Key Terms

Flue: Part of the combustion venting system below the draft hood.

Vent: Part of the combustion venting system above the draft hood that carries diluted combustion products to the outside.

Primary air: Air mixed with fuel before combustion.

Secondary air: Air introduced into the combustion chamber after combustion has begun.

Combustion air: Air that is used to provide oxygen for combustion.

Roll-out: Flames and combustion products that exhaust from the appliance at the burner area.

Spillage: Combustion products that spill out of a draft diverter and do not exhaust through the vent.

Draft: A measure of the potential of a system to exhaust.

Draft diverter: A safety device installed on atmospheric appliances to (1) allow draft to divert into the building if the vent becomes blocked, and (2) add dilution air for draft efficiency (sometimes called a draft hood).