Furnace Blower Motor Hums but Won’t Start

It’s likely a faulty capacitor. When the capacitor is damaged, the motor will hum, as if to come on, but eventually not run. It could also point to a malfunctioned blower motor.

However, a malfunctioning motor usually comes with several additional symptoms. Indeed, the fan will likely stop altogether rather than hum.

The biggest challenge with this issue is that it’s complicated to diagnose. If you’re entirely new to furnaces, you may want to call an HVAC professional immediately.

However, if you’re a DIY enthusiast, you can conduct a few tests to get to the root cause of the problem. Read on to find out how.

What’s a Furnace Blower Motor Capacitor?

The capacitor is a small electrical component attached to the motor. Its primary function is to store energy and discharge it whenever the motor needs it to run. So, already, you can tell that if the capacitor is damaged, the motor will have no source of power, thus won’t run.

You might be wondering why the blower motor shouldn’t just get power directly from the main supply. The reason is that the blower motor needs substantial amounts of power to start. The furnace alone cannot supply the amount of energy needed during the start.

Under normal circumstances, the furnace can only supply 110-120 volts. Meanwhile, the blower motor needs up to 300 volts to kick on. Capacitors store up to 400V+ to facilitate the process.

Most furnace blowers have only one capacitor, known as a run capacitor. However, some have the run capacitor and a second one known as a “start” capacitor, though dual-run capacitors are most common in ACs where you may need to run two fans simultaneously.

Diagnosing the Capacitor

Fortunately, diagnosing the furnace blower motor capacitor isn’t too difficult as long as you’re handy with electrical components.

Begin by re-confirming the issue. When you turn on the furnace, you should hear the blower humming without air coming from the vents. If it’s humming but producing air through the vents, it’s likely not the motor. Additionally, the blower fan shouldn’t be rotating. If it’s rotating, it’s likely something else – not the capacitor. After verifying these two conditions, it’s time to troubleshoot the capacitor.

NB: Only proceed if you’re fully prepared all the way. Otherwise, call a professional HVAC technician to handle the job.

The task is to find out that the capacitor is indeed the problem. So, we’ll use a two-step strategy. First, we need to determine whether you can start the blower motor manually (by bypassing the capacitor). Then, we also need to test the capacitor further to prove that it’s damaged.

How to start the blower motor manually.

You’ll need a wooden dowel, paper towel tube, and a long screwdriver for this process. We don’t recommend using your bare fingers.

  1. Figure out the way the blower should rotate. In most cases, you’ll find a directional arrow on the housing.
  2. Shut off the furnace from the switch. The switch should be located close to the furnace. The button is similar to a light switch. Then, lower the thermostat to the lowest setting.
  3. Using the wooden dowel, attempt to rotate the fan in the right direction (as determined in step #1 above). If it rotates effortlessly, it means it’s functioning optimally, further proving that the capacitor is likely at fault for the lack of air at the vents.
  4. With the furnace off (check again to confirm), turn on the “Fan-Only” mode on the thermostat and listen out for the humming noise. Does the fan rotate? It shouldn’t rotate.
  5. Turn the furnace back on and listen out again. Can you hear the humming noise? Also, does the fan rotate?
  6. If you can hear the humming but the fan isn’t rotating, use the wooden dowel to do a fast nudge or flick on the fan. This helps kick-start the fan rotation by overcoming the inertia.
  7. If the fan rotates after giving it a nudge, you’ve just proved that the capacitor is the problem. Most likely, it’s burned out. If not, then it’s possibly not the capacitor. Maybe the blower motor is damaged.

Conducting further tests on the capacitor

This step requires even more courage as it involves more electrical work. After turning off the furnace, proceed as follows;

  1. Locate the capacitor: Begin by removing the panel door to the furnace. The capacitor is typically located close to the blower motor and has wires running to the blower system.  
  2. Discharge the capacitor: Pick up an insulated screwdriver and place it horizontally along with the two terminals.
  3. Take out the capacitor: Unscrew the bracket and pull off the connectors. Then, slip out the capacitor and pull off the connectors. This shorts the capacitor.
  4. Read the capacitor charge: Check for a microfarad (MFD) number. It’s usually between 9 and 11. Then, verify with a multimeter dial.

If the multimeter returns an OK reading, the capacitor is in good working condition. However, you have a problem if the multimeter reading doesn’t reach the 9-11 range. It means the capacitor is wearing out.

As we mentioned earlier, the furnace blower motor needs upwards of 300 volts to start. If the worn capacitor only gives out, say, 200 volts, it wouldn’t be enough to kick start the blower. So you need to replace the capacitor.

Finally, you may also fail to get a reading. If you go an entire minute with no reading on the multimeter, it means the capacitor is dead and needs replacing.

How to Replace a Furnace Blower Motor Capacitor

Now that you’ve proved beyond doubt that you have a faulty capacitor, you need to replace it as capacitors cannot be repaired.

  • Always use a furnace capacitor: Ensure that the new capacitor is compatible with your furnace. This is very important.
  • Make sure it matches the old one: Does it match the old capacitor’s energy ratings? Does it have the same voltage and microfarad ratings?
  • Know where to buy: Never buy furnace capacitors from just anyone. Instead, to be safe, only purchase from verified HVAC stores.

How to Replace the Capacitor

Before you even think about removing the capacitor, make sure the furnace is off. Otherwise, you’re staring at electrocution. With the power off, locate the capacitor as discussed earlier and proceed as follows;

  1. Take pictures of the wiring: You need a reference point when replacing the malfunctioned capacitor. 
  2. Disconnect with wiring: Use an insulated screwdriver to remove the wires from their terminals. You can also use your fingers. 
  3. Remove any bolts holding the capacitor: This should also be a straightforward process. Most of the bolts come off with ease. 
  4. Insert and bolt the new capacitor: Use the same bolts you removed from the old capacitor to avoid damaging the threads. 
  5. Connect the wiring: This is where the pictures you took earlier come in handy. Be extremely careful as one mistake can damage the entire blower system. 

Now, you can power on the furnace and test drive the new capacitor. It should generally function if it’s the perfect match.

Next Steps

We strongly recommend having a professional technician replace the capacitor for you. However, if you decide to do it yourself, practice extreme caution. Additionally, as we mentioned initially, sometimes a blower motor may also refuse to run if the motor is damaged. If you’re replaced the capacitor, but the blower motor still won’t run, maybe it’s the motor itself. You’ll need an HVAC professional for this one.