Combustion Air Requirements for Gas Furnaces

Is there a quick and easy way to calculate how many cubic feet of combustion air a natural gas furnace in a room needs? This is assuming all the combustion air comes from that room. I know I’m not supposed to be out there making calculations like this. The reason I ask is every once in a while I run across a furnace in a room with out a source of combustion air (no flues or louvers in the doors). I need a rule of thumb to calculate this and help me decide If I need to write it up for a HVAC professional to assess and correct.

I’ve looked all over the Internet and I get conflicting information.

Thanks in advance.

Here’s what I got:
Formula for combustion air:Take the cubic footage of the enclosed room. (HxWxD).
Multiply by 20.
If the resulting number is greater than the INPUT btu of the unit, it is OK.

I don’t remember where I got it.:smiley:

There is more than one formula. For instance, outdoor air for combustion (according to CodeCheck) is:

Example: Appliance Btus = 120,000. If using outside air source only, opening (x) must be min 40 sq in (120,000 / 3,000).](

For a furnace located in an unconfined space, typically, 50 cubic feet of air is needed for each 1000 btu of total (add up all appliances) input.

For example with an 8 foot ceiling height:

50,000 btu = 18’ x 18’
75,000 btu = 20’ x 24’
100,000 btu = 25’ x 25’
125,000 btu = 26’ x 30’

And with “tight” construction outside air may still be required for the furnace to operate and vent properly.

Something I picked up with my recently needed furnace install. (I needed to bring in outside air.)[/font]

from Peoples gas
airforfurnace (Small).jpg

There are no formulas that cover all situations as our houses are now as loose as sieves to as tight as sealed plastic baggies. Most of the trade HVAC stuff is behind the times in this regard and are based on “good guesses” from when houses were less complicated. I have consulted and trained on this stuff many times including a furnace manufacturer having me in to speak to a class of 22 oil burner mechanics “in training” and their instructor .

There are tests using micromanometers or sensitive magnahelic gauges that should be performed to determine if certain levels of depressurization will occur in the furnace room/area during certain house events. Higher depressurization levels than determined “safe” for each type of appliance puts them at risk of combustion gas spillage into the home!!!

The best system now is the “sealed combustion”/“direct vent” / “balanced flue” in which the furnace/boiler + exhaust system are sealed and exhuast is power vented by positive pressure. The system has a dedicated air supply from the exterior to the burner.

Can’t go just by unconfined space. Especially if there are clothes dryers in the basement. Testing of the draft with a draft meter while all exhaust fans or clothes dryers are operating is a test , IMHO, that should be conducted to determine if sufficient combustion air is present.

Recommend draft testing by HVAC company.

I am seeing, more and more, induced draft cat 1 and cat 3 furnaces in small condos. Some have adequit are and some do not.

In any case, all of them (when the furnace and dryer are run (air fluff)) cause significant air leakage around the windows.

The question is about more than just combustion air. If there is that much of a pressure differential, the conventionally drafted water heater, in the same utility closet, always back drafts.

Many variables, here.

Hence, depressurization testing recommended and CO detectors be made mandatory.

CO detectors are mandatory in Illinois:

Keep out of an argument, just report that it is not there. !


I have to agree with you on this one…hard to believe, HUH!

If you have noticed that the mechanical area(s) in question has no designed combustion air openings, note that in your report. If/when you start trying to design or quantify the necessary amount (trying to be helpful to your Client) needed, you have “STEPPED IN IT”, “CROSSED THE LINE”, ETC.

We hear it all the time, stick to what you know, what you can prove, leave the guessing to others.

IMHO and Good Luck

What Linas said.

I note the problem and defer it to a licensed and insured HVAC contractor (this passing on the liability).

I have run across many city code inspectors who OK it, and the contractor feels that they don’t have to fix it (or, they get their own HVAC, the guy who installed the thing in the first place to OK it).

In such cases, I recommend that the client gets a signed, letterhead (with license number and insurance cert) from the codie or the HVAC contractor stating that it is OK. Some times, I go back (new construction) and re-check for backdrafting and get CO readings during normal usage. Many times, I get high CO readings. Then I have the client (and their lawyer) call the HVAC guy (the codies NEVER put in writing or sign anything!) and have him come out and fix it, on his dime.

BTW: The local gas utility (People’s Energy) puts on a 12 hour (2 day, 6 hour lab) course for HIs to teach them this stuff. We all get the VP of operations personal cell phone number and are urged to call him, anytime, if we see something that is funky. I have done this, many times, and the PE guy agreed with me. Three times (new construction) he has turned off the gas to the building (condos) until the combustion air problem is fixed. It’s nice to have that jind of backup.

This is a BUMP to this thread.

		 			Came across this combustion air calculator at State of Kentucky Dept  of Housing web site.…calculator.xls

If this does load go to and scroll down.

As they state "This is for estimating only. DHBC is not responsible for accuracy "

At another site that I posted this info at one person stated that they required a password after they downloaded it. I did not. So give it a try.%between%

(WAP) Weatherization Assistance Program (administered by DOE)

Guidelines we were required to calculate an inspect were:

Recycled Air (no ventilation) - 50 cubic foot per 1,000btu of appliance
Horizontal to Fresh - 1sqin per 2,000btu of appliance
Vertical to fresh - 1sqin per 4,000btu of appliance

Where were you 7 years ago when I was looking for this answer? :slight_smile:

  1. GO HERE;
  2. Click the link ONLINE BUILDING CODES (not the photo);
  3. Choose your state;
  4. Choose the Fuel Gas Code, 2015;
  5. Go to Section 304 (IFGC) Combustion, Ventilation, and Dilution Air;
  6. Scroll down to 304.5.1 Standard Method

So there’s all these great numbers and codes being thrown around. Can I use the same numbers at sea level and 10,000 ft above sea level? Do I need to derate the fuel gas? Do I need more combustible air at higher altitude and if so how much? Does one calculation fit all?

The answer is 42. everybody knows that