What Is Make-Up Air?

Here’s a rule buildings can’t escape: air that gets pushed out must be replaced. Every bathroom fan, dryer duct, garbage-room exhaust and rooftop fan removes air from the building — and that air has to come from somewhere. Without a plan, “somewhere” means gaps under doors, elevator shafts, parkade connections and whatever path of least resistance exists. With a plan, it means make-up air.

The unit on the roof

A make-up air unit (MAU) is typically a rooftop or mechanical-room unit that draws in outdoor air, filters it, tempers it (heats it in winter), and supplies it to the building’s corridors and common areas. Beyond simply replacing exhausted air, it does something subtle and important: it keeps hallways at slightly higher pressure than the suites around them, so air flows from corridor into suite — never the reverse.

Why you should care about that pressure

That corridor-to-suite pressure gradient is the building’s odour and air-quality discipline. When it holds, cooking smells stay in the suite that made them. When the MAU is weak — clogged filters, dirty coils, a failing fan — the gradient collapses, and the building starts to tell on itself:

• Hallways feel stuffy and smell like a rotation of everyone’s dinner
• Odours migrate between suites
• Suite entry doors whistle or resist opening
• Common areas feel stale even when it’s fresh outside

Maintenance is mostly housekeeping

The good news is that most MAU decline is dirt, not damage. Filters load up (they often go years past their change interval), coil surfaces coat over, intake screens collect debris, and cabinets accumulate grime — all of it recoverable with proper cleaning and filter maintenance. A unit that’s been serviced also gets its real problems documented: worn belts, failing dampers and corrosion caught while they’re maintenance items rather than replacements.

For councils and property managers, MAU filters on a fixed schedule plus an annual full service is one of the highest-return line items in a building maintenance plan — invisible when it works, expensive and unpleasant when it doesn’t.