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Is Your Furnace Blower not Working?

Tim H.Verified

Lead Building Engineer

Residential and Commercial HVAC

5,194 positive ratings
Furnace Blower Troubleshooting

Eliminate common problems, like a clogged filter, first. Next, move on to broken belts and busted blower motors. If none of these seems to be the issue, contact an expert.

A furnace uses fuel such as electricity, oil or gas to heat a series of coils. The heated coil, called a heat exchanger, has a fan that blows cold air over the coils and into the ducts. This is how your HVAC unit can blow warm air into your house. Even though the basic process is simple, there are several problems that can happen with its machinery. The following troubleshooting steps work for both natural gas and propane furnaces.

Furnace blower problems

A furnace is made up of many different components; filters, electrical, ductwork and more. Any of these can cause furnace problems that result in your fan not working. Check the following problems yourself before calling an HVAC professional.

Thermostat issues

First, check that your thermostat is on the heat setting instead of cooling. If the thermostat is not set 3-5 degrees higher than the room temperature, the heater will not go on. It is also important to adjust the setting to “AUTO.” The fan will keep going even when the heat is off if “ON” is selected.

One thing that you can do (after shutting power to the unit off) is go up to your thermostat and take the front section off and look at the wiring terminals. Find the R terminal and the G terminal. Take the wires off and twist the metal ends together, then go down and turn it back on. If the fan now runs, you have a bad thermostat and it will need replacement.

If you have a programmable thermostat, check to see if the time and date are correct. Since the point of this type of thermostat is to save energy, it may lower the temperature in the winter when it thinks you are away from home. If there is a miscommunication between dates, the furnace may not blow air. It is also possible for a thermostat to reset its dates if the batteries are low. Replace them when needed.

Power lapses

Eliminating circuit breaker problems in the beginning when troubleshooting will save you time. You can check for power by locating the service panel.

An electric outage, storm or power surge can cause the furnace’s breaker to trip or the fuse to blow, so check that first. It is not uncommon for the blower to have a separate circuit breaker, so check this too. If any switches are off, turn them on once and allow the system to run. If they trip off again, the wiring of your system will need to be checked by a HVAC contractor. For your safety, do not keep resetting faulty circuit breakers. This is dangerous and can start an electrical fire.

Blower motor broken

Is the fan actually running? You can usually look inside near where the air filter goes and determine this. Look for the blower wheel to spin. Another way to do this is to fire up the furnace. Does the furnace go on, produce heat, but then go off after half a minute because there's no fan? This happens when the internal control board notices the temperature getting too hot and not being regulated by the fan. 

If this happens, start by checking the breaker to see if it is tripped. If it is, reset it. If it immediately kicks out again, it is time to call for repairs.

A typical furnace blower motor

Also check to see if the blower motor is frayed or cracked. If this is the case, the belt will need to be replaced. Check the motor to see if it is too hot; a unit heating up too much needs to be replaced.

Filters and vents clogged

If the blower is working but there is no air movement, check all the vents to make sure they are open. Locate the cold air return and check for dirt or anything that is clogging it. A clogged filter, or one that is improperly installed, can significantly affect the air coming through ventilation ducts.

Icy coils

If the fan is actually running, but with little or no accompanying air movement, you may have an iced coil. Note that this generally only happens if the unit is in the air conditioner mode. If the coil is iced, set the system switch to off, but leave the blower on for an hour or mode by putting the fan switch in ON position. This will melt the ice; you will, however, need to contact an HVAC pro as your AC system is probably low on charge.

Plugged A-coils

If the unit is in the heating mode, the blower is running, your filter is clean, but you feel little or no air movement, your A-coil (the part that sits above the furnace) may be plugged and need cleaning. Typically most reliable duct cleaning companies can clean this as part of their service.

Bad run capacitor

If the fan is not running, open the door in the bottom of the furnace and find the blower and its wheel. Try to spin the wheel by hand. If it is hard to turn, you have a bad motor. If it spins freely, the motor can still be bad or you may have a bad run capacitor. The run capacitor is a small round or oval silver cylinder mounted near the blower. There are many articles on the internet on testing a run capacitor. If you decide to test this yourself, make sure power is off first.

Bad board

Another possibility for a blower not working is a bad circuit board. Look for the main furnace board on the unit. Visually inspect it. Are there burned off wires or signs of corrosion? If so, the board is most likely the cause of the problem and you will need to call a technician for repair.

Limit control switch fried

The limit control switch senses there is no heat being generated by your heat exchanger and tells your fan to turn off. When it gets warm from your furnace turning on, the limit control switch will turn the fan ON to circulate the warm air. It is helpful to clean the limit switch lightly with a mildly abrasive paper, like a crisp dollar bill or a very fine sandpaper. If this technique does not work, and you have exhausted all other troubleshooting tips and the fan operates by itself, the limit control switch may be the problem.

Fixing broken belts or overactive fans

Broken belt

If you have a very old gas stove (there are not many of these around anymore), check and make sure the belt or one of its parts is not broken or extremely loose. This can cause no air movement through the house. If the belt is broken, replace it. It should have markings indicating the size to replace it with.

A broken belt can prevent the fan from forcing air through the ductwork even though the motor is still working. If it is broken, follow these steps:

  1. Turn off all power to the unit. Also, turn off the valve to the gas if it is a gas furnace. Note the number stamped on the belt to get an exact replacement.

  2. Then, slip the new belt on the smaller pulley then start it on the blower motor pulley. By rotating the pulley by hand, it should slip right into place. Adjust the motor mount if it seems to be too tight and re-tighten once it is in place.

  3. Oil your fan motors and fans. Check your maintenance manual for the manufacturer’s directions.

Fan running constantly

If your fan is running constantly, it could either be the thermostat or the limit switch located on the furnace. The limit switch shuts it off if the air in the plenum (the box that distributes heated air to all the ducts) gets too hot. If the thermostat is set to OFF or AUTO, the furnace’s limit will need to be adjusted.

Replacing a furnace blower motor

After exhausting all troubleshooting for problems affecting the blower, the next step is to replace the motor.

Disconnect the power

The inside of a circuit breaker panel

Since the motor is commonly hardwired into a home’s main electrical supply, it is important to shut off the power first. Afterward, use a non-contact circuit tester directly on the motor’s wiring. This will immediately tell you if the circuit is live or off. After you are sure there is no danger of shock, you can start working on the assembly.

Remove the blower assembly

Look for the blower motor behind the furnace’s protective outside cover. It looks like a small barrel on its side with three wires coming from the housing. Note the motor’s wire color connections then cut or remove them from their connection point. There can be 2-4 bolts holding the blower assembly to the furnace’s main body. Remove these parts and slide the assembly from the unit.

Remove the fan motor

The motor is fastened to the interior of the blower assembly. Remove the set screw holding the wheel to the outside and the motor’s mount screws. This will allow you to pull the motor from the assembly.

Replacement blower motor information

Install the new motor in reverse of the above order. Before installing, verify the new one is compatible with the blower assembly by matching the specifications. Once it is installed within the assembly, the furnace can be turned on at the circuit breaker.

When troubleshooting problems with your blower, it is always best to start with the basics and work your way through the more complex problems. If you have exhausted all troubleshooting attempts on your own, and are still having trouble, you can ask a certified HVAC technician.

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