What Does a Make-Up Air Unit Do?

Restaurant owners usually meet the phrase "make-up air" in one of two ways: a contractor mentions it while quoting a new hood, or the kitchen gets so cold and drafty in January that someone finally asks why. Both roads lead to the same piece of equipment on the roof. This guide explains what a make-up air unit does, how it differs from other ventilation equipment, when one is needed, and what it looks like when one stops doing its job.

What does a make-up air unit actually do?

A make-up air unit replaces the air that an exhaust system pushes out of a building. A commercial kitchen hood removes a large volume of air every minute it runs, carrying grease, heat and smoke up the duct and out the roof. That air does not disappear quietly. Every cubic foot the hood removes has to be replaced by a cubic foot coming in from somewhere.

If nothing is designed to supply it, the building supplies it on its own: around entry doors, through window frames, down chimneys and flues, and through every crack in the envelope. A make-up air unit gives that air a deliberate path instead. It draws outdoor air through a filter, heats it, and blows it into the kitchen or near the hood in a controlled amount, so the building stays close to balanced rather than being sucked strongly negative.

The same principle applies to other big exhaust loads, such as paint booths, dryer rows and some industrial processes, but the commercial kitchen is where most Twin Ports owners run into it.

Is a make-up air unit the same as outside air or an ERV?

No. A make-up air unit is a dedicated supply of heated outdoor air sized to match a specific exhaust, while an ERV or HRV is a balanced ventilation device that trades stale indoor air for fresh air and recovers heat in the process. They solve different problems.

• Outside air on a rooftop unit is the fresh-air damper on your regular heating and cooling equipment. It brings in enough air for the people in the space, not enough to replace a kitchen hood.
• An ERV or HRV moves roughly equal amounts of air in and out and passes heat between the two streams. Our guide to HRV vs ERV in a cold climate covers how they work in homes and small buildings.
• A make-up air unit supplies raw outdoor air in large volume, heats it with its own burner, and does not recover anything from the hood exhaust, because greasy kitchen exhaust is not something you want passing through a heat exchanger.

Does a make-up air unit heat, cool, filter and run all the time?

A make-up air unit in this climate heats, filters, and runs only when the exhaust it serves is running. The table below answers the questions restaurant owners ask most often.

Common questions about how a make-up air unit operates

Question	Short answer
Does it heat the air?	Yes. Here it must, so units carry a gas burner, either direct-fired or indirect-fired.
Does it cool the air?	Many units are heating-only. Some include a cooling section; whether that is worth it depends on the kitchen.
Does it run all the time?	No. It is interlocked with the hood so it starts when the hood starts and stops when the hood stops.
Does it have filters?	Yes. The intake has a filter section that needs checking and changing on a regular schedule.
Does it replace the rooftop unit?	No. The rooftop unit still heats and cools the dining room; the make-up air unit only replaces exhaust.

Direct-fired units pass the incoming air across the flame and are very efficient; indirect-fired units heat through a heat exchanger and vent combustion products outside. Which one suits a given kitchen depends on the occupancy, the jurisdiction and the airflow the hood needs replaced.

When is a make-up air unit required?

A make-up air unit is required when an exhaust system removes more air than the building can safely replace on its own, and for a commercial kitchen hood that decision belongs to the local code official working from the hood design. Mechanical codes in both Minnesota and Wisconsin address replacement air for exhaust systems, but whether your particular hood needs a dedicated, tempered unit depends on the hood type, how much air it is designed to exhaust, what else is in the building and how the local authority applies the code.

In practice that means the question gets settled during design and plan review, not at the counter. If you are adding or replacing a hood, ask the hood designer and the mechanical contractor to show how the exhaust will be replaced, and confirm with the building department before the equipment is ordered. Adding a hood without that step is how kitchens end up with every symptom in the next section.

What are the signs a make-up air unit is missing or has failed?

The signs of missing or failed make-up air all come from one condition: the building is strongly negative, so outdoor air is being dragged in wherever it can get in. Operators usually notice several of these at once.

• Entry doors that are hard to pull open or whistle around the frame while the hood is running.
• Cold drafts at the cookline and a kitchen that is miserable to work in all winter.
• Smoke rolling out from under the hood instead of being captured.
• Pilot lights blowing out, or a water heater or boiler that smells of exhaust.
• A dining room that will not hold temperature no matter how hard the rooftop unit runs.
• A make-up air unit that sits idle while the hood runs, which usually means a failed interlock.

Our guide to restaurant negative pressure and make-up air walks through why the dining room is often the first place the problem shows up, and why a bigger rooftop unit does not fix it.

Why does make-up air matter more in a Twin Ports winter?

Make-up air matters more here because the air being replaced has to be heated from a −17°F to −21°F design temperature, over a heating season that commonly runs from late September into May. A kitchen that is merely drafty in a mild climate becomes unworkable in Duluth or Superior, and an undersized or unfired unit shows its weakness the first cold week of the year.

Fuel adds another wrinkle outside the gas-served towns. A café in Barnum or a roadhouse near Duquette is likely to be heating its make-up air with propane, so the gas supply, regulator and tank capacity all need to keep up with the burner on the coldest days. Across the state line, a restaurant in Brule follows Wisconsin rules for the same equipment, which is one more reason to confirm requirements locally before a hood project starts.

What can kitchen staff check, and when should you call?

Kitchen staff can confirm that the make-up air unit starts when the hood starts, note whether doors get harder to open during service, keep the rooftop intake clear of drifted snow, and keep a log of when drafts, smoke or pilot outages happen. That record shortens any service call.

Burners, gas trains, interlock wiring, airflow measurement and backdraft testing belong with a commercial technician. The fix is often a balance problem on equipment that is already there, found by measuring hood exhaust against make-up air. If your kitchen shows two or more of the signs above, ask for an assessment of make-up air unit repair and balancing along with the kitchen hood and ventilation system it serves.