Why does my Toronto home smell musty when the furnace or AC turns on?

A musty smell when your HVAC system kicks on almost always indicates mould or mildew growth somewhere in the duct system, on the evaporator coil, or in the area around the air handler — and Toronto's humid summers make this an extremely common complaint across the GTA. The smell is caused by mould spores and volatile organic compounds released when air blows across contaminated surfaces and carries them into your living spaces.

The most common source is the evaporator coil and condensate drain pan inside your furnace or air handler. During cooling season, the evaporator coil gets cold and moisture from the air condenses on it — exactly like a cold glass sweating on a humid day. This moisture drains into a pan and out through a drain line. If the drain line becomes clogged with algae, dust, or debris (which happens frequently in GTA homes), standing water accumulates in the drain pan, creating a perfect breeding ground for mould and bacteria. When the system runs, air blows across this contaminated standing water and distributes the musty odour throughout the house. Having an HVAC technician clean the coil and flush the drain line is a straightforward fix that typically costs $150 to $300.

Mould growth inside the ductwork itself is the second most common cause. In Toronto's humid summers, with humidex values regularly exceeding 40 degrees, any duct surface that is cooler than the surrounding air will develop condensation. Supply ducts carrying 12 to 15 degree air through a humid basement or attic sweat heavily if they are not insulated with at least R-8 duct wrap and a vapour barrier. Over time, this condensation promotes mould growth on the exterior of the duct, on surrounding insulation, and eventually on the interior duct walls. The musty smell is strongest when the system first turns on because stagnant air in the ducts has been in contact with mould colonies during the off-cycle.

Older GTA homes with panned joist return air plenums — where floor joist cavities are used as return ducts — are especially prone to musty odours. These improvised ducts pull air from crawlspaces, wall cavities, and other damp areas, bringing moisture, dust, and mould spores directly into the air stream. They cannot be effectively sealed or cleaned, and replacing them with proper return ductwork is the permanent solution at $300 to $800 per return run.

Before spending money on duct cleaning, identify and fix the moisture source first. Duct cleaning alone will not solve a mould problem if the conditions that caused the mould are still present. Check for condensation on duct surfaces, standing water in the drain pan, humidity levels above 50 percent in the basement, and any signs of water intrusion near the ductwork. A dehumidifier in the basement, set to maintain 40 to 50 percent relative humidity, can help prevent condensation on cold duct surfaces during summer.

If the smell persists after addressing moisture issues, professional duct cleaning by a NADCA-certified company using truck-mounted equipment costs $300 to $700 and can remove accumulated mould, dust, and debris from inside the duct system. Be wary of companies offering whole-house duct cleaning for $99 — these are bait-and-switch operations that will not resolve your problem.