How To Test A Furnace Blower Motor

Furnace blowers malfunction all the time. Therefore, it helps if you know how to test a furnace blower motor, as this would allow you to pinpoint the problem and fix it sooner.

Below we discuss how to test your furnace blower motor using a multimeter as well as how to bench-test the device.

First, a recap of how the blower motor works, its purpose within the furnace, and how to tell that you have a bad blower motor.

How Does a Furnace Blower Motor Work?

The furnace blower motor is a unique fan system that helps “pump” warm air from the furnace into your rooms/home. It comprises a fan, the fan’s housing, and wires connecting to the furnace’s main control board. The fan itself consists of a motor and shaft with metal blades.

When you power the AC, the fan gets power from the control board. Typically, it draws 24V from the circuit board. The electric power turns the motor which, in turn, turns the blades.

As the blades rotate, they create air movement (airflow) that drives warm air out of the furnace through the home’s ductwork. The same airflow also helps circulate warm air throughout the designated room or area.

Where is the Furnace Blower Motor Located?

Blower motors are located in the blower compartment in the furnace’s basement next to the air filter. You’ll find it supported on rails within the bottom compartment.

How to Tell if Your Furnace Blower Motor is Bad

The first sign that you may have a bad furnace blower is loud and strange sounds. Typically, the fan makes a low humming sound. However, you probably have a bad fan if you start heating loud banging, whining, or grinding sounds. Other signs of a bad furnace fan include;

  • Poor or no airflow: If your furnace produces very little or no warm air, the fan is the most likely culprit as the appliance relies almost entirely on the fan to blow air into your home.
  • Overheating: Whenever the blower motor malfunctions, the furnace experiences increased internal heating, potentially resulting in overheating. So, watch out for overheating signs, such as relevant error codes and sudden furnace shutdown.
  • The furnace hums nonstop: This is usually a sign of a bad blower motor capacitor. It means that the motor works but the capacitor isn’t powerful enough to kickstart the fan.
  • High energy bills: A defective furnace typically works harder to keep up with heating demands. It will certainly run faster and longer. As a result, you may get a notably higher heating bill.

What Happens When the Blower Motor Relay Goes Bad?

The blower motor relay, i.e., the circuitry that provides electrical power necessary for the fan to run, essentially keeps the fan on or off. The following will happen if the relay is compromised;

  • The fan’s fuse may be blown.
  • The furnace may shut down.
  • The relay wires can become hot and melt the plastic insulation.
  • The panel or fuse box can be damaged.

How to Reset a Furnace Blower

If your furnace is has become bothersome, the first thing you should consider is resetting it. Sometimes resetting the device can resolve the internal issues thus return the fan to normal function. Here’s how to reset a furnace blower;

  1. Switch off the furnace at the breaker and turn off the gas at the valve.
  2. Lift the front panel or open it with a screwdriver, then lift it off.
  3. Locate the blower box. It’s a small silver box towards the top of the cabinet.
  4. Locate the reset button – the white or red button on the raised part of the blower box.
  5. Push/press the reset button and hold for at least three seconds. Then release it.
  6. Return the front panel, turn on the gas, and restore power.
  7. Now, restart the furnace.

How to Test a Furnace Blower Motor Using a Multimeter (Step by Step)

If resetting the furnace blower doesn’t fix the original problem, you need to test it to confirm whether it still works.

Required Tools and Materials

Non-contact voltage tester

Non-contact voltage testers are inexpensive devices that you can use to quickly and reliable check that power has been shut on your appliance. Use the tester to check that the furnace is indeed off before touching any electrical parts of the furnace. Otherwise, you risk electrocution and possibly electric shock.

You can choose from a wide range of testers, but we recommend non-contact models.

A multimeter 

Multimeters are used to test whether power is getting through an electrical device or part. You can choose either an analog or digital multimeter. However, we recommend digital models. Although analog multimeters are more affordable, digital models are more accurate and have an LCD screen to facilitate easier reading.

You also have two digital options to choose from – fluke and clamp. Fluke digital multimeters are best for beginners as they’re simpler to set up and read. However, more experienced HVAC professionals prefer clamp multimeters as they can measure a broader range of electrical metrics, including resistance, voltage, watts, amperes.

How to Test a Furnace Blower Motor (Step-By-Step)

Note that you may also need screwdrivers depending on the furnace. Once you have the above tools, it’s time to begin the test.

Turn off all power to the furnace

Before you begin testing the furnace blower motor, make sure the power and gas supply are off. Otherwise, you’re at significant risk of electrocution and gas poisoning. The best way to switch off furnace power is to turn off the breaker. You can also turn off the furnace’s onboard switch.

But turning it off at the breaker is safer in case the furnace switch is faulty. Use the non-contact voltage tester to confirm that the power is off. All you need to do is touch the tester to any wires or even surfaces within the furnace.

An LED at the end of the tester will light up if electricity flows through the furnace.

Remove the furnace’s access panels 

The furnace’s access panels (or doors) protect the appliance’s internal components from damage. They also prevent users, especially the young ones, from reaching the internal components. You may or may not need a screwdriver to remove the access panel(s). just remember that the blower fan is located behind the bottom door in the bottom compartment of the furnace.

Disconnect the furnace and common wire

You’ll find more than a dozen different wires in the blower compartment as it also houses the furnace’s main control board. Your job is to identify the two or three wires that run from the board to your blower motor. Then, check the control board to determine which wires are connected to the heating components and disconnect them.  

Next, locate the control panel’s common wire. The easiest way to identify it is to check the control panel codes. Specifically, look for “CIR” or “CIRC” marks. These two are short for “circulator.” Disconnect this wire too.

Restore furnace power 

Switch the power back at the breaker and the furnace’s onboard switch (where applicable). Then use the voltage meter to confirm that power is back on the furnace.

Test whether the blower is getting power

Now, you have two open wires and have confirmed that the blower is getting power. The next step is to confirm whether the power reaches the blower motor. Begin by setting the multimeter to volts.

Then touch the first multimeter leading to the fan’s heating wire and the other to the circulating (control) wire and observe the reading. You should get a 120V reading if your blower is getting power. Otherwise, the fan is possibly good but isn’t getting power. In this case, the control board could be compromised.

However, if you get a 120V reading but the fan still won’t run when you start the furnace, the motor is defective and needs repair or replacement.

How to Replace a Furnace Blower Fan

If the two tests reveal that the blower motor is damaged beyond repair, the only logical solution is to replace it. It’s a fairly straightforward process if you know how to remove the fan. Installing a new unit is pretty much the reverse process.

The most important thing is to purchase the correct fan. Otherwise, you may have trouble down the line. Ideally, you want to buy the same type and model as the damaged blower.

If you’d love to upgrade, consult your HVAC technician and the furnace manufacturer first. Also, make sure to install it in the proper position and the correct rotation direction. If you feel overwhelmed, find a professional to help.

Cost to Replace a Furnace Blower Motor

The cost to replace a furnace blower fan ranges from $200 for older models to over $1,200 for modern variable speed fans. Costs also vary by furnace type, size, brand, and labor charges in your area. The average replacement cost is around $450.

Summary

That’s all. Now you know how to test a furnace blower motor. Again, you don’t always have to do it yourself. Many times, professional servicing is cheaper in the long run.

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