Thread: Combustion Air

I inspected a house today that is drawing combustion air into a utility closet from the attic through a six inch pipe. The closet has a water heater and boiler for radiant heat which are both rated for closet installation. The door has no vents. Is this OK?

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"rated for closet installation"?

John, by any chance are these direct vent heaters? They may be drawing combustion air directly from the exterior.

I'm unclear about this as well.

For example I know that for conventional (non direct-vent) appliances you need 1 sq/in net free per 2K BTU (Input) if air is provided from the outside via horizontal ducts.

But sites like this one

Combustion Air

seem to be stating that a gas appliance installed in a confined space requires the same connection to a interior unconfined space irrespective of whether it is also supplied with exterior air.

Hopefully someone thoroughly familiar with the NFGC will be along to set us straight.

It is acceptable to obtain combustion air from an attic which is well vented to the exterior. Your particular case is up in the air, with the information given.
Do you have a picture?
There should be both a "high" and "low" vent with the size based on the BTU rating of the appliance(s).

Hello. There are three ways to provide combustion air for gas appliances. 1) All air from outside. 2) All air from inside. 3) Combination of air from inside and outside.

Direct vent furnaces will use option #1, via a pipe from the appliance to an appropriate location outside. Older furnaces (and most gas fired water heaters) will generally use either #2 or #3. If the appliance is installed in an unenclosed space (see definition - deals with permanent openings to the exterior), generally you would need about 50 cubic feet of air per 1,000 BTU of gas appliance to qualify as an Unenclosed space. Usually this is not the case, however most basements even have at least one window that would qualify as a permanent opening to the exterior. The attic would also generally qualify as an area that has a permanent opening to the exterior. If the gas appliance is installed in a room adjacent to the space (attic or basement), and the adjacent space has permanent openings to the exterior, then you would generally need two openings provided for the room where the appliances are installed. In your case there is a duct (opening) into the attic. This would be an acceptable way to provide make up air for combustion, however you need a second opening (one within 12 inches of the floor and one within 12 inches of the ceiling). Each would need to be a minimum of 100 square inches. In your case you would need an opening that extends from the attic penetration down to within 12 inches of the floor.

This is my take on your situation, hope it helps. Please refer to Chapter 17 of the IRC for requirements.

Regards, Bob

Originally Posted by Robert Koch

generally you would need about 50 cubic feet of air per 1,000 BTU of gas appliance to qualify as an Unenclosed space. Usually this is not the case, however most basements even have at least one window that would qualify as a permanent opening to the exterior.

Robert - You're not suggesting a basement window can be considered a source of combustion air are you? It would have to be removed or permanently welded open or something like that.

Originally Posted by John Thompson

is drawing combustion air into a utility closet from the attic through a six inch pipe. ... Is this OK?

Absolutely not.

The minimum size of *each* combustion air vent is 100 square inches. A round pipe would need to be a 12 inch diameter pipe (a 10 inch diameter pipe would be too small and they don't make an 11 inch diameter pipe (or, if they do, those 11 inch diameter pipes are had to find).

And, they need two of those 12 inch pipes. One within 12 inches of the ceiling and one within 12 inches of the floor.

Jerry,

Does the two pipe >= 100sq/in requirement of exterior supplies still apply if you are making up a shortfall of interior combustion air, as in he example given below this calculator:

House of Craig - Online Combustion Air Calculator ?

I'm assuming this example is an instance of the general sort of situation depicted in the diagram of he third situation in the link in my post above:

http://www.blueflame.org/images/combustair4.gif
Combustion Air

but in all three examples there is only one exterior supply pipe - is this because it's only making up for interior shortfall, and the majority of combustion air is being supplied from the interior via an unconfined space or by compliant interior vents near the floor and ceiling?

Thanks

Jerry, where are you getting the minimum 100 Sq Inches figure from?
I have searched the IRC and find nothing except the formula of each opening being 1 square inch per 4,000 Btu on vertical attic ducts and the 3 inch minimum cross section on rectangular ducts.
Thanks, Jim

Just to make things more interesting:

http://commerce.wi.gov/SBdocs/SB-UDC...yWorksheet.pdf

and

Combustion Air Advisory For Fuel Burning Appliances

Combustion air from outside the dwelling

1) Direct opening to outside requires one square inch per 4000 BTU’s (no duct).
2) Opening via horizontal duct requires one square inch per 2000 BTU’s.
3) Opening via vertical duct requires one square inch per 4000 BTU’s.
Note: Two openings required – one within 12 inches of floor and one within 12 inches of the ceiling .
(If all appliances are gas, then only one opening required within 1 foot of ceiling & one square inch per 3000BTU’s but not less than the combined cross sectional areas of the appliance flue collars or draft hoods.