Heat comes on initially, but won't come on again


  #1  
Old 01-09-10, 07:59 PM
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Heat comes on initially, but won't come on again

Here's what happens:
I turn on heat.
Both gas furnace and blower come on (furnace first).
Actual temperature comes up to Set Temp.
Furnace shuts off, but fan NEVER shuts off.
Actual Temp eventually falls to well below Set Temp, but furnace never relights.
If I then lower the Set Temp to BELOW actual temp, the blower shuts off.
I can then raise Set Temp to ABOVE actual temp, and both furnace and blower come on again, and the above will repeat.

I changed the thermostat and that did not fix the problem.

What's going on?
 
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Old 01-09-10, 09:31 PM
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What is the make and model of the furnace? The model can be obtained from the rating plate in the burner compartment of the furnace.

Approximately how old is the furnace?
 
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Old 01-11-10, 06:27 AM
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It's a Trane XE 78 (don't have full model number). Installed 1992.
 
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Old 01-11-10, 08:53 AM
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sounds like you are tripping on high limit. there is a sensor/switch called high limit or sometimes just "limit" that measures the temp near the heating elements. If it gets too hot it will open and turn off the burners. The blower remains to run to cool the compartment down.

You should have a diagnostic indicator LED on the front of the control module that will flash a code indicating the problem. There is usually a small clear window on the outside of the lower compartment so you can see the LED. Its easiest seen with the lights off. Report back here what the code is. I am guessing it will be 4 flashes which means "Open temp limit circuit."

There are a few things that can cause this, if you have too many registers closed in the house essentially it can over heat the compartment. Dirty/clogged air filter could cause lack of air flow across the elements which in turn will over heat them. General dirty compartment which should be cleaned every year. Those are the easy things to check. Next will be a failing limit switch, control board, etc. If you are not comfortable checking from here you should call a professional.
 
 

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