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indago
 
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041121 1403 - Fleemo posted:

While working in the garage yesterday, I noticed that my furnace is
cycling on and off *constantly*. I hear the gas jets kick on and they
burn for about 15 seconds, then shut off. About 30 seconds later it
repeats the process, over and over and over again. Only occasionally
did the blower kick on where hot air was pushed through the house.

I don't know a dang thing about furnaces, but this can't be normal
functioning of the furnace, can it?



In general, there are two limit switches in the furnace plenum: the fan
limit, and the gas solenoid high heat limit. It is obvious, in your case,
that the fan limit is set too high and the heat in the plenum is rising
above this setting to a point that the gas valve high limit is tripping the
gas valve closed. When it cools, it turns the gas valve back on again and
this cycle repeats. Find the fan limit and set it lower, maybe around 140
degrees and see if that will make a difference. From your description, it
is obviously not the room thermostat. If you cannot change the settings on
the fan limit switch, maybe it is broken and needs replacing.