Trane Furnace Blower Running Nearly Continuously

Old 01-02-10, 09:47 AM
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Trane Furnace Blower Running Nearly Continuously

I have a Trane Furnace UE060A936K0

Beginning at the end of the last cooling season (and continuing into this season), our furnace blower fan is running through the heating cycle and past the FAN-OFF period when it should be de-energized. The fan will turn off eventually... just before the igniters heat up and the burners catch, at which time the entire heating cycle starts up again. The tstat is set to "Auto", but if I switch it to "Off" the blower will continue to run just like it does when the tstat is set to "Auto". After a long period of time (between 10 min and 45 min) the blower will eventually turn off, and stay off. If I turn off the power switch directly to the unit, the blower turns off, and will not turn on again, until a heating cycle starts it again.

The end result of all of this is a that floats between cool and warm, with the blower motor pushing air out at us nearly continuously. If the tstat is set at 70, the house warms to 70, the burners turn off and cool air comes into the house until it reaches about 66, then the burners reignite, and warm air flows until the house gets back up to 70...

The service manual clearly states that "The control module controls the indoor blower. The blower starts approx. 45 seconds after ignition. The FAN-OFF period is approx. 100 seconds from the interruption of gas flow" and elsewhere "The indoor blower motor will continue to run for the fan off period (fixed at 100 seconds), then will be de-energized by the control module."

My initial thought was that the thermal limit switch may have gone bad. In a big foul up, I tried to replace it, but ordered the switch for the UE080A936K0, rather than the UE060A936K0. I noticed the error after I replaced the switches, and the unit ran exactly the same, I switched them back, and nothing changed. I'm not sure what that means...

My second conclusion is that the control board has gone bad. It is a White-Rodgers 50A50-571 (CNT2181), (which I believe has been discontinued and replaced by the White-Rodgers 50A55-486-04 (CNT3797)). I don't have a multi-meter, so I just observed the self-diagnosing light which seemed to be normal. It blinked slowly while the unit was off, and when the call for heat came, the blinking increased. All of this matched the label on the control.

I just wanted to see if there was a seemingly obvious answer besides the control board that I can check easily, before I replace it without cause.

Thanks for your time.
Old 01-02-10, 09:54 AM
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Sounds like a bad board to me.

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