Symptoms
- Fan runs all the time, even when temperature is satisfied
- No heat or cool air coming out, just moving air
- Higher electric bill than normal
Likely causes
- CommonFan setting left on "On" instead of "Auto"The On setting intentionally runs the blower continuously for air circulation.
- PossibleStuck fan relayA relay in the furnace or air handler can stick in the closed position.
- PossibleWiring issue shorting the fan circuitA pinched or crossed wire can force the fan to run continuously.
- Less commonThermostat software glitchRarely, a glitch can cause the fan command to stay active.
What to do
- Check the thermostat's fan setting and switch it from On to Auto if needed.
- Restart the thermostat by removing it from its base or cycling batteries for 30 seconds.
- Turn off power to the furnace or air handler at the breaker for one minute, then restore it and recheck.
- Check wiring behind the thermostat faceplate for a wire touching the wrong terminal, especially the G (fan) wire.
- If the fan still runs nonstop with the setting on Auto after a reset, the fan relay in the equipment may be stuck.
Tools and parts
- Screwdriver
- Flashlight
When to call a pro
Call an HVAC technician if the fan keeps running after confirming the Auto setting and resetting power, since the fan relay may need replacement.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.