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Problem: Your fan on your furnace runs all the time and will not shut off

Problem: Fan on your furnace runs all the time and will not shut off

Solution:

1. Check to make sure the fan is not in the fan “ON” position on your thermostat.

2. If your furnace goes off on high limit or if one of the rollout switches are open then the furnace fan will run all the time because the furnace control board is telling the furnace that the furnace has over-heated and the fan runs to cool the furnace down. Check to make sure the limit and rollouts are all closed. You will need a volt meter to test. We sell a good volt meter on the following page: Please click here for a real good meter and testing instrument. You might want to purchase an less expensive volt meter from Radio Shack, Walmart, Home Depot.

3.  Check to make sure your air conditioning fan relay or control board fan relay is not stuck or welded in the on position. The contacts on your air conditioning fan center relay or control board could be welded closed, causing the fan to run all the time. You might try lightly tapping on the Air conditioning fan relay to see if the fan will go off? We sell Fan Center Relays on the following page: Please click here for fan center relays.  Many of the new furnaces have the control board control the fan off and on timing. If you would like for us to see what control board your furnace or air handler uses please email anytime at: support@arnoldservice.com We have programs where we can look up parts.

4. Please make sure you are not getting 24 to 28 volts AC between the G (green) and C (common) wires on your control board. Set your thermostat to “Fan Auto”.  Your thermostat should not be calling for heating or cooling.  If you are getting 24 volts between the G (green) and C (com) thermostat wires on your control board then you might have a bad thermostat or shorted thermostat wires.  You should only get 24 volts between G and C when the fan is set to “ON” or if the air conditioner is turned on.

5. Sometimes the fan limit control or rollout switches will  stick open. You might need a new fan limit control. On many of the rollout switches they are manual reset and all you have to do is press a button on the switch in. I have been on many service calls when all that I had to do was press a button in on the rollout. The rollouts and limits are safety devices to protect you and your family. Please make sure you see why the limit or rollout has opened up. Most of the time limits and rollouts open because the furnace has over-heated. Sometimes the heat exchanger is leaking or the vent is stopped up and you will need to have an HVAC Tech inspect to see why the limit or rollout opened. This can be a dangerous condition for you and your family.  You might need a new Fan Limit control. There are many different types of controls. We sell some fan limit controls and rollout switches on the following page: Please click here to see our limits and rollout switches page. Please send us your furnace’s model number if you would like for us to look it up and see which limit and rollouts your furnace uses. We would love to try and help you out!

We have a YouTube video that Acservicetech was kind enough to make which shows how to troubleshoot this problem with the fan on a furnace that will not shut off. Thanks Acservicetech for taking the time to make this video!


11 thoughts on “Problem: Your fan on your furnace runs all the time and will not shut off

  1. Hello.
    I have a two year old 5 ton Rheem heat pump. In the cooling mode everything works fine the inside fan turns off outside fan and compressor shuts off when it reaches the set point . In the heating mode everything works fine except when it reaches the set point and the compressor shuts off the inside fan will run all of the time. I changed my thermostat to a Honeywell RTH5160 series and followed the set up options and the fan still runs all the time in the heat mode after the compressor turns off and the set point is satisfied. With the new thermostat I tried to turn the fan to the on position and it will not turn on. I also tried to turn the thermostat to the off position and the fan would not turn off. The only time the fan will shut off is in the cooling mode. I have a stack able relay and I changed it with no results. I have not changed my board or the thermostat wire yet would that be my next step. Thank You

    1. Hi David! I would suggest that you test with a volt meter to see if the thermostat is sending 24 to 28 volts AC through the green wire. So you would set the thermostat to where the heat should be off and the fan not blowing air. If you are getting 24 volts AC between the G and C (com) thermostat wires then I would say you have a thermostat problem or a shorted thermostat wire problem. If you are not getting 24 volts through the green wire “G” then I would think you have an air handler control board or fan relay problem. You could also try removing the G green wire from the thermostat and see if the fan stops. If it stops then you have a thermostat problem. I hope you can easily find and fix the problem. Steve

  2. So my fan turns on then the igniter glows it fired a for a second then flamesstop and it keeps repeating every about 15 second with the fan staying on but won’t stay lit

    1. Hi Joshua! If you are referring to the fan as the draft inducer running then this would be normal as long as the thermostat is calling for heat. This sounds like you have a flame sensing problem. If you have not cleaned the flame sensor then I would suggest that you do that. We have a post about cleaning the flame sensor on the following page: https://arnoldservice.com/problem-your-furnace-main-burner-ignites-the-main-burner-stays-on-for-8-to-10-seconds-then-shuts-right-back-off/ If the blower motor is staying on then you might have an open limit, rollout, thermostat or thermostat wire shorted out. We have lots of furnace troubleshooting advice on the following page: https://arnoldservice.com/gas-electric-furnace-troubleshooting-simplified-2/ I would suggest going through the troubleshooting flow chart to see what is wrong: https://arnoldservice.com/wp-content/uploads/2014/12/furnace-troubleshooting-flow-chart.pdf We also have lots of troubleshooting questions and answers on the following page: https://arnoldservice.com/furnace-troubleshooting-questions-answers-index/ If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem. Steve

  3. Had a new Goodman AC unit installed first month it ran ok but now when the inside temp reaches the cool set point, the inside units shuts off but the outside units stays running and sends out cold air and then I see frost start to build. Please advise

    1. Hi! More than likely the contactor contacts might be welded closed or one of your thermostat wires are wired wrong or shorted out. A thermostat can also cause this problem where the thermostat has failed to turn off the outdoor unit. We have a post about this problem on the following page: https://arnoldservice.com/problem-air-conditioning-outdoor-condensing-unit-or-heat-pump-unit-will-not-shut-off-it-continues-to-run-no-matter-what-you-do/ I hope you can easily find and fix the problem. Steve

  4. indoor fan motor runs quarter speed even after thermostat is in the off position

    1. Sounds like you possibly have a stuck relay on the control board that is keeping the fan running. I would suggest lightly tapping on some of the relays on the control board. If this does not work then you might need a new control board. I would suggest taking the green “G” wire off the control board to see if the fan still continues to run. A furnace limit that has opened up (furnace over-heated) will cause the fan to run continuously because the control board thinks the furnace has over-heated and needs to cool down. Please check your limit to make sure it is not open. I hope you can easily find the problem. If you want me to look up parts then please send your furnace’s model number to our gmail email account: arnoldservice@gmail.com

  5. heil gas furnace mod # nule 125 ak01 its used in a church i attend,we leave the thermostat set on 55 deg when not in use..but when we turn the thermostat up to 70 and it reaches 70 the blower fan motor turns off like it should after about 3 minutes .but when the heat cycles on again and the gas burner eventually turns off the blower fan motor runs all the time…is it the fan limit switch bad? it has a dial switch set at 100 deg off , 150 deg on and hi limit at 200 deg…thanks….

    1. Hi Mr. Collier!
      Thanks so much for your interest in our website! I would suggest that you watch the dial on the fan limit control and see if it moves below the 100 degree off setting. If it doesn’t move below that off setting then you might need a new fan limit. I would try changing the fan off to a higher temperature just to see if the control will turn off. I show how to adjust the fan on/off tabs on the following page: https://arnoldserv.wpengine.com/problem-your-fan-shuts-off-too-soon-cycles-off-and-comes-back-on-within-30-seconds-to-a-minute-or-runs-too-long-and-will-not-shut-off-until-it-is-blowing-cold-air/ It looks from the parts list like your church furnace has a 5″ fan limit. Here is a screen shot showing the parts for your church furnace. We sell a 5″ fan limit control on the following page if this turns on to be the problem: https://arnoldserv.wpengine.com/product/honeywell-5-inch-insertion-fan-and-limit-control-l4064-b-2228/ I would also check to make sure that the thermostat is not sending 24 volts to the fan control. I would suggest testing with a meter between the G and C terminals to make sure there is not 24 volts coming to the control board from the thermostat causing the fan to run. If you do not have a meter you can remove the green wire from the control board to see if the fan stops. I would suggest making sure that all your limits and rollouts (if equipped) are staying closed. If one is open it will cause the fan to run all the time. Thanks so much for trying to help fix your church’s furnace and thanks for your service to God’s Kingdom. I hope you have a blessed day! Steve Arnold, arnoldservice.com

  6. I will be happy to try and answer any questions you have. Thanks so much for your interest and support in our website! Hope you have a blessed day! Steve

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