Furnace cycling off due to thermal protection.

Old 11-03-13, 11:01 AM
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Furnace cycling off due to thermal protection.

I'm helping someone troubleshoot a Trane XE80 gas furnace.

The majority of the time it will come on, the flame will kick in, stay on anywhere from 30 seconds to 5 minutes, then suddenly cut off. The blinking error light code will usually say it is a thermal protection error.

Occasionally it would not come on, and give an error code that it is an internal control error, but this is much more rare. In either case, it seems once it gets the error code, it gives up, we must turn off power to the furnace, and turn it back on for it to try again. Which most of the time would result in the furnace coming on for a minute or so, and quitting again.

I'm not completely sure what sort of flame sensor / thermocouple / flame rectifier device is used in this model, but it appears to be an elongated U shaped part with 2 wires going to it, and that part was supposedly replaced just a few months ago, it does looks new. Is it possible the the specifications on the new part may not quite be right, triggering a false no flame condition, or perhaps its positioned just a little wrong?

Anyways, any advice for testing and fixing this? Is there a way to at least temporarily bypass the flame sensor / thermocouple / flame rectifier in a safe manner just for brief testing purposes, so that we can figure out whether the control board itself is bad, or if its just recieving a bad signal?
Old 11-03-13, 01:58 PM
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Welcome to the forums.

XE-80 is a trade name not a model number.

That part you are describing is the ignitor. That's the part that glows and lights the gas. The flame sensor rod has a single wire going to it and is attached to the burner with one screw.
It's located near the part of the burner that lights last.... I believe left side on a Trane.

That rod can be cleaned by removing it and cleaning it with a scotchbrite pad or steel wool. It needs to be clean and shiny with no residue on it. The flame rod can not be disconnected to run a test. It also must be IN the flame as the flame actually carries the circuit. Make sure the burner flame near the rod is robust and not weak or wavering. It's possible that the burner itself may need to be cleaned.

However... you are getting a thermal code ? That would not be caused by the flame sensor but by a possible airflow problem. Dirty filter, closed vents, slow blower fan operation.

One style of flame sensor.
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Old 11-06-13, 08:43 PM
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Thank you for the detailed response. I will defiantly check the "real" flame sensor, to make sure its not dirty, and is in contact with the flame, and check for any intake filters that may be dirty when I go over there next.
Old 11-07-13, 09:27 AM
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In some appliances the hot surface ignitor doubles as a flame sensor. There will be nothing else in the flame. My water heater is like this and I believe my gas dryer is as well...not sure about furnaces.

You mention 2 wires...the high limit might be of the type in the picture. A bad limit can also cause the furnace to shut down early, and keep re-starting until the thermostat is satisfied, or quit--depending on the programming.
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