Make-Up Air Units — Restaurants & Commercial Kitchens, Twin Ports

What goes wrong when make-up air fails

A kitchen hood might exhaust several thousand CFM. That air leaves the building whether or not anything replaces it, and if the make-up air unit is not doing its job, the building pulls that air in through whatever gaps it can find.

The symptoms are distinctive and every restaurant operator recognises them. Entry doors that are hard to pull open, or that whistle. A hood that no longer captures smoke at the cookline, so the kitchen fills and the dining room smells. Pilot lights that blow out. Water heaters and boilers that backdraft — which is a carbon monoxide risk, not just a nuisance. And in January, outdoor air at −15°F being dragged in around every door and window, making the kitchen miserable and the dining room next to it unusable.

Negative pressure also defeats your dining-room HVAC entirely: you cannot heat a space that is continuously pulling in subzero outdoor air, no matter how large the rooftop unit is. We are sometimes called about a 'dining room that will not heat' and the actual fault is kitchen make-up air.

Getting this right is a code requirement and an insurance matter, not an optimisation. A hood running without adequate tempered make-up air is a finding waiting to happen.

Direct-fired versus indirect-fired in a −20°F climate

Make-up air has to be tempered here. Introducing thousands of CFM of −15°F air into a kitchen unheated is not an option, so the unit has a burner.

A direct-fired unit passes the incoming air directly across the flame. It is very efficient — nearly all the fuel energy ends up in the airstream — and it is compact and lower cost. The combustion products enter the space, which is permitted for this application within defined limits, but it constrains where it can be used and demands correct commissioning and adequate dilution.

An indirect-fired unit heats the air through a heat exchanger and vents combustion products outside. No combustion products in the space, more flexibility for sensitive occupancies, and simpler compliance in some situations — at lower efficiency, higher cost, and a larger footprint.

Which is right depends on the kitchen, the occupancy, the jurisdiction and the CFM. We will tell you which your situation calls for and why, rather than quoting whatever we happen to stock.

Balance is the whole job

The point is not to run a make-up air unit; it is to balance the building. Exhaust and make-up have to be measured and set against each other, with the building held slightly negative relative to outdoors in most kitchen designs so cooking odours do not migrate into the dining room — but only slightly, not the strong negative that causes every problem above.

That requires actual airflow measurement at the hood and at the MUA, interlocked controls so make-up air runs whenever the hood runs, and verification with the doors closed and the kitchen in its real operating state. A unit installed but never balanced is common and is why a restaurant with a brand-new MUA can still have all the symptoms.

We measure, balance, verify the hood's capture at the cookline, confirm no combustion appliance backdrafts under worst-case conditions, and leave you the numbers. For existing kitchens with chronic complaints, that measurement is usually where the answer has been hiding.