Gas furnace stops/restarts randomly

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  #1  
Old 02-22-10, 12:51 PM
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Gas furnace stops/restarts randomly

My 15-year-old GMP100-4 gas furnace recently stops or restarts randomly after the burners ignite and the blower starts. Sometimes it will restart the ignition process immediately one or more times and succeeds after that; sometimes it won't even try to restart at all. If it does not restart, it seems to go into lockout mode. During the whole process, I have never seen the red status light blinking at all (light always on ). I already cleaned the flame sensor though it looked clean without any buildup even before cleaning. The manual says if the flame sensor does not detect flame, it will retry for 4 times and then go into a one-hour lockup mode. Then I turned the power off/on and restart the thermostat, the exhaust blower tries to start, but stops immediately even before it gets to full-speed. Eventually it will come on again by itself randomly after a lot more than one hour. However, sometimes, it starts and completes a full heating cycle normally. My questions are:
1. It looks like a flame sensor failure, but why did it burn for so long that even the blower started before the flame sensor reported "no flame"? Why does it retry less than 4 times (as the manual says), or even no retry at all, before it stops?
2. Why does it not get out of lockout mode after resetting power and t-stat and no flashing light, though the exhaust fan moved a bit? Why is the "one-hour" lockout period not in effect and it takes randomly longer time to come back up?
 
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Old 02-22-10, 02:01 PM
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I'm inclined to think that your problem involves the draft proving switch(s) rather than the flame sensor. On my Lennox the connection to the induced draft fan will sometimes corrode to the point where it won't allow the draft to the pressure switch. I suggest that you concentrate your efforts to the pressure switch hose connections as the switches themselves rarely fail from what I have read.
 
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Old 02-22-10, 02:12 PM
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If it is the pressure switch, shouldn't it stop the process before ignition? It always ignites if it starts. The manual mentions "restart" only when flame sensor does not pass. I guess if any other switch fails, it never tries to restart. Am I right?
No being able to get it out of lockout mode as I want to is really making things worse.
 
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Old 02-22-10, 02:20 PM
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I am not familiar with your particular furnace, I only passed on my own experience.

Sorry, I have nothing else to offer.
 
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Old 02-22-10, 04:00 PM
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The furnace follows a series of events which lead to the burners staying on, the fan starting to circulate heat and remaining on until the thermostat is satisfied and shuts off the furnace.

The fact that the inducer motor operates erratically suggests a bad thermostat, bad thermostat wiring or a bad circuit board that is failing to keep the inducer motor on.

You need to use a voltmeter to check and see if the 24 VAC voltage to the "W" terminal is on and staying on while this erratic operation of the inducer motor is happening.

Alternatively, you need to find out if the "W" voltage shuts off when the inducer motor shuts off.
 
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