Coleman/Evcon Furnace won't keep running.

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  #1  
Old 11-21-08, 04:24 PM
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Question Coleman/Evcon Furnace won't keep running.

Model BGM12520B. Digital thermostat.
Keeps coming on and shutting off. Eventually stays on until blower comes on and thermostat reaches temp.
Heating and plumbing guy looked at it. Said to change the filter and charged her $125.
New filter. No change. Asked me to look at it.
Unit looks like new inside and out.
Starts normally. Draft fan comes on. Ignitor glows. Gas comes on. Beautiful flames on all jets. Flame sensor glows red. 30 seconds or so, shuts off. Repeats.
The red light was flashing when I first opened the unit, but I didn't get a count.
ok, I just learned that the single wire ceramic with the prong protruding into the flame is a flame sensor, and they can be cleaned carefully with steel wool, but I don't know how to test one.
I also see two steel buttons, one above the flame and one below, with 2 wires each. Bimetal heat sensors? Normally open?
I need to know what all to test and how.
 
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Old 11-21-08, 04:58 PM
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They are heat sensors (flame roll out switches) and they are normally closed. All of the safety devices along the 24 volt pathway remain closed when running, unless something causes them to open.

Condensate water can back up and cause the pressure switch to lose it's suction and kill the flame. Remove the condensate hose and blow it out toward the drain, and also back up toward the furnace.

If you can easily see your burner flame, stand there and watch it at a call for heat, and watch the flame before the house air blower kicks in and keep staring at it when the blower comes on and see if the flame jumps at all when the blower comes on. The flame should not move. If it does, that indicates a bad heat exchanger.

With sporadic cycling situations, you also always want to discount that the stat is shutting off also. You can test for this with a voltmeter at where the low voltage wires are hooked up in the furnace on a strip where they have small wires attached to terminals that have letters. If you are still getting 24 volts from W terminal to metal ground, then the stat is still calling for heat. If not, then we must stop right there and assess your stat, or a heat situation surrounding it. If you do not have a voltmeter, you could take some wire and jumper from R to W terminals, to see if flame goes out, still.

DID you clean the flame sensor?
 

Last edited by ecman51; 11-21-08 at 05:09 PM. Reason: added more
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