Glossary · Heating, cooling & ventilation
Make-up air
Also called: make-up air system · MUA · makeup air damper
Outdoor air deliberately brought into a house to replace air removed by big exhaust fans, range hoods, dryers or fireplaces.
What it means
In a tight house, a powerful range hood or other exhaust can't pull air out unless the same amount can get in. Without a planned path, replacement air may come down a chimney or flue, causing backdrafting, or the fan simply won't work well. Make-up air systems open a damper, sometimes with heating, when the exhaust runs.
Why it matters in an audit
Auditors look at exhaust capacity during a combustion safety test, and adding a big range hood after air sealing may call for make-up air.