[lm6y]

Matt, my old furnace did the same thing.

It was a combination of a bad igniton control unit, a small pinhole in the heat exchanger, and an intermittent flame sensor.

1. Gas burner ignites and heats (it's supposed to)

2. Relay clicks in ignition controller, gas fluctuates, fan DOES NOT operate (happens approximately 3 times) ( Flame sensor is not detecting a flame, shutting off the gas to prevent an explosion)

3. Gas burner shuts off ( Supposed to if no flame)

4. After a brief pause, fan activates ( To help clear the gas, and, or, cool the heat exchanger if there was a flame, and a bad flame sensor)

5. Fan runs for a short period of time, then gas burner lights ( can't explain this one)

6. Heater runs for a while, shuts off, then does not come back on ( The control unit hasn't gotten a response from the flame sensor, and it has entered a safety shutdown mode. if you kill the power it will start the cycle all over)

I found the hole in the heat exchanger AFTER I replaced the other parts. I had that same EXACT ignition unit until this spring. I tossed it in the trash. I would have gladly given it to you. They are right at $100.

Instead of waking up dead from CO poisoning, I just got a new furnace, and installed it myself. It paid for itself in energy savings alone in two years. I actually went from a 100,000 BTU unit to an 85,000 BTU unit, and it keeps a more even temp than the old furnace.

That was 10 years ago, so the savings would be even better now.

The limit switch should probably be closed until tripped, so it can shut the whole system down.