My wife had our ac set on 78, well it has gotten pretty cold here lately so today while I was gone she turned the heat on. Now 6 hours later I had to turn off the power for it on the breaker to get the heat to turn off. The furnace was set on 73, but when I got home the thermostat was sitting at 90 and my wife was freaking out. I tried changing the unit to off, but it kept blowing, I also tried turning it down, but that didn't work either. My only option was to turn it off by the breaker. I was wondering if anyone knew what to do??

Hi,
It sounds like a switch of some kind isn't working properly, probably some type of solenoid switch. I suggest calling a repair person to look at it. He/she can test it, and find the problem quickly. The same type of situation happened with my ac/heat pump. A solenoid switch went bad, causing the "defrost" cycle to not start. I would NEVER have found that myself. It took a repair person.

You do have a furnace, not a heat pump? It is fairly common for the blower relay to stick, but it would a real coincidence for the heat valve to stick too. I am guess it is the thermostat, cheap and simple to replace. First, try switching it to cool. If that doesn't work, shut the power off and remove it from the wall. Remove the red and white wires noting if they come off terminals marked R and W. With them not touching, turn the power back on. If the heat doesn't come on, the problem is in the thermostat. Note, the burner will come on until the furnace heats up and the blower, which also runs a little after the burner goes off. If the heat comes on, go to the furnace and disconnect the white or red wire. If that fixes the problem, it is the wiring between the furnace and thermostat. If that still doesn't stop the heat from coming on, likely the control board is bad. Harder and more expensive, but still can be a DIY project.

Check the things I suggested, and post back if you need more help correcting the problem once you identified it. If you click on the Heating and Air Conditioning link, you can look back and see where I have helped many people work out minor problems. Thermostats work on 24 volts AC. About the worst you can do if you work with the power on, is blow a small fuse that protects the transformer, or the transformer itself, if no fuse. There is no need to call a professional unless these simple checks don't locate the problem.

I have found out that my problem is the wiring between the furnace and thermostat. I disconnected the white wire turned the power on, the heat did not come on. I reconnected it and disconnected the red wire and the heat still did not come on. How do I know which wire is bad and needs replaced?