I have a Honeywell Chromotherm III programmable thermostat (probably 10 years old) that controls my heat pump. Since the temperatures in Phoenix have become moderate, the thermostat was in the off position. Yesterday I left the house in the morning and returned about 6 PM. The temperature in the house was in the mid 90s with heat coming from the ducts. I was unable to shut the heat off with the temperature control. I turned off the breaker to stop the heat. A short time later I turned on the breaker and put the thermostat in the cool setting, and the A/C came on. Once the temperature had come down I turned off the breaker. Has anyone ever heard of a thermostat turning on the heat when it is in the off position. I assume that the thermostat has become senile and should be replaced.

About half of the calls I get where the people ore blaming the thermostat it turns out not to be something with the thermostat but something in either the furnace or the AC.
You surely you tried the furnace after the temp had come down to see if it was going to work, yet you don't even mention trying the heat after you turned off the breaker the first time. If you want some help you have to so a little testing to find out what is causing the problem. For instance if you turn the furnace on again and it will not turn off, disconnect the red wire from the thermostat sub base and see if the furnace turns off, if is does not it sounds like you may have a wire shorted out somewhere between the circuit board and the thermostat. Let us know what happens.

After getting your response, I turned on the breaker and the HP came on at once. I think it was still in A/C mode. I didn't want to switch back and forth from cool to heat, because that is probably not good for the HP. I will have to find the book for the thermostat to find the pinch point to separate it from the base, without breaking it. When you mentioned a short, were you referring to the thermostat/base, or the HP? The thing that puzzles me is to have an electrical device do things on its own when it is turned off.

A heat pump can go into a defrost mode and if it does and you have a wiring problem is can cause the system to do some crazy stuff.

Tomorrow in the light of day I will pull the thermostat from the base. That should remove the TS from the circuit and the HP should stop if the problem is in the TS; correct? If that fails, I will disconnect the red (24v power supply from HP transformer) wire from the base, which would eliminate any wiring problem from HP to TS; correct? If the HP continues to run then that would indicate a problem in the HP? Thanks for your assistance.

Today I pulled the TS from its base and turned on the breaker to the HP; it started to run. After shutting off the breaker I disconnected the red wire from the TS base; turned on the breaker and the HP started up; turned off breaker. Would I be correct in assuming that some relay etc in the HP is stuck in a closed position? At the start of summer an A/C tech replaced a relay and redid the TS wiring to the HP that had become frayed as preventative measures. At this point is it time to call the A/C guy back, or is there anything more that I can do?

Sound like a possible stuck contactor. You'll prob, have to call a pro. Mosat wharehouses don't sell these parts to the general public

One last thing... disconnect the two wires that come from the HP to the air handler, they should ce connected to a circuit board at the air handler or furnace which ever you have. One will be on the C, Cor common terminal and the other will be on the Y terminal. This should stop all low voltage from getting to the HP. If it then continues then you will know that the problem is in the low voltage system in the HP itself.

Last night I had my a/c on an woke up my heat had kicked on tried to mess with my thermostat but it kept acting crazy couldn't fix it even changed the batteries in the face of the thermostat still didn't work its going crazy don't know what to do