What Is a Makeup Air Unit (MAU)?
A makeup air unit replaces the air exhausted from a building — supplying tempered outdoor air to balance kitchen hoods, industrial exhaust, and other high-exhaust spaces. Without makeup air, exhaust fans depressurize the building, backdrafting water heaters, slamming doors, and starving the hoods of the air they need to capture contaminants. The MEP engineer sizes the MAU from the total exhaust rate.
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Why buildings need makeup air
A commercial kitchen exhausts 3,000-10,000+ CFM through its hoods; that air must come from somewhere. Without a MAU, it comes through every crack and door — cold in winter, hot in summer, unfiltered always — while the building goes negative and combustion appliances backdraft carbon monoxide. Code requires the makeup air; physics punishes its absence.
Design components
The MAU includes outdoor-air intake with filtration, heating (gas, electric, or heat pump) and sometimes cooling to temper the air near room temperature, a supply fan, and distribution ductwork delivering air near the exhaust sources (kitchen designs often discharge makeup air at the hood). Controls interlock the MAU with the exhaust fans — the makeup runs whenever the exhaust runs, with airflow balanced to keep the building slightly positive or neutral.
Energy and code notes
Tempering thousands of CFM of outdoor air is a major energy load — energy recovery between exhaust and makeup airstreams (where codes and practicality allow; grease-laden kitchen exhaust limits recovery options) cuts it substantially. Energy codes increasingly require demand-controlled kitchen ventilation (hood airflow varying with cooking activity), which lets the MAU turn down too. Size from the exhaust schedule, not rules of thumb — and verify the existing building's MAU before adding exhaust.
The bottom line
MAUs replace exhausted air with tempered outdoor air — required wherever commercial exhaust operates. Interlock with exhaust fans, temper the air, and size from the exhaust schedule.
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