The Mathematics of Building Depressurization
If a commercial Exhaust Air Unit extracts 10,000 CFM from a kitchen, 10,000 CFM must be reintroduced. If the Fresh Air Unit (FAU) only supplies 7,000 CFM, the building goes into a 3,000 CFM deficit. This severe negative pressure causes exterior doors to become un-openable, pulls conditioned air from the dining room, and causes deadly back-drafting of carbon monoxide from gas-fired water heaters.
Tempering the Make-Up Air
Injecting raw, untempered ambient air directly over a chef's head creates an unworkable environment. Kitchen FAUs are engineered with direct-fired gas burners or evaporative cooling modules to temper the incoming air, ensuring the Make-Up Air (MUA) is introduced at a neutral 21°C (70°F) without disrupting the thermal capture plume of the exhaust hood.