Quick answer
The most common reason a furnace fan runs constantly is the thermostat fan switch set to On instead of Auto, which tells the blower to run around the clock. If the setting is on Auto and the fan still never stops, the furnace is either overheating and running the blower to cool itself, or a fan relay, fan-limit control or thermostat wire has failed and is holding the blower on.
When a furnace fan is running constantly, the blower has been told to run by one of three things: you, the furnace protecting itself, or a failed part — in that order of likelihood. Sorting out which takes about two minutes at the thermostat and a look at the light on the furnace, and it matters because a blower that never stops shows up on a Burlington Hydro bill even if the furnace is otherwise fine.
Check the thermostat fan setting before anything else
Every thermostat has a fan setting with two positions, On and Auto. Auto runs the blower only while the furnace is heating or the AC is cooling; On runs it continuously. The switch gets knocked, a child presses it, or a smart thermostat's Circulate or fan-schedule option gets enabled during setup and runs the fan a set number of minutes each hour. Set it to Auto, wait a minute or two for the current cycle to end, and if the fan stops, that was it.
One more thermostat trap: if the thermostat is calling for heat but the burners never light — a dirty flame sensor, say — some boards run the blower in a fault mode. In that case the fan is a symptom, and the real problem is on the ignition side. Why is my furnace not turning on covers those checks.
The furnace is overheating and cooling itself down
If the setting is on Auto, the next most likely cause is a high-limit trip. When the heat exchanger gets too hot, the limit switch shuts the burners off and the control board keeps the blower running to move that heat out. On a furnace starving for air this happens every cycle, so the burners run briefly, stop, and the fan carries on for a long time — sometimes indefinitely if the board has locked out on repeated limit trips. Look for these airflow restrictions:
- A filter you cannot see light through, or a filter that is too dense for the blower — see can a furnace filter be too restrictive.
- Supply registers closed in unused rooms, or return grilles blocked by furniture, a rug or a finished-basement wall.
- A dirty evaporator coil sitting above the furnace, which chokes airflow in heating season even though the AC is off.
- A blower wheel packed with dust, so it moves less air than the furnace was designed for.
The furnace light will be flashing a limit code while this is happening, which is your confirmation. Clear the restriction, power-cycle the furnace, and see whether the fan behaviour returns to normal.
A stuck relay, fan-limit control or shorted wire
When the thermostat and airflow check out, a part has failed closed. On mid-efficiency furnaces from the 1980s and early 90s — still working in some original Glen Abbey and Millcroft houses — the blower is switched by a mechanical fan-limit control, a round dial with a bimetal probe into the plenum and a small manual-fan tab on its face. The tab gets bumped to the on position, or the dial's contacts weld together, and the fan runs forever.
On newer furnaces the blower is switched by a relay on the control board. A relay whose contacts have stuck closed keeps the fan running regardless of what the thermostat asks, and the only fix is a new board. A shorted G wire between thermostat and furnace — nicked by a picture hook, chewed in a crawlspace, pinched under a baseboard — does the same thing electrically, and is a much cheaper repair once found.
Is it bad to run the furnace fan all the time?
It will not hurt the furnace, and there are good reasons to do it on purpose: evening out temperatures between floors, or filtering the air through a good pleated filter in wildfire-smoke weeks. The costs are electricity and filter life. On continuous fan, an older PSC blower draws far more power than a modern ECM motor idling at low speed, and on time-of-use pricing — the default for Burlington Hydro and Oakville Hydro residential accounts — that motor is running through every peak-rate afternoon. Filters load up faster, and in summer a fan that keeps running after the AC cycle ends re-evaporates water off the coil and pushes humidity back into the house.
What a constantly running furnace fan costs to fix
- Thermostat setting or a schedule fix — free over the phone, and no extra charge if we are already there for something else.
- Limit trip from a clogged filter or blocked return — often resolved within a $90–$150 diagnostic visit, which we credit toward any repair.
- Fan-limit control or thermostat wiring repair — a small part plus some tracing; usually within the $150–$450 most fixes land at.
- Control board with a stuck fan relay — $500–$1,200 depending on the furnace, which on an older unit is a fair moment to read our repair-or-replace guide.
If the fan is on Auto and still will not quit, ZK Mechanical traces it anywhere from Aldershot to Stoney Creek. We check the thermostat, the limit history on the board and the blower amp draw before we quote a part, so you are not paying for a board when the problem was a filter. Book furnace repair in Burlington, Oakville or Hamilton or contact us to arrange it.