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Problem: I changed out my control board and my furnace blower motor continues to run constantly.

Problem: I changed out my control board and my furnace blower motor continues to run constantly. Even if I disconnect the thermostat wires it continues to run the blower. The only way I can shut the blower off is to turn the power to the furnace off. What could be the problem?

Answer: Most of the time when a blower will not shut off it is an open limit or rollout switch problem. Many furnaces (Bryant & Carrier) when first powered on will run the blower for a minute or two then shut off. This is a safety function of the furnace to eliminate any heat that is in the heat exchanger before the furnace begins its heating cycle. If your blower is running all the time… I would suggest that you test your limit control and roll-out switches to make sure they are not open. I have some pictures of limit controls and roll-out switches below. If a rollout switch or limit is open, the furnace thinks it has over-heated and the control board makes the blower run all the time to cool the furnace down. You would need to test these with a meter to make sure they are not open. Is your control board flashing a blink code? If it is blinking a fault code, try to read what the code says the problem is. The fault code key is usually located on the furnace door. Please click here if you would like to see the limits and rollout switches that we sell.  Another problem that could cause the blower to run all the time would be a faulty thermostat that is sending 24 volts through the green fan on wire. Please make sure your thermostat is set to “Fan Auto” and not “Fan ON”.  I would like to suggest that you take the green wire off the “G” terminal on the control board and see if the blower stops. If the blower stops after the green wire is removed then this indicates that you have either a thermostat problem or a thermostat wire problem. If the green and red thermostat wires are shorted out then this would cause the blower to run all the time. A stuck relay (contacts welded shut) on the control board can cause this problem too. Below we have a very informative YouTube video that explains how furnace limits and rollout switches operate and how to test limits and rollouts. If you have any other questions please feel free to email us at: support@arnoldservice.com or comment in our comments section below. We would love to have your business.

 

Limits

Above-two limits and one rollout switch in the middle:

2 thoughts on “Problem: I changed out my control board and my furnace blower motor continues to run constantly.

  1. Does this apply to electric furnaces too?

    1. Hi! Yes, if one of the limits on an electric furnace are open then the blower could run all the time. This could also be a stuck fan relay on the control board or a thermostat that is sending 24 volts on the green wire all the time. So you might need a new thermostat, control board or limit. A shorted thermostat wire could also cause the fan to run all the time. You should be able to test with a volt meter and find out where the problem is. Thanks for asking this question. Steve

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