Why Furnace Blower Motor Hums But Won’t Start?

Keeping your furnace running properly is essential to provide needed heat to warm interior spaces and create hot water.

When you hear a humming sound, which is often louder and more buzz-like than the typical operating noise, there is likely something wrong. A problem is apparent when you hear the noise, but there is no motor fan rotation. 

When a furnace blower motor hums but won’t start, the most common issue is a faulty run capacitor. This electrical part stores energy to boost the blower motor into starting. If the capacitor does not require replacement, the problem may be with the blower motor itself. 

Read below to learn the likely causes of the humming and troubleshoot why the furnace blower motor will not start.

Knowing where to look in the first place will make it easier to address the issue and save you time and energy in getting the furnace back to working condition

Reasons Your Furnace Blower Motor Hums But Won’t Start 

When a blower motor hums but doesn’t start, this indicates that the furnace itself is working, but the blower motor is unable to start and do its job correctly. There are two common reasons for this: 

  • A faulty capacitor
  • A blower motor that no longer works
Image Credit: RepairClinic.com

The capacitor is by and large the most common reason you will have problems with a furnace blower motor.

Let’s look at how both of these problems can prevent heat from distributing throughout the home. 

A Malfunctioning Capacitor Is the Main Culprit in Blower Motor

The most common reason that a furnace blower motor will hum and not start is a malfunction with the run capacitor. This is an electrical part attached to the motor used to store energy and discharge it when the furnace triggers the motor to run. 

Furnace blower motors require large amounts of energy to start, and the furnace itself cannot provide the necessary energy levels.

The added electricity stored in the capacitor quickly releases to start the blower and its fan. Capacitors may need to be replaced over time as they no longer hold their charging capabilities. 

Source: HowStuffWorks

How to Check the Capacitor on Your Blower Motor

First, to address this issue, remove the outer paneling from your furnace to reveal the parts underneath. Your capacitor will be attached to the outside of the drum with a fan inside. 

While the furnace is off and the thermostat is set to low, try rotating the fan yourself with a dowel or similar tool for protection. If the fan can easily spin, the capacitor is the likely culprit. 

If Your Capacitor Needs to Be Replaced

The capacitor can and should be replaced with a new one, which is not incredibly inexpensive to do.

When replacing a capacitor, you should keep this important information in mind: 

  • Carefully remove the capacitor: Ensure that the furnace is not running when you remove the old capacitor from its place. When removing, you will need to use an insulated screwdriver to remove the wires from their terminals and avoid any electrical current accidents. Check out this video for step by step instructions.
  • Use furnace capacitors: Ensure the capacitor replacement is compatible with the furnace and matches the previous capacitor’s energy requirements. You will also want to look at the voltage and microfarad ratings to ensure they are the same number (Source: Electronics Notes). 
  • Know where to buy new capacitors: You can find capacitors online and in HVAC appliance stores. Finding a capacitor with the exact specifications is found more easily online, but you can consider calling an HVAC parts distributor as well. RepairClinic.com is a popular site for finding parts online. 
  • Consider professional help: While replacing a capacitor is relatively straightforward, you can also call a furnace company. They know the typical problems associated with humming and can quickly address the issue. The part itself is inexpensive, but installation will drive costs up significantly, which is important to consider when hiring technicians.

A Bad Motor Will Keep Your Blower Motor from Starting

If your furnace blower motor would not start after running tests on the fan and replacing the capacitor, there may be something wrong with the motor itself. This repair will be more expensive but is necessary to get it working again. 

You can either hire a professional service to replace it for you or follow step by step instructions for removal and replacement. 

Replacing a Faulty Motor 

To remove and replace the motor yourself, you should do the following: 

  • Turn the furnace off: For any removal or repair to a furnace, make sure the switch is turned off to avoid any electrical accidents and eliminate contact with hot surfaces. 
  • Remove blower: With many designs, you should be able to remove the blower entirely from the unit. This will give you better access to the motor for removal. Clean the unit while it is removed, including air filters, which can lead to buildup and malfunction. 
  • Remove the motor from the blower: You will want to disconnect the wires and any bolts that attach the motor to the blower. That includes the ground wire that allows for electrical flow. This can be done with most screwdrivers and wrenches if needed. The bracket that is attached to the motor can also be removed and used for the new motor. 
  • Replacing the motor: Make sure the new motor has the same power metrics and specifications as the old one. Working backward, you will want to perform the same steps used for removal to put the motor back in. Make sure you read the instructions for the new motor to determine precisely where wires should be placed.  

The most common signs that the motor itself will need to be replaced is when you notice humming or other odd noises, the airflow from the vents is either low or nonexistent, and your energy bill is higher than usual. 

Because this replacement is often expensive, getting a professional opinion may be helpful. A professional can determine if a replacement is even needed, saving you time and money if the problem lies elsewhere (refer back to the capacitor in most cases!).  

Source: The Cooling Company

Know Enough About Your Furnace Blower Motor to Determine the Problem 

Checking the furnace settings when you hear the humming noise will help determine where the issues are.

You will likely hear noises above the typical running hum when there is no air flowing from the motor fan, and the thermostat is set higher than the temperature reading.

The blower needs to rotate to distribute the heat being ordered by the thermostat. 

What Exactly Does the Blower Motor Do?

Understanding what the furnace blower motor does will help visualize the issue you have and identify the problems more quickly. 

The furnace blower motor is critical because it is the distribution center for heat.

As the name suggests, the furnace blower takes the heated air from the furnace and distributes it through a space to correspond with the thermostat’s request.

It also pulls in cold air from the space that will be recirculated and heated in the furnace. 

The blower motor uses a fan to push the air created in the furnace through your vents.

When the motor doesn’t start, the thermostat has triggered the furnace to kick on, but the fan will not allow for the distribution. This is the reason you hear that humming noise – the blower is unable to do its job. 

Preventing Issues With a Furnace Blower Motor

While not all prevention and maintenance are directly tied to the furnace blower motor itself, taking care of the furnace is the best way to avoid premature replacement and keep it running correctly.  

Maintenance and Prevention Tips

Consider the following maintenance efforts to keep the furnace blower motor and entire unit in the best shape: 

  • Test run capacitor proficiency: Using a multimeter, you can check the capacitor’s status after being removed. This will give you a better idea of how much juice it has left. This can also be done when you have problems with the motor and need to diagnose if the capacitor is the problem. 
  • Clean the blower: Using a damp cloth, wipe down the blower and surrounding belts to remove any dust or debris that could lead to buildup. Make sure also to include the blades of the fan. Cleaning is one of the most effective ways to avoid unnecessary problems with running a furnace and blower motor. 
  • Replace air filters: One of the most crucial general maintenance tips is to replace air filters regularly. Not doing so can lead to motor burnout. The intervals at which you replace will depend on how often the furnace is used. You can check to see how much debris has accumulated and replace it when it is nearing full. 
  • Run furnace in the off-season: You should try to run your furnace at least once in the off-season to check on its functionality and keep it in use. Like a car that sits in a garage over time, you want to give it some life so it will be ready when you do need it in the winter.

Sources: Electrical Technology, Absolute Air, Air Ductors

Addressing Furnace Blower Motor Humming and Starting 

Checking on your furnace regularly in the colder months can help you address humming noises quickly, avoid further problems with the furnace, and allow your home to receive needed heat.

If you do hear humming without a starting motor, be sure to check any debris buildup and look into replacing your capacitor to restore the furnace to working condition.