In short
Make-up air is outdoor air brought in on purpose to replace what exhaust fans, range hoods and dryers push out. Without it a building goes into negative pressure, and the missing air comes down the chimney or water heater vent instead, carrying carbon monoxide with it. Rule: every large exhaust needs a planned, tempered supply of roughly the same airflow.
Every litre of air a fan pushes out of a building has to be replaced by a litre coming in. In a leaky pre-war brick house that replacement air finds its own way through cracks. In a tight 2000s-and-newer home, an older house with new windows and air sealing, or a restaurant with a large kitchen hood, it has to be supplied deliberately, and that supply is called make-up air.
Why make-up air matters in a tight Ontario house
Negative pressure is the problem make-up air solves. A high-output range hood, a clothes dryer, bathroom fans and a central vacuum running together can pull out more air than the house lets in. The house then draws air through the easiest opening, which is often the vent of a natural-draft water heater, a wood fireplace or an older furnace.
That reversal is backdrafting, and it brings combustion gases, carbon monoxide among them, into the basement. The ventilation section of the Ontario Building Code (Section 9.32) deals with this through protection-against-depressurization requirements for houses where exhaust equipment could make a fuel-fired appliance spill. Sealed-combustion appliances are far less exposed, because they draw their combustion air from outdoors through their own pipe.
Signs a building is short of make-up air
- Exterior doors that are hard to pull open, or that slam shut, while the exhaust is running
- Whistling at door seals and window weatherstripping
- A range hood that is loud but will not capture smoke or steam
- Pilot lights that go out, or a fireplace that smokes into the room
- Soot, moisture or melted plastic around a water heater's draft hood, the marks of a backdrafting water heater
- Cold drafts at floor level and a dining room that will not stay warm in winter
A carbon monoxide alarm that sounds while the hood or dryer runs is not something to look into later. Shut the exhaust off, get everyone outside and call 911. Ontario's Hawkins-Gignac Act requires working CO alarms in homes with a fuel-burning appliance for exactly this kind of failure.
How make-up air is supplied in homes and commercial kitchens
The fix depends on how much air is leaving. The number to look for is the exhaust rate in CFM (cubic feet per minute) on the hood or fan nameplate; make-up air is sized against that figure.
- Houses: a motorized damper on an outdoor-air duct, interlocked so it opens whenever the hood runs, is the usual form of range hood make-up air; larger hoods need the incoming air tempered by a duct heater so the kitchen does not fill with air at −15 °C
- Commercial kitchens: a dedicated make-up air unit, usually gas-fired and roof-mounted, matched to the hood exhaust and interlocked with the exhaust fan, as described on our restaurant HVAC page
- Not a fix: an HRV or ERV, which exhausts as much air as it supplies and so cannot make up for a hood
What a make-up air job involves and what drives the cost
There is no single price for make-up air, because a damper kit in a house and a rooftop unit on a restaurant are different jobs. Cost is driven by the airflow required, whether the air must be heated, the gas and electrical supply to the unit, roof or wall penetrations and the controls that tie it to the exhaust. A gas-fired unit is gas work for a TSSA-certified technician, and new mechanical ventilation can need a permit from your municipal building department.
ZK Mechanical measures the exhaust and quotes make-up air units in writing before any work starts. More airflow terms are explained in the HVAC glossary. To have a backdrafting or door-slamming problem looked at, contact us.
Questions people ask about Make-up air
Do I need make-up air for a residential range hood?
It depends on the hood's exhaust rate, how airtight the house is and what burns fuel inside it. A low-CFM hood in an older, leaky house rarely causes trouble. A high-CFM professional-style hood in a tight house with a natural-draft water heater or a wood fireplace is the combination that needs an interlocked make-up air supply. On a permit job, your building department has the final say.
Can I open a window instead of installing make-up air?
An open window does relieve negative pressure, and it makes a fair test: if the hood suddenly captures smoke and the doors stop sticking, the building is short of air. It is not a solution in an Ontario winter, and nobody remembers to do it every time. A commercial kitchen cannot rely on it at all; the supply has to be mechanical and interlocked with the exhaust fan.
Why is my restaurant dining room cold and drafty in winter?
The usual cause is a kitchen hood exhausting more air than the make-up air unit delivers, because the unit has failed, its burner is locked out, its filters are plugged or it was never sized for the hood. The building then pulls unheated outdoor air through the front door every time it opens. Servicing the unit and rebalancing supply against exhaust is the fix.