The description of your trouble points to a defective fan limit control. If it is a Honeywell rotating dial with number settings, they are subject to a ratchet operation (jerky or sticky) after many years of service. If you have the main burner operating but no blower fan on occasion it is probably this control. The fan limit switch usually will start the blower fan at 145deg exchanger temperature and shut it down at 115deg. Now this is the important part. THIS CONTROL WILL SHUT THE GAS OFF AT THE HIGH LIMIT OF APPROX 200DEG. If it is not reliably controlling the blower I would be concerned about it not shutting off the gas burner circuit. This repair would not cost a bundle if down by an honest service company. I would recommend it. Good Luck.

I agree that this is dangerous. If the blower fan does not start the heat exchange plates will become much hotter than normal. The heat exchange plates of some high efficiency condensing furnaces are anchored into plastic that can melt and even burn if the burners continue to pour heat into the furnace with no blower fan to remove it. The fan limit switch does a good job of sensing temperature in an air stream. Usually the air does not get hot enough to shut off the flame before the furnace is done heating, but with a clogged filter this may be possible.

With no air movement through the furnace, the full power of the flame applied to the metal heat exchange pates has nowhere to go, and the furnace can get way too hot before the fan limit switch finally acts. I recently participated in a fire investigation where this had occurred. There was no structure fire or damage, but it spread a lot of smoke through the house before it burned out. Needless to say the whole furnace had to be replaced.

Don't play with it too long, get it repaired or replaced very soon.

Good to know, but since I've made this thread it has not had any issues what so ever... It will get replaced anyways, we will be calling a tech next week.

~Nate

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