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posted on 12/31/2008

This has been ongoing and intermittent for about 5 years. I've had six different techs come here and none can fix it.

It's a Lennox Elite furnace - approx 9 years old.

Furnace runs fine when "rebooted" (power off/on) until at the end of a certain number of  heating cycles the inducer motor fails to turn OFF and runs until the next heat request from the thremostat. Upon next heat request, the blinking lights indicate "Heat Request" for about 5 seconds, the ignitor does NOT glow and it slips into a "Pressure Switch" indicator fail state (blinking lights indicate Pressure Switch). No heat until I come home and reboot again.

* I've had the pressure switch replaced once before in the last 4 years.
* I changed thermostats upstairs and it still happens.  
* I've had a tech check the pressure on the system and it's fine.
* I cleaned out the inducer motor housing blew through the hose.
* The ducts are all clear. I checked on the roof and we disassembled above the furnace.

It's not wind over the chimney causing a pressure issue because it also happens on completely calm days and does NOT occur on some days with 50 mph winds.

At this point I don't see how this CAN'T be the circuit board causing the problem. How would the inducer motor continue to run when power to the motor should have been shut off. Unless there's a switch in the motor itself controlling it, power to the motor is constant and that SWITCH is failing, I would think it's the circuitry that's at fault. But then again what do I know.

Seems to happen mostly during the night and early morning, although I'm not positive that was always the case.

Any help would be greatly appreciated.

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