At what CFM rating does a range hood require make-up air in an Ontario home?

Under the Ontario Building Code, a make-up air system is required when total exhaust capacity exceeds 75 litres per second, which works out to approximately 159 CFM. In practical terms, this means any range hood rated above roughly 400 CFM will almost certainly trigger the make-up air requirement, because kitchen exhaust is rarely the only exhaust in the home — bathroom fans, dryer vents, and other exhaust sources all contribute to that total. Most standard range hoods in the 200-300 CFM range will not trigger the requirement on their own, but the moment you step up to a professional-style hood at 600, 900, or 1,200 CFM, make-up air becomes mandatory.

The reason this rule exists is straightforward and safety-critical. When a powerful range hood pulls large volumes of air out of your home, it creates negative pressure inside the building envelope. In a well-sealed GTA home — and modern homes built to Ontario's increasingly strict energy codes are very well sealed — that negative pressure has nowhere to draw replacement air from except through the path of least resistance. That path is often the chimney flue of a gas furnace, gas water heater, or gas fireplace. When exhaust gases are pulled back down the flue and into the living space instead of venting safely outside, it is called backdrafting, and it introduces carbon monoxide into your home. This is not a theoretical risk — it is a well-documented cause of carbon monoxide poisoning in tightly sealed houses across the GTA.

The make-up air system solves this by providing a controlled pathway for outdoor air to enter the home, replacing the air being exhausted by the range hood. The system must be interlocked with the range hood so it activates automatically whenever the hood runs. In Toronto's climate, where winter temperatures drop to -20 degrees Celsius or colder, the incoming make-up air also needs to be tempered — meaning it passes through a heater before entering the living space, so you are not blasting freezing outdoor air into your kitchen every time you cook. A typical make-up air unit for a GTA home costs between $2,000 and $5,000 installed, including the tempering coil and dedicated ductwork.

Before purchasing a high-CFM range hood, confirm with your contractor that the make-up air system is included in the project scope and budget. Many homeowners renovating kitchens in older Toronto neighbourhoods like the Annex, Leslieville, or High Park install beautiful commercial-style range hoods without realizing the make-up air requirement adds significant cost and ductwork. Your contractor should also confirm that all gas appliances in the home have adequate combustion air supply after the make-up air system is installed. This work requires a building permit from the City of Toronto, and any gas connections must be performed by a TSSA-registered contractor. If you are planning a kitchen renovation that includes a powerful range hood, get matched with a ductwork contractor through the Toronto Construction Network to ensure the make-up air system is properly designed from the start.