In yesterday’s article, I wrote that atmospheric combustion appliances should not be inside the conditioned space of a home. If you have a furnace or water heater that draws air from the room, uses it in the combustion process, and then exhausts it to the outside, you could have serious health and safety risks in your home.
One of the solutions I listed at the end of the article is to enclose the combustion appliances inside a sealed combustion closet. The diagram above (from the Georgia State Supplements and Amendments to the International Energy Conservation Code) shows how this works.
The closet is completely air-sealed to the house, as shown above. The door must have weatherstripping and a threshold to prevent air leakage between the closet and the house. All penetrations must be air-sealed as well.
The key is that you have to install air inlets so that the furnace or water heater gets the combustion air it needs when it’s running. Called high-low vents, one must terminate within a foot of the ceiling and the other within a foot of the floor.
The size of the inlets depends on the capacity of the furnace or water heater. There must be one square inch for each 4000 Btu/hour of input capacity. For example, a 100,000 Btu/hr furnace would need 25 square inches of air inlets. These requirements are from the International Mechanical Code (IMC).
Put your atmospheric combustion appliances in such a closet, and you’ll be much safer in your home than if you’re running those appliances in the conditioned space. Remember this basic rule:
Keep people air and combustion air separate.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.
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