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.

What is the Makeup Air Unit (MAU) CFM Calculator?

Sums total exhaust CFM from hoods, restrooms, and labs, subtracts an infiltration credit, and computes required makeup air supply CFM to maintain slightly negative or neutral building pressure. Also sizes the MAU heating capacity from the design temperature rise at that flow rate. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Makeup Air Unit (MAU) CFM Calculator

Name: Makeup Air Unit (MAU) CFM Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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

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.

Why building pressurization matters

A building that is strongly negative (much more exhaust than supply) creates several problems:

Door-opening force increases. Code limits door-opening force for accessibility (often 5 lbf max for fire doors). Negative pressure can make it nearly impossible to open a door into a low-pressure space.
Combustion appliance backdraft. Gas water heaters and boilers that rely on natural draft can spill flue gases into the building if the space is negative enough to reverse the chimney.
Infiltration-driven moisture. In humid climates, forced infiltration through wall cavities and ceiling penetrations drives moisture into the structure, leading to condensation and mould.
Hood capture degradation. An exhaust hood in a strongly negative kitchen loses effective capture velocity as makeup air rushes in through any available opening rather than controlled paths, reducing cooking-fume containment.

Providing 80–95% replacement as dedicated makeup air (rather than counting on accidental infiltration) controls all four of these risks.

MAU supply arrangement

Where makeup air is introduced matters as much as how much:

Short-circuit risk: if the MAU supply register is too close to a kitchen exhaust hood, make-up air is captured directly by the hood and does not displace air in the occupied zone. Locate supply away from exhaust hoods or use an interior diffuser arrangement.
Tempered vs direct-fire heated: direct-fired heaters (a burner inside the airstream) are highly efficient (near 100% combustion efficiency captured in the airstream) and common for large commercial kitchens. Indirect or heat-pump systems avoid combustion products in the supply air — preferable when supply air enters a sensitive zone.
Supply height: supplying makeup air at low level near the cooking line can improve hood capture by providing an air curtain below the exhaust plenum, a technique used in some commercial kitchen designs to reduce required hood exhaust CFM.