I have a 15 year old 70000 BTU high efficiency gas furnace, and it is cycling every 2-3 min and the house is COLD (about 10 degrees lower than comfortable)

The small exhaust fan comes on, the igniter turns on, the pilot turns on, the pilot sensor heats up and then the main gas goes on and burns, then the main fan comes on and then a minute later it shuts off and 1-2 min after it starts the same cycle over again.

The temp limit system does not reach max temp, and it does not look or feel like the system is remotely overheating.

What could be wrong?

My guess is the main limit, make sure 24 volts are going to and from it during the heat cycle.

My guess is the main limit, make sure 24 volts are going to and from it during the heat cycle.

Oops, Put an "it" in the wrong place. :( "It" should refer to "burner" turns off, not "blower".

Main blower fan upper limit switch works fine.

On a hunch, I installed a separate ground wire for the flame sensor. (from the sensor mounting bracket to the burner ground on the control module.) Now burner stays on, but exhaust fan won't quit when the flame turns off! Darn thing is chock full of gremlins.

But, I have found a clue for that problem too. It seems if I tap the exhaust fan relay the exhaust fan will turn off. I am guessing the relay is sticking and needs to be replaced. The relay is located up near the top of the burner chamber albeit on the control panel side. Does it incorporate a high temp limit too? If so, it might explain everything. If not, a marginal flame sensor and a sticky relay make a nice combo explanation.

Anybody know of a good on-line high effiency Gas furnace "theory/repair" manual?

Susquatch

Is this a Carrier/Bryant furnace?

Is this a Carrier/Bryant furnace?

No, It is an Armstrong.

But I think I have found the problem. The exhaust fan blower relay is sticking closed. If I tap on the relay, it will release and the exhaust fan quits. I believe the relay contacts are pitted from arcing and are fusing.

Cheers!

Sounds like a plan