Symptoms
- Electrical hum from the furnace but no fan sound
- Fan blade may be still or barely turning
- Smell of hot electrical components possible
Likely causes
- CommonFailing start/run capacitorA weak capacitor can't give the motor the extra push needed to start spinning.
- CommonSeized or worn blower motor bearingsOld bearings can bind up enough that the motor hums without turning.
- PossibleObject jamming the blower wheelDebris that fell into the blower housing can physically block rotation.
- PossibleFailed motor windingsInternal motor damage can cause humming with no rotation at all.
What to do
- Turn the furnace off immediately; don't let it keep humming, as this can burn out the motor.
- Listen and look for any visible obstruction in the blower compartment if you can safely access it with power off.
- Try gently spinning the blower wheel by hand (with power fully off) to see if it moves freely or feels stuck.
- Don't attempt to test or replace a capacitor yourself unless experienced, since it can hold a charge even when the furnace is off.
- Leave the furnace off and call a technician to test the capacitor and motor.
Tools and parts
Furnace blower motor capacitorSearch: “furnace blower motor run capacitor replacement”
When to call a pro
Call an HVAC technician promptly; a humming motor that won't start can burn out quickly if left running, and capacitors require safe handling.
Safety
Use caution
Capacitors can hold an electrical charge after power is off; leave this repair to a technician unless you know how to safely discharge one.