What type of air should not be mixed with the supply and return air in a confined space?

Combustion air refers specifically to the air that is necessary for the combustion process, which occurs in heating equipment or appliances. In confined spaces, mixing combustion air with supply and return air can lead to several safety and operational issues. Combustion air needs to be distinct because it can contain products of combustion which, if mixed with the supply and return air, can compromise air quality and pose health risks to occupants.

Ensuring that combustion air remains separate helps maintain proper ventilation and prevents the introduction of pollutants or carbon monoxide into the breathing air of the confined space. Proper separation of combustion air also aids in the efficient functioning of combustion systems, ensuring they receive sufficient oxygen for effective operation without negatively impacting the rest of the indoor air quality.