Problem: Fan on your furnace runs all the time and will not shut off
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: firstname.lastname@example.org 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!