Do I need make-up air if I install a powerful range hood in my Toronto home?

Yes — if your range hood is rated above approximately 400 CFM, the Ontario Building Code requires a make-up air system to replace the air being exhausted, and this is not optional. The specific threshold in the OBC is 75 litres per second of total exhaust capacity, which works out to roughly 159 CFM. In practice, because the OBC looks at the rated capacity of all exhaust equipment combined — not just the range hood — a powerful range hood in a home with bathroom fans and a dryer vent can trigger the make-up air requirement even below the 400 CFM mark. But as a practical rule, any range hood rated at 400 CFM or higher will almost certainly require make-up air in a Toronto home.

The reason make-up air is required goes beyond code compliance — it is a combustion safety issue. A powerful range hood exhausting 600 or 1,000 CFM pulls an enormous volume of air out of your home. That air must come from somewhere. In a tightly sealed modern Toronto home, the hood creates strong negative pressure that pulls air through every gap and crack in the building envelope — and critically, it can reverse the draft in your furnace flue, water heater vent, or fireplace chimney. This is called backdrafting, and when it happens with gas-fired appliances, it pulls combustion gases — including carbon monoxide — into your living space instead of sending them up the chimney. This is a potentially fatal hazard, and it is the primary reason the OBC mandates make-up air.

Even in older, leakier GTA homes where natural infiltration might seem like it provides enough replacement air, a high-CFM range hood overwhelms that leakage. The signs of inadequate make-up air are immediate and obvious: doors become hard to open or slam shut on their own, you hear whistling from windows and doors, the range hood itself underperforms because it cannot pull enough air to reach its rated CFM, and a cold draft sweeps across the floor as air rushes in through gaps at the bottom of exterior doors.

A make-up air system consists of a duct from the exterior that brings fresh outdoor air into the home, interlocked with the range hood so it only operates when the hood is running. In Toronto's climate, the incoming air must be tempered — you cannot dump -20 degree air directly into your kitchen in January. Most make-up air units include an electric or gas-fired heater that warms the incoming air to a comfortable temperature before it enters the living space. The system is interlocked with the range hood through a pressure switch or wired relay so that the make-up air damper opens and the heater activates automatically whenever the hood turns on.

Budget $2,000-$5,000 for a professionally installed make-up air system in the GTA, depending on the complexity of the duct routing and whether electric or gas tempering is used. This is in addition to the range hood and its exhaust ductwork. The installation requires a building permit from the City of Toronto, and the gas connection for a gas-tempered unit must be done by a TSSA-registered contractor. Many homeowners are caught off guard by this cost when they purchase a high-end range hood — a $2,000 hood can easily require $3,000-$4,000 in make-up air infrastructure to install legally and safely. Factor this into your kitchen renovation budget from the beginning.