Combustion air may be required when appliances are installed in small enclosures. Combustion air is a calculated condition in such that it takes the appliance needs and is compared to the volume of the space it is located in. There are many variables in methods to accomplish this. The most common is two air openings (one within 12” of the ceiling and one within 12” of the floor) that communicate directly to the outside. Newer direct vent appliances utilize a piped system to deliver the combustion air to the appliance’s combustion chamber (sealed combustion).
Unvented gas heating appliances are prohibited.