Makeup Air

When a powerful exhaust fan blows air out of a house, that air has to come from somewhere. Makeup air is the controlled supply of outside air brought in to replace it. The most common reason a custom home needs it is a large kitchen range hood. A professional-style hood can pull a lot of air out fast, and without a planned way for air to return, the house fights back.

The building code puts a hard number on it. Under the 2018 International Residential Code, Section M1503.4, any kitchen exhaust hood that can move more than 400 cubic feet per minute (CFM) must be provided with makeup air at roughly the same rate, through a damper that opens automatically when the hood runs. Many high-end ranges come with 600, 900, or 1,200 CFM hoods, so makeup air is a real design item on a luxury kitchen, not an option.

The reason this matters more in a modern home is air tightness. New Arizona custom homes are built tight and sealed (you can confirm tightness with a blower-door test), which is great for energy bills. But a tight house has no easy gaps for replacement air. Run a 900 CFM hood in a sealed house and you create negative pressure that can pull air, and combustion gases, backward down a water-heater or fireplace flue. That backdraft is a carbon-monoxide hazard.

For example, a Paradise Valley kitchen with a 1,200 CFM hood needs a dedicated makeup air unit, often a ducted, damper-controlled, sometimes tempered air supply, sized to match the hood so the house stays balanced when the hood is on high.

Plan makeup air early, with the HVAC and hood selection together. It ties into the home's overall air balance and Manual J load planning, and retrofitting a makeup air path into a finished, tight house is far harder than designing it in from the start.