Combustion air from attic

I recently found the combustion air being supplied from the attic space. I feel it is wrong due to stack effect, no fire break, IAQ, economics (losing the heat and AC into the attic). I did write it up and will stand behind it.What are your thoughts on this practice?

Combustion air being supplied from the attic space is allowed. But there should be two sources; at ceiling and floor.

Heat shield (fire break) is required at the exhaust (B-vent).

2 sources as stated prior and more important, does this attic have enough air source itself to provide combustion air. CFM for BTUs ?? Also top of inlet pipe should be at least 6" above insulation

There should be another source of a combustion air 12 inches from the floor. The opening in the attic should be at least 6 inches above the insulation and shouldn’t be screened.

Other than that, in my opinion, you should reconsider standing behind it.

CF per BTU. I meant

There was adequate attic space. There was no other source of combustion air. It is greater than 6 inches above the insulation and has no screen. This was a warranty insp. I did preface my comment in the report that this is within code allowance. I made recommendation for louver doors instead of solid.

I love that you can just cut a hole to the attic and expect air to be sucked into the house automatically. This is a stupid design for combustion air and I would flag that all day long.

Louvered doors will make for a loud noise in the home when the furnace is running.

Ooooooh. That must be why they aren’t allowed in bedrooms! :wink:

A one-opening method is allowed, but rarely used.

“G2407.6.2 (304.6.2) One-permanent-opening method. One
permanent opening, commencing within 12 inches (305 mm)
of the top of the enclosure, shall be provided. The appliance
shall have clearances of at least 1 inch (25 mm) from the sides
and back and 6 inches (152 mm) from the front of the appliance.
The opening shall directly communicate with the outdoors
or through a vertical or horizontal duct to the outdoors, or
spaces that freely communicate with the outdoors (see Figure
G2407.6.2) and shall have a minimum free area of 1 square
inch per 3,000 Btu/h (734 mm2/kW) of the total input rating of
all appliances located in the enclosure and not less than the sum
of the areas of all vent connectors in the space.”

“Code commentary: Research has shown that for modern appliances, a
single opening to the outdoors will perform as well as
the traditional two-opening method. The one-opening
method described in this section depends on a reduced
pressure being created in the enclosure by the
draft created by the venting system. This reduced
pressure causes combustion air to enter the enclosure
through the single opening. The opening must be
properly sized considering both sizing criteria: the
square-inch-area-per-Btu/h ratio and the area minimum
based on the sum of the areas of all vent connectors
in the enclosure. This method allows for fewer
openings, fewer ducts and fewer objections by the

Why is that a stupid design? The room is physically connected to the attic air. Please explain using science why this would not work.

IMO, when you cite something as a defect that is allowed by model codes, you are making yourself look ignorant about the subject.

So what tells the air it has to be used for combustion purposes? Is there smart air in the attic that knows this? What happens when the attic is under negative pressure and pulls air out of that room? This may be allowed by code or AHJ but the design is stupid. I am no scientist but I dont think smart or stupid air has been invented yet.

You are not scientist but as a CMI you should know that the difference in temp of the air is what makes the air move from attic to enclosed closet thus the pipe one foot from the ceiling and one foot from the floor. Get in the attic above these two pipes with the furnace closet door closed take a piece of paper and place it over the two pipes and come back and tell me if there is air movement:shock:

Sorry that question was probably not on your CMI test. OHHHH dumb A** me there is no CMI test

Where did you get your PHD from ?..WalMart ?

school of hard knocks I earned it:roll:

Arn’t you a Redneck for OK ?

Yes I am a Redneck with a capital R and its my time of the month OK

Hahaha !
I like you …Maybe a little.
Have a great day brother.

2012 IRC


What are the circumstances which allow for the use of only one combustion air intake from the outdoors? The section of the IRC after what I just quoted details how to install a single intake but it doesn’t detail the answer to when it could be used.

I have no clue about a single horizontal pipe for C air we don’t use them here and I have never seen one in use with gravity feed , I guess I should travel more