My Furnace Blower Motor Hums But Won’t Start

Does your furnace blower make a humming sound when you change the thermostat? Like it’s trying to start, but it just won’t turn on? If this is the case then one of two things is wrong with your furnace.

The furnace blower itself has gone out and will need to be replaced – or the capacitor attached to the blower is bad, or is going bad, and will need to be changed out with one of the same specifications.

How the Furnace Blower Works

The blower fan motor on your furnace, when functioning properly, is designed to push a lot of air throughout your home’s ducts. In order for the blower to run it needs large amounts of energy to get it started and to keep air moving.

The energy requirement for the fan motor is more than the amount of energy supplied to the furnace – by way of the capacitor, the blower is able to work. The capacitor stores the necessary amount of energy required to get the motor turning and pushing the air.

Checking and Changing the Capacitor

More often than not this problem can be tied to a bad capacitor. This is good for you because a capacitor is not only significantly cheaper than a new motor, but it is easier to change and can be done without the help of a professional.

This job can be done with a select few tools – [amazon link=”B08BXJTQCV” title=”screwdrivers” /], [amazon link=”B07MSJ48RT” title=”nut drivers” /], and a [amazon link=”B01N9QW620″ title=”multi-meter” /]. Before beginning to work on your furnace you need to make sure that you have shut the furnace power switch off.

Remove the furnace’s cover

This may be secured to the furnace by any variation of screw head.

Locate the blower

This will more than likely be at the bottom of the furnace. The capacitor is going to be mounted on the blower with a bracket.

Remove the Capacitor

Once you have located the capacitor you need to discharge any potentially stored energy by placing an insulated screwdriver across both of the capacitor’s terminals. After you have discharged the capacitor remove the mounting bracket that is holding it to the motor.

Check Capacitance

On the side of the capacitor, there is going to be a plus-minus capacitance range that your capacitor needs to test between. 

To test this set your multimeter to the capacitance setting and place a meter-lead on each of the capacitor’s terminals. 

If the reading is within the range you can install that same capacitor back into the furnace. If the reading is not in between the specified range, then you need to replace the capacitor with a new one. These can be found online for around $10.


If your blower isn’t turning on but just makes a humming sound instead, then you can bet on having to replace a capacitor or an entire blower. If there is nothing wrong with the capacitor when you test it then that means your blower is broken. This repair is more difficult and should only be taken on by a locally licensed HVAC technician.