All fuel-burning appliances require oxygen to burn. They require sufficient oxygen to burn clean and safely, without soot and CO (carbon monoxide).

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.

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.

That's why all open combustion appliances that utilize combustion air from inside the space must be in an “unconfined space” or connected to an unconfined space or the outdoors using an approved method.

I see many furnaces jammed into tight closets and mechanical rooms with little thought or planning regarding combustion air.

The Code

The NFPA contains three standards that apply to combustion heating appliances: 31, 54 & 58 (for oil-burning, natural gas, and LP equipment, respectively). According to the NFPA, an unconfined space is a space that has at least 50 cubic feet of open area for every 1,000 BTU of input. That means that a 100,000 BTU furnace must be in a 5,000 cubic ft space to be considered unconfined.

If the appliance is not unconfined, then additional combustion air must be made available to the space. We usually do this with one opening at the ceiling level and another one near the floor.

If the air is coming from another unconfined space, then the openings should be at least 1 square inch per 1,000 BTU and 1 square inch per 5,000 BTU if it is connected to the outdoors.

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.

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 an unconfined space, bring air in from an unconfined space or the outdoors, or have a direct vented appliance.

—Bryan