I went on vacation for the weekend and made sure my thermostat was turned off. When I got home, the furnace was running and the house was literally 109 degrees. It was obvious the heater had been running. When I went to the thermostat, it was still in the off position and I was unable to control the AC/Heater. I couldn't stop if from running until I went to the breaker panel and turned off the breaker.

Since then, I've turned it back on at the breaker and the same problem is occurring. The heat turns on but I can not control it with either the temperature setting or the "off" switch - it just runs. Has anyone heard of this problem? If so, can I hope that this is only a problem with the thermostat and not the AC/Heater?

First try disconnecting the thermostat wires from the inside unit. Turn off power to the inside unit,label and disconnect thermostat wires from the inside unit,then turn back on the circuit breakers/power switch. If the unit does not start up and run then you know the problem is in the thermostat or the thermostat wiring. If the unit does start up without the thermostat connected then you know the problem is in the furnace/indoor unit. Basically you are eliminating the thermostat and thermostat wiring to help you discover exactly where the problem is. Thermostat or furnace/indoor unit.

I did what you said and the unit did not kick on. So, I'm hoping that's the problem. Thanks... off to the store to get a replacement.