On a natural gas or propane furnace, there are typically 6 main reasons why a blower fan motor may not shut off and just continues to run. Before turning the power off to the furnace or opening the doors to the furnace, check the LED status code light (if equipped) for any error codes. On the furnace, there should be a correlating error code list. This will help in the initial diagnosis of the problem.

Reason #1: One of the 24v Electrical Safety Switches Inside the Furnace Has Tripped

A furnace may have the following safety switches. If any of these are electrically open, the blower motor may turn on and stay on until the switch automatically resets or is manually reset.

· A draft temperature switch may be installed in a furnace to ensure that there isn’t a clog in the exhaust pipe. This should automatically reset when the temp lowers.

· A thermal limit switch ensures that the heat exchanger area does not overheat. When the temp increases too high, this opens. This switch should automatically reset and close.

· A flame rollout switch ensures that the flames don’t pop back in the combustion chamber due to a crack in the heat exchanger. This switch stays open and must be manually reset to close it. Before resetting, check for a crack in the heat exchanger along with the presence of CO. This is very important and a major safety concern. If a crack is present, the heat exchanger must be replaced for safety.

· With a propane gas furnace, a low pressure (lp) gas switch may also be installed. This opens up the electrical circuit if the propane tank is low on propane, or if the gas valve supplying the furnace is turned off.

Safety switches can be measured with a multimeter (like this one used) with one probe on the ground frame or common terminal of the 24v transformer and the other probe on a spade terminal of the switch. The meter should display 24v. Next move the probe on the switch spade terminal to the other side of the switch on the other spade terminal. 24v should be displayed. Repeat this step for each safety switch until you find the switch that is electrically open and 24v is not present. Determine why the switch has tripped such as overheating and fix the problem. If the switch is the problem, replace the switch with one of the same rating.

Reason #2: A Bad Thermostat

If the thermostat is not calling for heat, air conditioning, or fan to turn on, yet the fan is running, a quick test can be performed to see if the thermostat is the problem. Simply remove the face of the thermostat. If the blower motor stops, this indicates that the problem is in the face of the thermostat.

Reason #3: Bad Thermostat Wiring

· This may be due to a thermostat wire being flattened by a staple, or having been chewed by an animal (e.g. mouse, squirrel, etc.), which leads to the R and G wires accidentally connecting.

· Use a multi-meter to check whether there is any power from G to common on the furnace control board while the door switch is temporarily held closed.

· If there is power from G to common, turn the power off to the furnace and remove the thermostat face from the thermostat back plate.

· Check the electrical resistance between R and G. There should NOT be any electrical resistance. If there is resistance between R and G, that means that the wires are accidentally connected and the thermostat wiring needs to be replaced.

Reason #4: The Control Board Has Closed Contacts that are Welded Together

· Turn on the power to the furnace

· If there is no power measured on the G, Y, and W terminals, but there is 120 volts on relay contact going to the blower fan motor either on the heat or on cooling terminal, this indicates that the contacts inside the relay are welded together. This may have occured due to high current crossing the contacts which has melted the contacts together, resulting in a normally open switch being closed.

Reason #5: Fan Limit Control Incorrectly Set or the Bimetal is Worn Out

· Some older model furnaces have a fan limit control rather than a control board. The fan limit control determines when the blower motor turns on or off in heating mode, and acts as a limit switch to turn off the electrical gas valve in the case of overfiring and excessive heat.

· The fan limit control may have a button which when pushed in manually turns on the blower motor. When the button is pulled out, the control is in auto mode. Make sure that the button is on auto, otherwise the fan will continue to run.

· When the cover of the fan limit control is removed, a dial with three temperature settings is revealed.

· If the bimetal on this unit senses the temperature is greater than the highest temperature setting, the blower motor will continue to run but the gas will be shut off.

· The middle temperature setting is the temperature in which the blower motor is supposed to turn on.

· The lower temperature setting is the temperature in which the blower motor is supposed to turn off. The problem could be that the bimetal is worn out due to expansion and contraction, and/or the lowest of the three temperature settings is too low. The setting should be 90-95 degrees F. If this setting is set at 90 or 95 degrees F and the blower fan continues to run, turn the dial a little to see if it temperarily shuts off. If it does, the fan limit control bimetal is worn out and the fan limit control should be replaced.

Reason #6: Bad ECM Motor Module or Control Board

Furnaces may be equipped with an ECM motor instead of a PSC motor equipped with a capacitor. Some common ECM motors are the Genteq 2.3 and 3.0 versions which are true variable speed motors. Other motor versions exist that are ECM constant torque (otherwise known as ECM multispeed motors) such as the X-13 and Broad Ocean models. If there is no power measured on the G, Y, and W terminals, but the ECM blower motor continues to run, the blower motor module may be at fault. Testing must be performed to determine if the motor and/or module are bad.

For variable speed motors, use the TECMate Pro or Supco Tester to test the motor module to determine whether it needs to be replaced. If it is an ECM constant torque or multispeed motor, use the testing method in this video- https://youtu.be/AHAbkElOceY

Still looking for an answer why the furnace blower fan motor wont shut off? Check out our video! "Furnace BLOWER FAN Motor WON'T SHUT OFF! 6 Reasons Why the FAN KEEPS RUNNING!"

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Follow us on Instagram! Published: 5/3/2022 Author: Craig Migliaccio

About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel