First check the thermostat. If the fan is set to On instead of Auto, the fan runs all the time, even when the furnace isn’t heating. If the fan still won’t stop on Auto, or the burners keep firing after the house is warm, turn the furnace off at its switch and call.
Which One Is Happening?
| What you notice | Likely cause | What to do |
|---|---|---|
| Fan runs all the time, air isn’t warm | Thermostat fan set to On | Switch the fan to Auto. |
| Fan runs nonstop on Auto, no heat | A stuck relay on the control board, or the furnace cooling itself after overheating | Check the filter. If it keeps happening, turn it off and call. |
| Heat keeps running past the set temperature | Thermostat placement, wiring, or a thermostat fault | Lower the set point. If it still won’t stop, turn it off at the furnace switch and call. |
| Burners stay lit after the thermostat is satisfied | A gas valve not closing | Turn the furnace off at its switch and call. Don’t keep using it. |
Fan On vs Auto
Most thermostats have a fan setting. Auto runs the fan only while the furnace heats. On runs it all day. Someone in the house may have switched it, or a smart thermostat may have a circulation schedule. Change it to Auto and wait one cycle.
The Fan Can Run to Cool an Overheated Furnace
When a furnace gets too hot, a safety switch (the limit switch) shuts off the burners and keeps the fan running to cool it down. A clogged filter, closed vents or a weak blower can cause this. If the fan runs long after the burners stop, check the filter first, then see limit and rollout switch problems.
Burners That Won’t Shut Off
If you can see the flame through the furnace window after the thermostat has stopped calling for heat, the gas valve may not be closing. That is a safety problem. Turn the furnace off at the switch beside it or at the breaker, and call. See gas valve problems.
Thermostat Problems
A thermostat in direct sun, near a cold window or on an outside wall reads the wrong temperature, so the heat runs too long or not long enough. A thermostat replaced recently may also be wired or set up wrong. Tell David the thermostat model and when it was changed.
Questions About A Furnace That Won’t Turn Off
Why does my furnace fan run all the time?
Most often the thermostat fan setting is on On instead of Auto. If it still runs on Auto, a stuck relay on the control board or an overheating furnace cooling itself can be the cause.
Is it dangerous if my furnace won’t stop heating?
If the burners stay lit after the thermostat stops calling for heat, yes. The gas valve may not be closing. Turn the furnace off at its switch and call.
How do I turn my furnace off?
Set the thermostat to off, then use the switch beside the furnace (it looks like a light switch) or the breaker labelled for the furnace.
Can a dirty filter make the fan run too long?
Yes. A clogged filter can overheat the furnace, and the safety switch then runs the fan to cool it down. Replace the filter and watch the next cycle.
Talk to David
Fan or burners still running after you changed the setting? Turn it off at the switch and call.
SkyLimit Heating & Gas is owner-operated. David answers the phone, does the work and gives you the price before he starts. Parts we supply carry a 1-year parts and labour guarantee. We work across Metro Vancouver and the Fraser Valley, Monday to Saturday, 9 AM to 9 PM, and don’t run a 24-hour line.
Call 778-892-5119 or request service online. Find your city in the furnace services directory.
Sources
Published by SkyLimit Heating & Gas. Updated . General homeowner information; gas, electrical and combustion work inside the furnace needs a licensed gas fitter.
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