Furnace Fan Won't Turn Off? Check This One Thermostat Setting First
The blower runs and runs — sometimes blowing cool air — and never shuts off. This one has the highest "oh, that's it" rate of any furnace complaint. Check the thermostat before anything else.
Safety first, every page: if you smell gas anywhere in the house, leave and call your gas company from outside.
1. The fan switch: ON vs AUTO
On the thermostat, find the fan setting. It has two positions:
- AUTO — the fan runs only when the furnace is heating (or the AC is cooling). This is the normal setting.
- ON — the fan runs 24/7, whether there's heat or not.
If it's on ON, that's your whole problem. Switch it to AUTO. Someone hit it by accident, a kid was pushing buttons, or a previous occupant liked constant air. Cool air blowing from the vents between heating cycles is exactly what ON does.
Smart thermostats can hide this in a "Fan" menu, and some have a "circulate" mode that runs the fan part of every hour. Check for that too.
2. A cool-down that's just long
After the burners shut off, the furnace keeps the fan running for a minute or two to move the remaining heat out of the exchanger. That's normal and on purpose. If the fan runs a couple minutes after each cycle and then stops, nothing is wrong.
When the fan runs constantly on AUTO
If the fan is set to AUTO and still never stops, something inside is holding the blower on:
- A limit switch stuck open. The furnace thinks it's still too hot, so it keeps the fan on to cool. Often the cause is restricted airflow — pull the filter and hold it to the light. If you can't see through it, replace it, wait 15 minutes, and see if the fan settles.
- A fan relay or control board fault. The board's "fan on" signal is stuck. Technician job.
- A thermostat wiring issue (the G wire shorted). Also a technician job.
If the fan runs constantly and the house isn't heating, treat it as a no-heat problem: see furnace blowing cold air.
Before you call
Run the free troubleshooter. It captures the fan setting, the filter state, and the pattern you're seeing, and gives you a summary the technician can use on the phone — so you don't pay a trip charge to be told "your fan was on ON."
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