Furnace blower fan runs constantly and won't shut off

Good news first: the most common reason a furnace blower never stops is a single switch on the thermostat set the wrong way — a ten-second, no-cost fix.

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In Canadian homes, check the thermostat fan switch first. If it's set to ON, the blower is supposed to run all the time — slide it to AUTO and the fan will start cycling with the heat again. Free, ten seconds, and by far the most common reason a blower never stops.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order

Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

Hold your hand at a supply vent. If the air is room-temperature (not warm) while the blower runs, the burner isn't firing — the fan is just running on its own, which points straight at the fan setting.
Look at the thermostat for a fan switch or fan menu with ON and AUTO. ON means "run the blower all the time"; AUTO means "only during heating."
Slide or set it to AUTO. The blower should keep running for a short cool-down, then stop. If it stops and later cycles on and off with the heat, that was the whole problem.

buildNo tools schedule5 min

handymanHow to fix it

On the thermostat, move the fan setting from ON to AUTO. The blower finishes its cool-down and then only runs during a heat call. If the thermostat is dead or unresponsive, a fresh set of batteries often brings the fan setting back under control.

Find the fan control on the thermostat — a physical FAN switch with ON/AUTO, or a "Fan" option in the menu on a digital or smart model.
Set it to AUTO. Expect the blower to keep running for a minute or two of cool-down, then shut off on its own.
If the thermostat screen is blank or frozen, replace its batteries (usually two AA or AAA behind the faceplate) and set the fan to AUTO again.

schedule2 min savings$0–5

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Changing the thermostat fan setting and its batteries is routine homeowner maintenance in every province.

verifiedWhy this one goes to a pro

The fan limit switch and the control board sit inside the furnace's line-voltage wiring, next to the heat exchanger and the gas train. A stuck relay or a mis-wired limit switch can keep the blower on or, worse, let the furnace overheat. In Canada this is a licensed HVAC repair — a technician can confirm and swap the part quickly and safely.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

handymanHow to fix it

warning Kill the breaker, and know when to hand it upThermostat wiring is low-voltage, but it runs a few inches from line-voltage inside the furnace, and a wrong reconnection can energize the fan or the board unexpectedly. Switch the furnace off at the breaker before you open the thermostat, and take a photo of the wiring first. If the short is inside the furnace cabinet, involves line-voltage wires, or you're unsure which terminal a wire belongs on, stop — this is a quick, inexpensive job for a licensed electrician or HVAC technician.

With the furnace powered off at the breaker, pull the thermostat off its base and check the low-voltage terminals: look for a G (fan) wire touching another terminal or a frayed/pinched cable behind the plate. Re-terminate loose or shorted wires cleanly. If the wire is damaged inside the furnace or you find line-voltage involved, stop and route it to an electrician or HVAC tech.

Switch the furnace off at its breaker before touching any wiring — even the low-voltage side can share a panel with line voltage.
Gently pull the thermostat body off its wall base. If the blower stops the instant the thermostat comes off, the fault is in the thermostat or the wiring, not the furnace controls.
Inspect the terminals: a G/fan wire touching R or another terminal, a pinched cable behind the plate, or a corroded connector can all keep the fan on. Re-seat and re-terminate anything loose or shorted.
If the wire is chafed inside the furnace cabinet, or you cannot find the short at the thermostat, restore power and hand it to a licensed electrician or HVAC tech rather than guessing.

schedule30 min savings$0–40

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: licensed professionals only
Owner re-termination of low-voltage thermostat wiring is generally permitted; any in-furnace, control-board, or line-voltage work is a licensed HVAC/electrical job.

verifiedWhy this one goes to a pro

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.