Why does makeup air need to be sized separately?

Every cubic foot of air a building exhausts must be replaced. If makeup air is not provided, the building goes strongly negative, doors are hard to open, combustion appliances can backdraft, and exhaust hoods lose capture. The MAU deliberately supplies that replacement air.

Should makeup air equal 100% of exhaust?

Usually slightly less. Commercial kitchens are kept mildly negative — often supplying 80 to 90% of exhaust as dedicated makeup air — so cooking odors do not migrate into dining areas. The remaining air comes from adjacent conditioned spaces and infiltration.

What is the infiltration credit?

Some replacement air naturally leaks in through the envelope. Subtracting a modest infiltration credit from required makeup air avoids oversizing the MAU. Tight modern buildings have little infiltration, so keep the credit small unless you have a measured air-change figure.

How is the heating capacity calculated?

Heating BTU/h equals 1.08 times the makeup air CFM times the temperature rise from winter design temperature to the supply setpoint. The 1.08 factor is the standard sensible-heat constant for air at sea level.

Does this size cooling for the makeup air?

This tool sizes only sensible heating, the dominant load for tempered makeup air in cold climates. Cooling and dehumidification of makeup air in hot, humid climates require a separate latent-load calculation and are not covered here.

Makeup Air Unit (MAU) CFM Calculator

A makeup air unit must replace what the building exhausts while holding the intended pressure relationship. This tool totals exhaust, applies your pressurization target and infiltration credit, and sizes both the supply CFM and the winter heating capacity of the MAU.

How it works

totalExhaust   = sum of all exhaust CFM sources
requiredMakeup = totalExhaust × (target % / 100) − infiltration credit
heatingBTU/h   = 1.08 × requiredMakeup × (supply temp − winter design temp)

Targeting slightly under 100% replacement keeps the space mildly negative — standard practice for commercial kitchens so odors do not migrate. The 1.08 constant is the sea-level sensible-heat factor for air (0.075 lb/ft³ × 60 min/hr × 0.24 BTU/lb·°F).

Example and tips

Two kitchen hoods exhausting 1,500 CFM each plus 400 CFM of restroom exhaust total 3,400 CFM. At 90% replacement with a 200 CFM infiltration credit, the MAU must supply about 2,860 CFM. Heating that air from a 5°F winter design temperature to a 65°F supply setpoint needs roughly 1.08 × 2,860 × 60 ≈ 185,000 BTU/h. In hot humid climates, size cooling and dehumidification of the makeup air separately — sensible heating alone understates the load there.