No heat on Trane XB

Old 10-28-12, 10:20 AM
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No heat on Trane XB

I have a trane xb natural gas furnace that is integrated with my ac. It is an upflow model, but installed horizontally in my attic. It is 4 years old.

The temps are high 70s in the day and low 50s to high 40s at night around here. We just started needing a little heat the last few days, after having to use the AC every day a week ago. The first time we used the heat it had a burning dust smell (which we notice every year). Within a few cycles it started blowing cold air. It wouldn't turn off even after the thermostat attempted to turn it off, the fan just kept blowing. Last year this same thing happened within the first few days of using the heater, but only did it one night, then fixed itself.

This year, it seems the probablem isn't going away. A few days ago, I removed the air filter, cleaned the flame sensor, flicked the breaker, and it worked fine. Later that evening, same thing happened, thermostat kicked the furnace on and it blew cold air, but thermostat wouldn't turn it off. I have also vacuumed out the condensate line from the outside with a shop vac and disconnected the flue to try and shake stuff out.

Everytime I go up in the attic to observe, it works fine. This morning, it started to do the same thing. I climbed into the attic and saw the furnace running with no flame. I pulled the cover to check for error codes, and just got a call for heat, but no alarm. Pulling the cover caused the unit to restart and I saw the inducer motor start, the ignitor light, no gas fire, and then the fan motor comes on. I let go of the door switch to check resistance of the sensors, and all were about 2-3 ohms. When I closed the switch again, it fired up and cycled normally.

Can anyone point me in the right direction on what to check? I am competent in electronics.
Old 10-28-12, 10:24 AM
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You can not remove the cover to view the LED code. Their should be a window to look through.... If there is not then temporarily tape the door switch to all you to take the door off and not interupt the power to the control board. When power is removed it resets the control and the code. It sounds like a limit is tripping and causing the blower to run continuous.... You need the code to help tell you which way to take your diagnosis without sitting in front of the furnace waiting for it to happen.

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