I have a Lennox G21Q4/5-100-3 that is giving me a little trouble.
This problem went unsolved last year and we basically went without heat - there were busted water pipes and cold hands.
I've been looking at this issue on and off for the last two weeks. I have cleaned flapper assembly and replaced flapper and checked intake/exhaust for blockage. At this point, I believe the issue it that the furnace gets started and the blower may or may not start and, if it does start, shuts off when it shouldn't. I found the blower motor to have a ton of hair/crap caked in the side where the electric connects to it. Cleaned this all up but the problem persists. When the blower goes to start or when it cuts out prematurely there is usually a noticeable zzzt-zzzt-zzzt sound and I have seen a small electrical arc (not sure if that's correct terminology). I thought it was the transformer that was the problem (p/n 78H5501), but after seeing the electrical arc on what I think is a blower control (Heatcraft FTC5-EH04), I think that's it. I can usually bump this unit, or the red wire that connects to it, (sometimes zzzt-zzzt-zzzt) and the blower will fire up and usually run until the thermostat setting is reached (doesn't matter what the temp reading is on the limit control). After that, whos to say.
Next I'm going to reseat all of the connections on the blower control, this time with the power off so I don't get zapped. But otherwise I'm looking for some insight as to what going on here and if replacing the blower control is recommended (price?).
A couple of side questions:
1. Does the limit control read actual temperature in the heat exchange compartment?
2. What is the function of the expansion tank?
3. Is the heat exchanger or the furnace itself being 'beat on' by it continually reaching the cut-off point because of this blower issue?