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Old 01-14-2015, 09:46 AM   #1
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Burners light, but I don't hear the igniter


I have a Lennox 78UGF3-75-1 nat gas furnace from 1992. Normally I hear the snap-snap-snap sound of the igniter, then the burners light.

Lately the burners are lighting, but I do not hear the snap sound.

Could the pilot valve could be stuck open allowing the pilot to remain on, then the ignition module senses flame and does not need to run the igniter?

Or could the spark be weak so I can't hear it?

When the burners are off, and I look into the burner area I do not see
any light. Do I need to remove the cover to see it?

I am concerned that the lack of igniter sound is an early warning of a problem.

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Old 01-14-2015, 10:00 AM   #2
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Burners light, but I don't hear the igniter


Not familiar with that specific model but I am a Lennox tech. Post a pic of it with the doors off. It may be similar to the 80 MGF. The spark may be getting weak but still working. As far as I know they never used a pilot light and valve until we got to the G20 Whisperheat so I doubt you have one. Probably has a spark ignition in front of one of the burners that lights them. At that age the circuit board/spark igniter may fail from old age. If it has the old Heatcraft board those are obsolete and have a expensive and difficult retrofit board. I would invest my $$ in a new furnace as it may turn into a $$ repair pit soon.

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Old 01-14-2015, 11:14 AM   #3
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Burners light, but I don't hear the igniter


It has the same ignition/pilot system as the G20. The replacement ICM would
be the Lennox 30W33. Sometimes I hear the snapping, sometimes I don't.
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Old 01-14-2015, 11:19 AM   #4
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Burners light, but I don't hear the igniter


Well if it works it works. I have never seen one with a stuck on pilot flame and I have worked on thousands of them. The thermostat opens the damper door on a G20 and it is all interlocked so I doubt it could bypass the startup sequence even if the pilot stayed on.

Not a hard fix to change to the 30W33 so you can do it yourself if necessary. Remove the door and see if the pilot is stuck on. Never had it happen yet.
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