The technical guidance on this page reflects these published codes and standards, which your local authority (AHJ) adopts, amends, and enforces. It is educational, not engineering or legal advice — confirm the current requirements for your jurisdiction.
Make-up air replaces the air a kitchen exhaust hood removes. The IMC requires supplied air to be approximately equal to exhaust, and kitchens commonly introduce about 80 to 90 percent of exhaust CFM as make-up air to keep slightly negative pressure. In cold climates that air is tempered.
A commercial exhaust hood can pull thousands of cubic feet of air per minute out of a kitchen. That air has to be replaced, and the replacement is called make-up air (MUA). If you remove air without deliberately supplying it back, the building fights you: it tries to draw replacement air through every crack, door, and gap it can find. The result is a kitchen under negative pressure, which causes a long list of problems.
When a kitchen is starved for make-up air, exhaust fans cannot move their rated volume, so hoods stop capturing smoke and grease effectively. Doors become hard to open or slam, pilot lights and gas burners can be pulled off or backdraft, and combustion appliances risk drawing flue gases back into the space instead of venting them. Cold drafts pour in around openings, comfort suffers, and heating and cooling costs climb. Proper make-up air is what lets the exhaust system actually do its job.
The International Mechanical Code (IMC), Section 508, governs make-up air for commercial kitchens. The core rule is that make-up air supplied to the building from all sources must be approximately equal to the exhaust air for all exhaust systems. Make-up air must be provided during operation of the commercial kitchen exhaust and must not reduce the effectiveness of the exhaust system.
In design practice, kitchens are usually set up so that make-up air equals roughly 80 to 90 percent of the exhaust volume. Supplying slightly less than you exhaust keeps the kitchen at a small negative pressure relative to the dining room, so cooking odors, heat, and grease drift toward the hood and out rather than into the seating area. The remaining balance is typically drawn from adjacent conditioned spaces. Some jurisdictions add a specific requirement that a minimum of 80 percent of the make-up air be delivered directly into the space where the hood is located; always confirm the exact figure with your local authority having jurisdiction.
The goal is a deliberate, slight negative pressure in the kitchen, not a strongly negative or positive building. Think of it as a target range rather than a single number:
Because a kitchen rarely runs in isolation, MUA design has to account for other building exhaust, such as restroom and dishwasher exhaust, so the whole building stays close to balanced.
Dumping large volumes of raw outdoor air into a kitchen is a problem in any climate with real winters or hot summers. Tempered make-up air means the incoming air is conditioned, most importantly heated in cold weather, before it reaches the space. Without tempering, staff work in a cold draft, condensation forms, and the building heating system cannot keep up.
Codes and manufacturers generally expect make-up air to be heated when outdoor temperatures fall below a threshold, commonly in the range of roughly 50 to 55 degrees Fahrenheit, though the exact trigger varies by jurisdiction and design. Dedicated make-up air units with gas or electric heat are the usual solution; in some designs the air is also cooled or partially conditioned. Tempering adds equipment and operating cost, which is one reason MUA is engineered rather than left to chance.
Make-up air is not something you bolt on after the hood is installed. The exhaust rate is driven by the hood type and the appliances beneath it, and the MUA system is then sized to match that exhaust and distributed so it does not blow across the hood and spoil capture. Supply registers are placed to introduce air low and away from the hood face or through the hood's own perforated supply plenum so the exhaust still pulls a clean capture pattern.
ASHRAE Standard 154, Ventilation for Commercial Cooking Operations, is the reference many designers use alongside the IMC for sizing exhaust and make-up air and for capture-and-containment performance. After installation, the system should be air-balanced and tested so measured exhaust and supply match the design, doors operate normally, and the kitchen holds the intended slight negative pressure. If you later add appliances or change the hood, the make-up air has to be re-evaluated too.