Symptoms
- Fan never shuts off, even when heat isn't needed
- Air may feel room-temperature most of the time
- House temperature may be fine despite constant fan noise
Likely causes
- CommonThermostat fan setting on 'On' instead of 'Auto''On' runs the blower continuously regardless of heating calls.
- PossibleStuck fan relay on the control boardA relay can stick closed and keep the fan powered even after a cycle ends.
- PossibleLimit switch keeping fan on to cool the heat exchangerIf the furnace overheated, the fan may run extra to dissipate heat, which can point to airflow problems.
- Less commonWiring issue at the thermostat or furnace boardMiswired fan connections can force continuous operation.
What to do
- Check the thermostat fan setting and switch it to 'Auto' if it's on 'On'.
- Wait a few minutes after switching to see if the blower shuts off between cycles.
- Replace the air filter if it's dirty, since restricted airflow can trigger extended fan run time.
- Turn the furnace off and back on at the breaker to reset the control board.
- If the fan still won't shut off in Auto mode, the relay or control board likely needs professional repair.
When to call a pro
Call an HVAC technician if the blower won't stop running in Auto mode after a filter check and power reset; the relay or control board may need replacing.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.