Lennox Model 80 MGF

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Don, answers to your January 21, 2004 post -

From Harold hydronicnetwork.net

You need the flame sensor cleaned, the electrical connections cleaned, along with the gas pressure checked and the exhaust system checked for cleanliness and proper pressures EVERY YEAR. It is time.

From my post -

Several other things to check:
The inducer fan.
The inducer (vacuum) switch.
The spark module.

Have a qualified furnace technician come in check the furnace. It is expensive, but a lot better than replacing things hit and miss. Depending how cold it is in your part of the country, it could save lives.

To clarify -

What I call the inducer fan is a fan that checks to see if the flue is clear so it is safe to start the burner. This fan blows air out the flue and also creates a vacuum on input side of the fan. Mine is located at the top of the furnace where the flue pipe is attached to the furnace. This fan is the first thing I hear after the click of the solenoid when the thermostat calls for heat.

If the flue is clear, the input side of the fan produces a vacuum that activates a switch that tells the burner it can come on.

If the flue is obstructed (we had a bird fall into ours and partially plug it up), the fan cannot run fast enough to create sufficient vacuum to trip the switch and let the burner turn on.

If the fan is bad or going bad, it cannot run fast enough to create sufficient vacuum to trip the switch and let the burner turn on (this also happened to us).

If the vacuum switch is bad it won't trip and let the burner turn on. If the switch is marginal, sometimes it will work, and other times it won't (this also happened to us).

The amount of vacuum can be measured using the correct instrument (which I don't know how to spell) and determine if the inducer assembly is the problem.

A spark module can also be intermittent. This is harder to isolate. It might be wise to call a technician .

Good luck.