Where does combustion air come from in Toronto?

In Toronto homes with gas-fired equipment, combustion air comes from one of two sources: the surrounding indoor air (for older, naturally aspirating appliances) or directly from outside (for modern sealed-combustion or direct-vent equipment). Understanding which type your home has is critical — getting this wrong creates a carbon monoxide risk.

Older Atmospherically Vented Appliances

Most pre-2000 Toronto homes have furnaces, water heaters, or boilers that draw combustion air from the mechanical room or basement. These appliances are called atmospherically vented or natural draft — they rely on the buoyancy of hot flue gases to pull air through the burner and up the flue. The combustion air comes from the house itself, which in turn draws replacement air through gaps in the building envelope — around windows, doors, rim joists, and penetrations.

This worked reasonably well in older, leaky Toronto housing stock. The problem arises when homeowners seal and insulate their homes aggressively (spray foam rim joists, new windows, weather stripping) without addressing combustion air. A tight home can depressurize relative to the outdoors, causing the furnace or water heater flue to backdraft — pulling combustion gases back into the living space instead of exhausting them. This is a serious carbon monoxide hazard and is more common in Toronto than most homeowners realize, particularly in post-war bungalows in Scarborough, North York, and Etobicoke that have been partially upgraded.

The Ontario Building Code and TSSA require that atmospherically vented appliances have adequate combustion air supply. For a typical GTA mechanical room, this means either a direct outdoor air duct (minimum 1 square inch of free area per 1,000 BTU/hr of appliance input) or sufficient volume in the mechanical room to sustain combustion without depressurization. A TSSA-registered contractor must assess and design this — it is not a DIY calculation.

Modern Sealed-Combustion Equipment

High-efficiency furnaces (95%+ AFUE) and most modern water heaters are sealed-combustion or direct-vent appliances. They draw combustion air through a dedicated PVC pipe from outside the building and exhaust through a second pipe — completely isolated from indoor air. These appliances are essentially indifferent to indoor air pressure and are far safer in tight homes.

If your Toronto home has a high-efficiency furnace with two PVC pipes penetrating an exterior wall (typically through the rim joist or foundation), you have sealed combustion. These pipes must be kept clear of snow and ice — a GTA winter concern. Blocked combustion air pipes cause the furnace to shut down on a safety lockout, which is exactly what the safety system is designed to do.

The Ductwork Connection

Combustion air interacts directly with your duct system when large exhaust fans are running. A powerful kitchen range hood (400+ CFM) or multiple bathroom fans running simultaneously can depressurize a home enough to interfere with atmospherically vented appliances. This is precisely why the Ontario Building Code requires a make-up air system when total exhaust capacity exceeds 75 litres per second (approximately 159 CFM). Make-up air must be interlocked with the exhaust system and tempered in winter — bringing -20°C outdoor air directly into a mechanical room is not acceptable.

Any time you are adding a high-CFM range hood, sealing your home's envelope, or modifying exhaust ventilation, a TSSA-registered contractor must assess combustion air adequacy. This is not optional — it is Ontario law and a life-safety issue. Toronto Ductwork can connect you with local ventilation and ductwork professionals who work alongside TSSA-registered contractors to ensure your exhaust and make-up air systems are properly designed and integrated.