstood your post.

You understood it correctly. You will learn, if you pay attention, that he thinks an average home inspector is an utter moron and isn’t capable of applying any logic because they lack 60 years of field experience changing fans, capacitors, contactors, lubing things up, measuring the delta Ts, pumping more juice into the system, installing a line filter, brazing/solder the copper lines, spraying some soapy water to check for leaks, checking and setting the gas valve’s outlet pressure using a manometer, checking the combustion performance using a combustion analyzer, and the rest of the rocket science applied during HVAC installation and service :smiley:

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