Furnace blower motor hums but won’t start is a common furnace problems. There is likely something wrong when you hear a humming sound, which is often louder than the typical operating sound. If your furnace blower motor hums but does not start, this is most likely due to a faulty capacitor or a bad blower motor.
A furnace blower motor is one of the most critical components in your furnace. It takes the heated air from the furnace and distributes it through the house. If you’re having problems with the blower motor, your furnace will not be able to circulate any heated air.
Two main things can cause the furnace to humm and not start.
- A bad capacitor
- A bad blower motor
Troubleshooting a Bad Capacitor
A starting capacitor that has gone bad is the most common reason you will have problems with a furnace blower motor hums but won’t start. Starting caps provide a phase shift to help the motor start.
The capacitor(s) are usually close to the blower motor in an electrical box on the unit. Some motors have the caps mounted on the motor, but they are behind a panel more often in an HVAC system.
The second is the contactor (motor relay) that connects the high amperage to the motor using low amperage/voltage to energize the motor.
How to Test a Capacitor
To check a capacitor, it needs to be removed from the circuit power off for a few minutes before removal. The easiest way is to use a meter with a capacitor function and see if the value in uf (microfarads ) is within 10% of the value stamped on the cap.
A more complex and less accurate way is to use an ohmmeter to put the leads on the capacitor, and watch for the value to increase. Switch leads, and the value should start high and drop ( the meter is charging the cap in 1 direction and discharging in the other). It is a bad capacitor if there is no change or the cap doesn’t charge and discharge.
If the capacitor appears bulged or leaking, it should be replaced. The cost to replace a capacitor with a professional technician will be $75 to $200.
Troubleshooting a Furnace Blower Motor
Another reason for a furnace blower motor hums but won’t start is a bad blower motor. If this is the case, you need to check the motor. Before checking for any potential electrical issues, try lubricating the motor shaft. Like with many devices that are in motion regularly, the humming could result from something as simple as improper lubrication.
Oiling a furnace blower motor
This is how to properly clean and oil the furnace blower motor.
Gain access to the fan and motor. Spray a small amount of lubricant on the shaft where it exits the motor. Some models may have been equipped with a small oil port from the factory. You may have access to the opposite end depending on the design. Spin the fan by hand a few times. Apply a small amount of oil again. Wipe off any excess, reassemble and see if the fan starts.
When you lube the motor, you should also check the belts. If the motor belts are worn or frayed, replace them with new ones.
Troubleshooting a furnace blower motor is a moderate to difficult job for DIY. If you suspect that your furnace blower motor is having a problem, call a professional HVAC technician to diagnose the system.
Checking on your furnace regularly can help you address the problems quickly. If your furnace blower motor hums but won’t start, be sure to check the capacitor and the motor.