Hi there

I have a Lennox G26 furnace and a week ago, the furnace fan would just turn on by itself (the thermostat would be off at the time) and start blowing cool air. I would then try turning the thermostat to heat or AC, but it would continue to blow cool air. And it wouldn't turn off! The only way to get it to reset was to turn the power switch at the furnace itself off and back on.

Please help!

Thanks for your time!

Sounds like a bad upper limit switch. However, if it was bad, I would think the blower would come on again as soon as you turned the heat on. Try replacing the upper limit switch. If that doesn't fix the problem, you may need a new control board. $$$$$