Furnace source air requirements

I inspected a house that has a 32-year old 100K BTU furnace in a closet. There is a single 10x20 louvered opening above the door. I understand the need for an additional vent in the lower 12 inches of the door. My question is; Why do I need two openings? I’m thinking that one opening should provide enough air, but with only one opening there could be a CO build up, maybe? Or that not enough air would be supplied which could cause improper combustion which could lead to something bad happening? Just need a little more info as the “why” question.



I would check the manufacturers directions but you’re going to want to verify with the National Fuel Gas code, for your reference.
If I remember correctly the requirement is 1 square inch of free air per 2000 BTU’s. A 10" x 20" grill isn’t 200 sq inches of free air.

Also, is the opening into another room and is there anything else nearby that might require combustion air?

Hey Scott,

Thanks for the feedback! The opening is into another room that has a cathedral ceiling. There are no other combustion devices nearby.

I was going by the Code Check book which states “…100 sq. in. each & min 1 sq. in./kBtu if on same level, 2 sq. in. if on different levels.” In going by this I can understand my current situation doesn’t meet the requirements. I’m fine with that.

As it pertains to the current scenario, I can see why a second opening is needed to meet the need of total sq. in… My question is; what is the result of not enough sq. in.? Does it affect the combustion? Does it cause damage to the unit? Does it create a CO risk?

Many times when these units are in closets there are also other things stored in the closet. Did you check the ceiling and floor inside the closet on all sides for attic or crawlspace air?

2 different vents: Combustion air and Make -up air.

Hey Stephen,

Thanks for the response. The furnace is on the 2nd floor with a cathedral ceiling. I didn’t see any vents to allow outside air; only exhaust vents. What happens if I don’t have enough air vents? Will it damage the appliance or be a health hazard?


Hey David,

Thanks for your response. I think I’m slowly getting this figured out. Combustion air: that’s obvious. You can’t have a flame without combustion air. Make-up air: That’s to replace the air being sucked out in the draft. It keeps there from being a negative pressure resulting in a back draft. Is that correct?

On one online sight I found that the furnace closet cannot be in a bedroom. Is that right? I’m researching that right now.


The reason you need a vent opening at the top and bottom is to create air flow/circulation.

Yes, the make-up air prevents negative pressure.
As far as the bedroom goes, this is what I follow:
5. The appliance is installed in a room or space that opens only into a bedroom or bathroom, and such room or space is used for no other purpose and is provided with a solid weather-stripped door equipped with an approved self-closing device. All combustion air shall be taken directly from the outdoors in accordance with Section G2407.6.