Thread: Combustion air in small room with open ceiling

This furnace is a 100,000 BTU unit. The room size is only 7.5' x 6' but there is no ceiling, it is open to the floor trusses above.

Would this room be sufficient for proper combustion air or should louvers be installed?

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I run into that situation a lot, and my take is that there might be enough air, but since I don't KNOW that there is, I recommend that a qualified HVAC contractor check it out.

Originally Posted by Jon mackay

This furnace is a 100,000 BTU unit. The room size is only 7.5' x 6' but there is no ceiling, it is open to the floor trusses above.

Would this room be sufficient for proper combustion air or should louvers be installed?

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Yes for combustion air as it is considered outside the conditioned space and attic ventilation should be present.

However the walls need to have sheet rock installed and the door needs to be sealed to prevent air infiltration from the conditioned space.
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Originally Posted by Billy Stephens

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... the walls need to have sheet rock installed.
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Yes, I meant to mention that, too. The paper facing on the insulation says right on it that it shouldn't be left exposed.

OP said floor trusses, so no attic above it. It can be okay if there is some exchange to the outside at the rim board, or if there is enough air to qualify as non confined space. If not, then it should have an external air vent. You can use the joist space to bring the combustion air into the room.

Based on 2009 IRC G2407:

Given that the furnace is installed in a separate room. That room air must communicate through a door opening that does not have a door (2407.5) or through combustion air openings per 2407.5.3.

So, in my opinion, and the way I have settled this situation in the past, is to install a 100 square inch ( 100Mbtuh x 1 sq. in.) vent within 12 inches of the floor and the truss space is adequate for the high opening.

This is the code solution that settles the confined space problem without involving outside parties or other expensive solutions.

Any grilles used for combustion air are to be sized by their net open area and not the nominal size.

Yes, the vent will be bigger than 10 x 10. The vent needs 100 square inches of net opening in this case. Thanks.