Here in central Virginia, most houses use heat pumps with electric auxiliary heat. I have inquired with several different contractors about installing outdoor thermostats on my units to 1) disable the use of auxiliary heat above a set outdoor temperature and/or 2) stop the outdoor compressor from running below a set outdoor temperature. No one I talk to thinks it a good idea or they have not done it before.
Looking at the schematics and examining the current wiring it seems fairly simple and the parts are very cheap. It seems like running the compressor when it's too cold to do its job and running the expensive electric heat strips when the compressor is much more efficient and able to do the job just seem like a lot of wasted energy.
There are manufacturers that make outdoor thermostat controls that are designed to do exactly what I am looking for and can be purchased at my local HVAC supply. (one Example :TPI # LRD100A)
My question is why wouldn't you want to do this? Are there downsides that I'm not seeing? (besides possible personal comfort)