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Blower won't turn off after demand for heat.

snackerdog Member Posts: 2
edited September 2015 in Gas Heating
My furnace is a Trane XE-90 model TUC100C948B0. After I turn on power to it and adjust the thermostat to demand heat it ignites and after a while warms up and then the fan motor turns on. It runs okay and once temperature has been made the gas shuts off as it should. The blower continues to run and after a long while the room temperature drops well below the thermostat demand point. The fan continues to run unless or until I cycle power to the furnace. Note: It's not the blower "fan on" on the main thermostat. Which does not turn it off even though it says off. I've tried that. I think it's an internal thermostat however, I'm not sure which one. There are two, one on the plenum and one on the fan I think. Then there is one on the exhaust motor along with a flow switch. Could someone point me to the right part and maybe even offer a part number to order a new one? Thanks, Brian...


  • Ironman
    Ironman Member Posts: 6,868
    If it was a limit switch within the furnace, the blower way stay on and the burner would not light.
    You're gonna need a tech that has a DMM and knows controls.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • snackerdog
    snackerdog Member Posts: 2
    I have a "Digital Multimeter" and I know how to use it.. I'm an electronics guy not an HVAC guy. Just not sure which component to test. Seems to me there would be a thermostat in the fan circuit that would open when the temperature got cool enough maybe 70-90 degrees or so. The relay control system is a mystery on this furnace. Trane proprietary. There is no schematic of the control circuitry just the external sensors, switches and motors etc.. Thanks though, I appreciate your input..
  • Tim McElwain
    Tim McElwain Member Posts: 4,506
    Give me the numbers on the controls and I will walk you through the procedure for troubleshooting.
    The majority of control is on the Electronic Fan Timer or it may have an Integrated Furnace Control.

    All furnaces will have several "limits" that if there is one of them that is open the fan will continue to run to protect the heat exchanger from possible thermal stress.

    Seeing as you are in electronics this should be easy for you, find which switch is open when it should be closed then temporarily jump it out and see if everything reverts back to normal.

    If you still have trouble get me the numbers and I will help you that along with some pictures of your system would help.
  • wogpa67
    wogpa67 Member Posts: 238
    Nest thermostat?