The Short Answer
Make-up air is fresh outside air deliberately supplied to a building to replace the air removed by exhaust systems. When large exhaust fans — such as kitchen hoods or industrial ventilation — pull air out, that air must be replaced or the building becomes negatively pressurized. Make-up air, often delivered by a dedicated make-up air unit (MAU) that can temper the incoming air, keeps the building balanced so exhaust systems capture properly and occupants stay comfortable.
Why Exhaust Needs Replacement Air
Air is conserved: whatever leaves a building must be replaced. If a kitchen hood exhausts a large volume of air and nothing supplies it back, the building goes negative. Air then infiltrates uncontrolled through doors, windows, and gaps. This causes doors that are hard to open, whistling gaps, drafts, and — most seriously — backdrafting of combustion appliances that can pull dangerous exhaust gases back indoors. Balancing exhaust with make-up air prevents these problems.
What Happens Without Enough Make-Up Air
Under-supplied buildings show telltale symptoms. Exhaust hoods fail to capture smoke and grease vapor because the fan can't move its rated airflow against negative pressure. Comfort suffers as cold or hot outside air infiltrates uncontrolled. Combustion safety is compromised when negative pressure disrupts water heater or furnace venting. Energy costs rise because conditioned air is constantly lost. Proper make-up air design eliminates these failure modes.
How Make-Up Air Units Work
A make-up air unit brings in outside air and, in many climates, tempers it — heating cold air in winter or, in some designs, cooling and dehumidifying in summer — before delivering it to the space. The MAU is interlocked with the exhaust system so supply and exhaust operate together, keeping pressure balanced. Engineers size the MAU to match the exhaust it offsets and select controls that maintain the intended building pressure relationship.
Balancing the Building
Good ventilation design isn't just about individual units — it's about the whole-building air balance. Engineers account for all exhaust (hoods, restroom fans, general exhaust) and all supply (HVAC outside air, make-up air units) to achieve the desired pressurization. Many spaces are intentionally kept slightly positive or neutral; some, like certain kitchens, are held near neutral to balance heavy hood exhaust. Documenting the intended balance guides the contractor's test-and-balance work.
When Do You Need This?
Make-up air is required wherever significant exhaust exists — most commonly commercial kitchens, but also manufacturing, labs, and spaces with heavy general or process exhaust. If your project adds or increases exhaust, make-up air must be evaluated as part of the mechanical design. We size and document make-up air alongside the exhaust system it serves.
Frequently Asked Questions
Is make-up air the same as ventilation air?
They overlap but aren't identical. All outside air brought in is ventilation, but "make-up air" specifically refers to air supplied to offset exhaust. A dedicated MAU is common where exhaust volumes are large, such as kitchens.
Does make-up air need to be heated or cooled?
Often yes. In cold climates, unconditioned make-up air would make spaces uncomfortable and could freeze equipment, so the MAU tempers it. The degree of conditioning depends on climate, use, and code.
Can existing HVAC provide make-up air?
Sometimes the building's HVAC outside air can offset modest exhaust. But large exhaust loads typically need a dedicated make-up air unit to keep the building balanced. We evaluate this as part of the design.