This summer I replaced the mechanical thermostat on my AC/heat pump (an old Amana unit)
with a Hunter 44760. Everything went well during the summer months.
A few days ago, the weather in the area turned cold, and I switched the hunter to heat. Everything seemed to be working correctly (including second stage heating coming up when the set temperature is more than 2 degrees above the house temperature). However, when the temperature reaches the preset value, the fan shuts off, but the compressor does not. I have called Hunter and they sent me promptly a replacement. Same problem. The only way to turn off the compressor is to switch the thermostat off (or hit the reset button). Note that with the mechanical thermostat the compressor shuts off with the fan. Also, no problems in the summer with the AC on. I believe the wiring is correct, I have carefully checked the wiring diagram of the unit to make sure that the reversing valve and secondary/emergy heating are wired OK. Any idea what is going on?
Thank you
Alberto

I lack experience with heat pumps. I know they require more in switching from cooling to heating that separate furnaces and A/C units. My guess is that the new thermostat isn't sending all the signals the system needs to change over. Do you have a wiring diagram for the Amana? You may need to compare it to the Hunter and make sure everything is right.

I think the wiring is OK. After all, the unit works in heating mode, that means that the compressor and the reverse valve terminals are properly energized.
The problem seems to be that the hunter "forgets" to denergize them when the set temp is reached (it denergizes the fan terminal). I will double check which terminals are energized. I will also try to get in touch with somebody higher up in the technical food chain at Hunter. I wonder if some noise in the 24V line confuses the Hunter. After all, it is an old Amana unit...
Thank you for you help. I'll post a note when and if the matter is resolved.
Alberto