Our furnace won't turn off. I turned the heat off at the thermostat and no luck. I changed out the thermostat with a new one and it is doing the same thing. If the breaker is off, the fan and heat are off; is the breaker is on, the fan and heat are on even if the thermostat is set to off. We turned down the thermostat and the house just got hotter and hotter.

THis is a Lennox FAG, model G12RQ 3E-137.

This furnace was here when we bought the house; we estimate the age of the furnace at 20-22. It was originally wired so that the blower is on all the time, which is probably a good idea for a house with high ceilings ( some over 12' and 3 stories.

Start by reading the sticky at the top of this forum. Then with the heat shut off at the thermostat, and the breaker off, go to the furnace and remove the red or white wires that run to the thermostat. If the furnace doesn't start up when you turn the circuit breaker on, you have a short between the red and white wires somewhere. You can check them where you can get at them, round new wire, swap them with unused wires in the bundle, or buy a wireless thermostat.

If the furnace still runs, try giving the gas valve a rap. If it shuts off, it is bad. Replace it.

With a furnace that old, the heat exchanger should be carefully inspected. It is the part the gas burns in, and the supply air blows by. If it has any cracks, holes, or anything else allowing a leak, replace the furnace. If you don't feel you could spot a problem, call a professional.

You really should have that furnace safety instected, I would hope you did before the
Purchase. The serviceman should be able to cheaply repair the problem while he is there. Lennox G series with there duracurve heat exchanger is notorious for cracking in far less than 20 years. Although the CO release may be low in a hairline crack, cracks never get better. And even small amounts of CO may not make you sick according to the state of California and New York can cause cancer, birth defects and other health issues. I really don't go around pushing equipment, but your health is priceless.

PS don't let on you know of the potential for cracks, make him prove it honestly. Almost all servicemen push equipment because they get a commission or finders fee.