What happens if I do not have make-up air for my large range hood in Toronto?

Operating a high-CFM range hood without make-up air in a Toronto home creates a dangerous negative pressure situation that can backdraft gas appliances and pull carbon monoxide into your living space. This is the single most serious risk, and it is not hypothetical — it is a well-documented hazard in tightly sealed GTA homes, particularly newer construction built to modern energy codes and older homes that have been retrofitted with improved air sealing and insulation.

When a 600 or 900 CFM range hood runs at full speed, it is pulling that volume of air out of your home every minute. In a well-sealed house, the air has to come from somewhere. Without a dedicated make-up air system providing a controlled path for replacement air, the negative pressure pulls air through any available opening — and the easiest path is often down the flue of your gas furnace, gas water heater, or gas fireplace. This reversal of flue gas flow is called backdrafting, and it brings combustion byproducts including carbon monoxide directly into your home. Carbon monoxide is odourless and colourless, so you will not know it is happening until symptoms appear or a CO detector alarms. Even if your home does not have gas appliances, running a large range hood without make-up air pulls unconditioned air through gaps in the building envelope — around windows, through electrical outlets on exterior walls, under doors, and through plumbing and wiring penetrations — bringing dust, allergens, humidity, and cold winter air with it in unpredictable ways.

Beyond the safety risk, there are practical performance problems. Without adequate replacement air, the range hood itself cannot perform properly. A 600 CFM hood in a tightly sealed home may only achieve 300-400 CFM of actual airflow because there simply is not enough air available to exhaust. You will notice the hood struggling — it sounds strained, cooking odours linger, and grease accumulates on kitchen surfaces despite the hood running. Doors may become difficult to open or may slam shut on their own due to the pressure differential. You may hear whistling around windows or feel cold drafts near exterior walls in winter as air is forced through tiny gaps in the building envelope.

There is also a code compliance issue. The Ontario Building Code requires make-up air when total exhaust exceeds 75 litres per second (approximately 159 CFM). If your range hood alone exceeds this threshold and you do not have make-up air, the installation is not code-compliant. This can create problems when selling your home, during insurance claims, or if a home inspection reveals the violation. The City of Toronto requires building permits for make-up air installations, and inspectors will verify that the system is properly interlocked with the range hood.

The fix is to install a proper make-up air system, which typically costs $2,000 to $5,000 in the GTA including tempering for winter operation and dedicated ductwork. This is not optional — it is a safety requirement. If you already have a large range hood without make-up air, have a qualified HVAC or ductwork contractor assess your situation. Toronto Ductwork can match you with a local professional through the Toronto Construction Network for a free estimate.