It is one of the most common calls we get from restaurant owners, and it almost always arrives the same way: the kitchen is unbearable, the air conditioning was just serviced, and somebody has suggested adding another rooftop unit. Before anyone spends money on more cooling, it is worth understanding what is actually happening, because in most of the kitchens we walk into, the air conditioning is not the problem. The problem is the air leaving the building and what is, or is not, coming back in to replace it.
Quick answer
A commercial kitchen hood pulls thousands of cubic feet of air per minute out of your building. A make-up air unit has to put most of that back. When the make-up air unit is undersized, switched off, or not working, the building goes negative: the kitchen stays hot, outside air is dragged in through every door and gap, front doors become hard to open, and combustion appliances can backdraft. Adding air conditioning does not fix this, because the air you paid to cool is going straight up the hood.
What make-up air actually does
An exhaust hood over a cook line removes a large volume of air continuously, carrying off heat, grease, smoke, and steam. A single hood can move several thousand cubic feet per minute. That air has to be replaced, and a make-up air unit is the piece of equipment whose entire job is to bring in outdoor air and deliver it near the cook line in roughly the volume the hood is removing.
It is normal and intentional for a kitchen to sit slightly negative relative to the dining room, because that is what keeps cooking odors from drifting out to the guests. What is not normal is for the whole building to sit negative relative to the outdoors. That is the condition that causes the symptoms below, and it happens whenever exhaust substantially exceeds the replacement air being supplied.
The signs your building has gone negative
The front door is the easiest test in the building. If guests have to pull hard to open it, if it whistles, if it slams behind them, or if the door sweeps are worn down on one side, your building is pulling air in through that doorway because it cannot get enough from anywhere else.
Inside, the pattern is familiar: the kitchen is hot no matter what the thermostat is set to, the dining room is cold and drafty because outdoor air is being pulled in through the front of the house, the hood does not clear smoke the way it used to, and in winter there are cold drafts along the floor at the entrance. Staff often describe it as the air conditioning not keeping up, which is exactly how the real cause gets missed.
There are also symptoms you should treat as urgent rather than annoying: a pilot light that will not stay lit, a burner flame that pulls sideways, a water heater or boiler that smells like exhaust, soot around a flue, or a carbon monoxide alarm that has gone off even once. Those point to backdrafting, which is covered further down.
Why more air conditioning will not solve it
This is the part worth being blunt about, because it saves owners real money. Your hood runs the entire time the kitchen is open. If the building is short on make-up air, every cubic foot of conditioned air your rooftop units produce is being pulled out through that hood or replaced by hot outdoor air leaking in around the doors. Adding cooling capacity to that situation means buying a larger unit to condition outdoor air that leaves immediately.
It also shortens the life of the equipment you already have. Rooftop units in a negative building run nearly continuously, never satisfy the thermostat, and accumulate compressor hours at a rate that brings failure years early. We have replaced compressors in restaurants where the underlying pressure problem had been in place for so long that it had already taken out the previous unit too.
The Michigan complication: tempering make-up air in winter
In a warm climate, a make-up air unit can sometimes get away with delivering raw outdoor air. Here it cannot. Bringing several thousand CFM of 15°F air into a kitchen is not workable, so make-up air units in our area include a gas-fired heat section that warms incoming air to something the kitchen can tolerate, usually delivered near but not directly on the cook line.
That heat section is also the most common point of failure, and the failure has a predictable sequence. The heat section stops working, the unit starts blowing cold outdoor air across the line in January, the kitchen staff find the switch and turn the unit off, and nobody mentions it to ownership. The kitchen warms up, everyone is relieved, and the building has quietly gone negative. Four months later there is a comfort complaint, a spike in gas usage from the rooftop units running constantly, and sometimes a combustion safety issue.
Freeze protection, damper operation, and the low-limit controls all matter here. A damper stuck open in a cold snap can freeze a coil; a damper stuck closed means no replacement air at all while the unit appears to be running normally.
What we usually find on the roof
The list repeats itself from restaurant to restaurant. The make-up air unit switched off at the disconnect or the breaker. A broken belt. Filters and coils packed solid with grease, which is its own fire concern as well as an airflow problem. Dampers seized in position. An interlock between the hood and the make-up air unit that was never wired, or was disabled at some point, so the hood runs without the make-up air unit ever starting.
The other common one is a change nobody connected to the symptom. A new fryer or a larger range went in under the hood, the hood exhaust fan was upsized to handle it, and the make-up air unit stayed exactly as it was. The kitchen was fine before the remodel and hot afterward, and the cause is sitting on the roof with the original nameplate on it.
What a proper balance check looks like
Measuring, not guessing. We read the actual exhaust volume at the hood, the actual delivery from the make-up air unit, and the building pressure relative to outdoors with a manometer. We verify the interlock so the make-up air unit runs whenever the hood runs, confirm the heat section fires and the discharge temperature is what it should be, check the damper travel, and look at filter and coil condition.
The goal is a kitchen that is slightly negative relative to the dining room and a building that is close to neutral relative to the outdoors. When we finish, you should have written readings, not an opinion. That record matters, because it gives you a baseline to compare against the next time somebody says the kitchen feels hot.
The safety issue underneath all of this
A building under strong negative pressure will find air wherever it can, and one of the places it can find air is down the flue of a natural-draft appliance. If your water heater, boiler, or any atmospherically vented equipment is in the building, negative pressure can pull combustion gases back into the space instead of letting them rise out the vent. Those gases include carbon monoxide.
This is not a theoretical risk and it is the reason we treat pressure complaints as more than a comfort issue. If you have combustion equipment in the building, working carbon monoxide detection is not optional, and a pilot that keeps going out or the smell of exhaust near a water heater should bring a call the same day, not at the next scheduled visit.
If you operate more than one location
Multi-site operators tend to discover this problem one restaurant at a time, usually in July. The better approach is an inventory: every location, its hood exhaust volume, its make-up air unit, the age and condition of both, and whether the interlock is working. Once that exists, make-up air belongs in the preventive maintenance scope alongside the rooftop units you already service, with filter and belt service on a schedule and a heat section check before every heating season.
That single change, checking make-up air before winter rather than after a complaint, prevents most of the calls described here.
Frequently Asked Questions
What is a make-up air unit in a restaurant?
It is rooftop equipment that brings outdoor air into the building to replace what the kitchen exhaust hood removes, usually delivering that air near the cook line. In Michigan it includes a gas-fired heat section so incoming winter air is tempered before it reaches the kitchen.
Why is my restaurant front door so hard to open?
The building is under negative pressure, meaning more air is being exhausted than is being supplied. The door is being held shut by the pressure difference as the building pulls in replacement air through that opening. It is the clearest sign of a make-up air shortfall.
Will adding another rooftop AC unit cool my kitchen down?
Usually not. If the hood is removing more air than the make-up air unit supplies, conditioned air is being exhausted as fast as it is produced. Additional cooling capacity raises operating cost and equipment wear without fixing the underlying pressure problem.
Can negative pressure in a restaurant be dangerous?
Yes. Strong negative pressure can cause naturally vented appliances such as water heaters and boilers to backdraft, drawing combustion gases including carbon monoxide into the building instead of venting them outside. Pilot outages, exhaust odors, and any carbon monoxide alarm should be treated as urgent.
How often should a make-up air unit be serviced?
At least twice a year, with a heat section and damper check before heating season, and more frequent filter service in a high-grease environment. We generally recommend that restaurants put make-up air on the same quarterly schedule as their rooftop units.
We remodeled the kitchen and now it is hot. Why?
Adding cooking equipment usually increases hood exhaust volume, and the hood fan is often upsized to match. If the make-up air unit was not increased at the same time, the building loses the balance it had before, which shows up immediately as a hot kitchen and a hard-to-open front door.
If your kitchen has been hot for a season or two and adding cooling has not helped, we can measure what is actually happening and tell you what it will take to fix it. Kern Mechanical maintains rooftop units, make-up air, and exhaust systems for restaurants, retail brands, and multi-site operators across Oakland, Livingston, Genesee, Macomb, Wayne, and Washtenaw counties, with over 60 years of combined experience and emergency service available. Quotes and consultations are always free.
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