Why does my furnace blower run constantly?

A furnace blower running constantly is usually caused by one of four things: the fan setting on your thermostat is set to "ON" instead of "AUTO," a faulty limit switch, a wiring issue, or in some cases, an HVAC system intentionally running for air quality or zoning reasons.

The most common — and easiest — fix is checking your thermostat. If the fan switch is set to "ON" rather than "AUTO," the blower will run 24/7 regardless of whether the furnace is actually heating. Switch it to "AUTO" and see if the blower stops cycling on its own. This solves the problem for a surprising number of homeowners.

If your thermostat is already set to "AUTO" and the blower still won't shut off, the next likely culprit is a faulty high-limit switch. This safety component monitors the furnace's internal temperature and shuts off the burners if it gets too hot — but it keeps the blower running to cool things down. If the limit switch is malfunctioning or has tripped repeatedly, it can get stuck in a mode that keeps the fan running continuously. A dirty or clogged furnace filter is often the root cause here, because restricted airflow causes the heat exchanger to overheat and trip the limit switch. Check your filter first — if it's grey and clogged, replace it (Toronto's dusty winters are hard on filters). A 1-inch filter should be replaced every 1–3 months.

A wiring fault between the thermostat and the furnace control board can also send a constant "fan on" signal. This is less common but does happen, especially in older Toronto homes where wiring has been modified over the years.

Toronto-specific note: If your home has a heat recovery ventilator (HRV) — common in newer GTA builds and required by the Ontario Building Code in airtight homes — it may be integrated with your furnace blower and run on its own schedule. This is normal and by design. Check your HRV controller if you have one.

Continuous blower operation isn't always an emergency, but it does increase your hydro bill and can wear out the blower motor prematurely. In Ontario, furnace season runs hard from October through April, so it's worth addressing before the problem worsens.

This is a job for a licensed HVAC technician. If the thermostat fix doesn't solve it, a TSSA-certified contractor can test the limit switch, inspect the control board, and check wiring safely. Furnace control boards and limit switches typically cost $150–$400 in parts, with labour bringing the total repair to $250–$600 in the Toronto market depending on the furnace brand and age.

As a next step, start with the thermostat fan setting, then replace your filter if it's dirty. If the blower is still running constantly after that, it's time to call a licensed HVAC professional. You can find TSSA-certified contractors serving the GTA through the Toronto Construction Network directory — look for contractors with strong reviews for furnace diagnostics and repair.