What is the Ontario Building Code requirement for make-up air with range hoods?

The Ontario Building Code (OBC) requires a make-up air system when the total exhaust capacity of all equipment in a dwelling exceeds 75 litres per second, which is approximately 159 CFM. This threshold is found in OBC Part 6 (Mechanical) and applies to all residential construction and major renovations in Ontario, including every municipality across the GTA. The requirement is designed to prevent dangerous negative pressure conditions that can cause backdrafting of combustion appliances — a potentially fatal carbon monoxide hazard.

The key word in the OBC requirement is total exhaust capacity. The code does not look at the range hood in isolation — it considers the combined rated capacity of every exhaust device in the home, including bathroom exhaust fans, the dryer vent, and any other exhaust equipment. However, in practical terms, it is the range hood that typically pushes a home over the threshold. A standard bathroom fan at 80 CFM and a dryer at 150 CFM together total 230 CFM — already over the threshold on their own, though the code accounts for simultaneous operation factors. When you add a range hood rated at 400, 600, or 1,200 CFM, you are well past any threshold regardless of how the calculation is applied.

The make-up air system must meet specific OBC requirements. It must be interlocked with the exhaust equipment — meaning the make-up air supply activates automatically when the range hood turns on and shuts off when the hood turns off. The system cannot rely on the homeowner remembering to open a window or flip a separate switch. This interlocking is typically achieved through a pressure switch, current-sensing relay, or hardwired connection between the range hood and the make-up air damper. The make-up air must be delivered at a rate that offsets the exhaust volume, preventing the home from going into significant negative pressure.

In Toronto's climate, tempering the incoming make-up air is essential. The OBC does not explicitly mandate heating the make-up air in all cases, but bringing -20 degree outdoor air directly into a kitchen in January is impractical and creates its own comfort and condensation problems. Most make-up air installations in the GTA use a tempered system — either an electric resistance heater or a gas-fired inline heater that warms the incoming air before it enters the living space. Electric tempering is simpler to install but costs more to operate. Gas-fired tempering is more economical for high-volume systems but requires a gas line and TSSA-registered installation.

Practical implications for GTA homeowners planning a kitchen renovation are significant. If you are selecting a range hood for a new kitchen, understand that anything above approximately 400 CFM will trigger the make-up air requirement. The installation cost for a complete make-up air system runs $2,000-$5,000 in the GTA market, including the make-up air unit, ductwork to the exterior, tempering heater, interlocking controls, and the required building permit. The permit itself runs $200-$500 through the City of Toronto, and an inspection is required before the system is considered code-compliant. Some homeowners deliberately choose a range hood rated at 300-390 CFM to stay below the make-up air threshold while still getting effective kitchen ventilation — a pragmatic approach that saves thousands on the installation. If you do go with a high-CFM hood, get quotes for the make-up air system at the same time as the hood installation so you understand the true total project cost before committing.