Combustion Air

I see this everyday. Natural gas furnace in garage with no outside combustion air source. Is a combustion air source required in a garage that is not sealed shut? Will the small opening around a main garage door provide enough combustion air for the furnace? Like I said I see this every single day. I would like to know once and for all is it okay or do not need to note that a combustion air source is not present. Thanks in advance.

I would recommend a qualified HVAC Tech. evaluate the combustion air requirements for the unit. Even if there were enough gaps around the doors to provide the required combustion air. If the homeowner were to seal them to keep the bugs out heat in or for some other reason, the unit could start producing co2.

First off, it’s not living space so risk of issues is minimized.
Secondly you can use an open space for combustion air relative to the number of BTU per cubic feet of space, and most garages have a relatively large amount of cubic feet of space. (50 cu.ft / 1000 BTU/hr)
Third, a furnace uses a relatively small amount of combustion air, and I can’t imagine a vehicle garage door that would be so tight as to not allow that much air to pass.

Ralph’s post: Ditto.

Also new furnaces are all induced draft, they will win the draft game in most cases with an open garage. You can’t seal a garage door that good or it will not open.

Local and National codes require C/A design for this situation.
In my area, local codes meet/exceed the National, with specific design criteria. They basically account for TODAYS design of equipment and structual energy envelop design.
In our area, a typical two (2) bay garage with a gas-fired furnace/water heater requires introduction of exterior C/A ventilation, and if a second gas-fired furnace or water heater is added, additional sq inches of lovered ventilation is calculated into the installation of this fixed (high/low) louverd design.
I agree with David about induced-draft design capabilities in theory, but conditions vary so greatly with construction quality, design, interpretation of conditions existing, that design for external C/A is or should be standard procedure throughout.

We must view the situation in accordance with our local standards. Some localities here require makeup air in garages and others fall on the BTU capacity/free air space of the utility room/garage ratios.

Personally, because I have the equipment from a past life I do test the effects of inadequate makeup air. This is outside of the SOP. I generally address the combustion air issue when I can specifically document the deficiency through testing.

I hate to get involved in deciphering the building code dates and interpretations, so I avoid it as much as possible. We don’t really have a mechanical code in my area. They said have one but it’s just a combination of electrical/plumbing to formulate mechanical. It is nothing like the commercial mechanical codes I am familiar with.

I am just addressing points of perspective to be considered a home inspector who must make a decision in light of regional practices. It is always safe to report on the fact that there is or is not a source of outdoor combustion air. Remember, system performance evaluation is not in our job description.

Is this fan powered or passive?


No powered/mechanical ventilation is required normally, just a fixed ventilation louver allowing ambient movement of air.
My explanation above is just that, and explanation of basic/typical design for ensuring adequate C/A for the safe and reliable operation of gas-fired mechanical equipment.
I am a Home Inspector now, practicing as suggested by industry SOP, etc. I would always yield to the industry experts of today, and I ALWAYS recommend further evaluation/inspection to my Client.
Sorry if I wasn’t clear in my previous reply.

I don’t call this out. The garage itself and the opening should supply all the necessary air. If there are vents provided and they are obstructed, then I will mention that. An inspection of the unit should determine if insufficient combustion air is present.

Let me ask this, do you believe the unit will draw air through the garage/house entry door before the garage bay door? Basically, the garage floor space and bay doors work. In my opinion of course.

I appreciate the responses guys. I will need to find the local code out. I did not even think that modern systems may not require C/A in the garage space. Thanks again!

This style of required ventilation can fail and suck air out of the furnace room when the direction of strong winds put the vent hood in the lee of the wind where negative pressures develop.

The Wood Energy Technology Transfer org in Canada has a homeowner video of wood smoke actually being drawn out of a directly connected c/a duct/supply hood at the main floor joist level (wood appliance was on the main floor ) while combustion air was being drawn down the chimney and the fire burnt down through the wood!!** Not as designed!!!**

Section M1702 of 2003 IRC has some of the info you are looking for. There ar a lot of R309.2 violations around here with unducted vents. When are you going to buy lunch, Sonny’s works.:smiley:

I’d enjoy viewing that. Do you have a link?

Haven’t found a link yet but here’s some info on outdoor air supplies from a Canadian guy I call Mr. Wood Heat, John Gulland. He was one of my trainers in 1988 when I received my Master System Advisor certificate.


That’s CO not CO2

Your right! My bad

A red? Really? Someone simply responded with “wronge” to my post. and yes they spelled it incorrectly. What part of opinion didn’t someone understand? Call it out if you want, but until proven otherwise, opinions are not wrong.

Doesn’t two wrongs = a right?! :wink:

Just shut that green square thing off.

Actually that’s right on David. I did shut id down for a while. Now I know why. I’ll live

The 50cu.ft/1000 btu/hr would the the first step and if this is not enough then the differance needs to be coming from exterier.
when I figure sq " lets say from a closed mechanical room door. I can actually measure 1"x 30" under door 30 sq. BUT if you cant even do that on a closed door forget it. And we cant assume just because we see a tinyy air gap, here and there it cant even be figured into the calculation.
Ask your self when you see the tiny air gaps , how many sq." is that actually. with cumbustion air we can not just assume BUT must have calculations to verify it is or not enough.