Does my Toronto home need make-up air if I have a bathroom exhaust fan over 150 CFM?

No, a single 150 CFM bathroom exhaust fan does not trigger Ontario's make-up air requirement. The Ontario Building Code requires make-up air systems only when total exhaust capacity exceeds 75 litres per second, which equals approximately 159 CFM.

However, you're very close to that threshold, and there are important considerations beyond just meeting code minimums. The 75 L/s rule applies to the total exhaust capacity of all mechanical exhaust in your home combined — not just one fan. If you have multiple bathroom fans, a kitchen range hood, and other exhaust systems, those CFM ratings add up quickly.

Why the make-up air requirement exists is crucial to understand, especially in Toronto's climate. When you exhaust 150+ CFM of air from your home without replacing it, you create negative pressure that pulls replacement air through every crack, gap, and opening in your building envelope. In a typical GTA home, this replacement air comes from uncontrolled sources — through basement foundation cracks, around windows, through wall penetrations, and worst of all, down your chimney or furnace flue.

The combustion safety risk is the primary concern. Negative pressure can cause backdrafting of gas appliances — your furnace, water heater, or fireplace exhaust gets pulled back into your home instead of venting safely outside. This brings carbon monoxide into your living space, which is potentially deadly. Toronto's older housing stock, with gas furnaces and water heaters often located in basements, makes this risk particularly serious.

Even below the code threshold, a 150 CFM bathroom fan can create noticeable pressure imbalances, especially in tighter homes or during winter when windows stay closed. You might notice doors slamming shut, difficulty opening exterior doors, or cold drafts as outside air gets sucked in through unintended openings. In winter, this incoming air is -20°C and hasn't been filtered or conditioned.

Consider your total exhaust picture when evaluating make-up air needs. Add up all your exhaust: bathroom fans (typically 50-110 CFM each), kitchen range hood (often 200-600 CFM), dryer vent (around 100-200 CFM when running), and any other mechanical exhaust. If you're planning a kitchen renovation with a higher-capacity range hood, you'll likely cross the make-up air threshold.

Make-up air systems cost $2,000-$5,000 installed and include a motorized damper that opens when exhaust systems operate, an intake duct to bring in outside air, and often a tempering system to pre-heat incoming winter air. The system must be interlocked with your exhaust equipment — when the bathroom fan runs, the make-up air damper opens automatically.

For a 150 CFM bathroom fan specifically, focus on proper installation fundamentals: duct it directly to the exterior (never into the attic or soffit), use rigid ductwork where possible, minimize bends, and install a good exterior wall cap with a backdraft damper. Ensure your home has adequate return air pathways so the HVAC system can operate efficiently even when the exhaust fan creates slight negative pressure.

If you're planning any kitchen or additional bathroom renovations, factor make-up air requirements into your planning early — it's much easier to install during construction than retrofit later.