All fuel-burning appliances require oxygen to burn. They require sufficient oxygen to burn cleanly and safely, without soot and CO (carbon monoxide). I see many open combustion furnaces jammed into tight closets and mechanical rooms with little thought or planning regarding combustion air.

I live and work in Florida, where most of our fuel-burning appliances are 80% efficient with open combustion. (You can learn more about the basics of combustion on a podcast with Benoit Mongeau.) The systems utilize air and oxygen from the space for combustion.

With these low-efficiency appliances, whether the appliance is forced air or natural draft, combustion air leaves the space and exits the flue. Look at the closet below. The furnace pulls room air in through its burner compartment, the water heater pulls it in at the bottom, and every bit of it goes up the flue and out of the house. That air has to be replaced from somewhere.

This phenomenon causes negative pressure that must be allowed to equalize, and it consumes oxygen from the space. So, these open-combustion appliances must be in a sufficiently large space. Alternatively, they may need to communicate with (be open to) a larger space or the outdoors.

When you consider that other gas appliances also need to use oxygen and need to vent outside, you can see that negative drafts can occur on natural draft appliances like water heaters that lack sufficient communication to the outdoors. Shut the door on that same closet, and the first thing to give up is usually the water heater. Its draft hood is an open relief, so once the room goes negative enough, flue gas spills out of the hood and into the closet instead of going up the vent.

A shut door isn’t the only thing that can pull that space negative. Anything that pulls air out of the combustion air zone will do it. Return leakage is a big one. So is setting a furnace on a typical stand in a closet like this and letting it free-draw its return air right out of the closet. Every time the blower runs, it pulls the space down. That can cause backdrafting of the furnace or the water heater, and if there’s any combustion spillage at all, it lets carbon monoxide into the space. 

That's why all open combustion appliances that utilize combustion air from inside the space must have enough room around them, or they must be connected to a bigger space or the outdoors using an approved method. The older editions of the code called a big enough room an “unconfined space,” and you still hear that term in the field.

The Code

The NFPA has three standards that come into play with combustion heating appliances: NFPA 54 (the National Fuel Gas Code, which covers natural gas and LP appliance installations), NFPA 58 (LP gas storage and handling), and NFPA 31 (oil-burning equipment). The International Fuel Gas Code says the same things as NFPA 54 on combustion air, so the numbers below are the same.

The basic rule is that the space needs at least 50 cubic feet of volume for every 1,000 BTU/h of input. That means that a 100,000 BTU/h furnace must be in a 5,000 cubic ft space. The part people miss is that it's the total input of every appliance in the space, not just the furnace. Put a 40,000 BTU/h water heater in the same closet, and you're sizing for 140,000 BTU/h, which is 7,000 cubic feet. A typical furnace closet is about 100 cubic feet. Not even close.

If the space isn't big enough, then additional combustion air must be made available to it. We usually do this with two permanent openings, one starting within 12 inches of the ceiling and the other starting within 12 inches of the floor.

If the air is coming from the rest of the house, each opening needs at least 1 square inch per 1,000 BTU/h of total input, and never less than 100 square inches. For our 140,000 BTU/h closet, that's 140 square inches each. If the openings go straight outdoors, or to the outdoors through vertical ducts, each one only needs 1 square inch per 4,000 BTU/h, which is 35 square inches each for the same closet. Horizontal ducts need 1 square inch per 2,000 BTU/h each. These are net free area numbers, so a louver or a grille that covers the opening counts against you.

While these openings are needed to allow for proper combustion and venting in many cases, it helps illustrate why modern sealed combustion “direct vent” appliances that take all of their combustion air from outdoors make so much sense. Two pipes, both going outside, and the closet doesn't matter anymore. Just remember that if there's still an atmospheric water heater in that closet, it still needs its air.

Not only are direct vent appliances more efficient on the fuel utilization side, but they also prevent the negative home pressures or thermal losses associated with having vents in walls and ceilings.

So, either make sure you have enough space, bring air in from a bigger space or the outdoors, or have a direct-vent appliance. And whatever you do, check the code your area has adopted. Local amendments have the final say.

—Bryan

Illustrations by JD Kelly