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Furnace fan What does a fan limiter switch do? When they fail can the fan run continuously?

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  • Jim Beyer
    Jim Beyer Nov 05, 2007

    It's a Carrier and the thermostat is an electronic programmable I installed about 12 years ago.

  • Jim Beyer
    Jim Beyer Nov 05, 2007

    It's a gas furnace with a separate a/c unit (outside.) The fan will continue running after the heat cycles off and will blow cool air (obviously) until the heat comes on again. The only way to stop the fan is to kill the power at the furnace. The thermostat won't control the fan.
    I was thinking maybe the tstat crapped out on me because when you switch to "auto" and there is no call for heat the fan should switch off. The limit switch got my attention because I see a thermal device on it that I assume keeps the fan off until warm air is detected.

  • Jim Beyer
    Jim Beyer Nov 06, 2007

    OK-Looks like the tstat isn't causing a problem-disconnecting the green wire and restarting the system didn't help. The fan runs as soon as you power the unit up. Turning the system off at the tstat doesn't turn the fan off.

    I looked at the limiter wondering if the thermal device could be the cause.Is this a normally open switch that closes when the heat heats it up?



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What type of system do you have? The auto/on switch on the thermostat will cause the fan to run continuously.

Posted on Nov 04, 2007

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  • Rob Luna
    Rob Luna Nov 05, 2007

    O.K. assuming you programed the thermostat that shouldn't be the problem unless its defective. The fan limiter you are referring to is I believe the time delay relay. When you turn the thermostat off does it continue to run? Or does it come on instantly when heat is selected? Also is it a gas furnace, heat pump, electric or what?

  • Rob Luna
    Rob Luna Nov 05, 2007

    Remove the furnace panel. You should find 4 small wires coming from the thermostat and connecting to the furnace somewhere. They will be red, white, green, and blue (yellow is sometimes used for blue). unplug the cord. Disconnect the green wire from the furnace, make sure its not touching anything. Its only 24 volts, but it will short the transformer out if it touches something. turn the power back on. If the fan does not come on, set the thermostat as you would for heat, if the unit functions correctly now, its either the thermostat or the low voltage wires are jumping across somewhere.

  • Rob Luna
    Rob Luna Nov 06, 2007

    Yes, some are time delayed, yours sounds heat activated. Try removing one of the wires in the same fashion as the thermostat and see what happens.



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