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)
Best Answer
When a heat pump is installed it should be set up to turn on the electric heat strips when it drops below the point where the heat pump is more efficient. This is calculated on the units performance which should be specified based on the predicted (Charted) lowest anticipated temperature for your area. If it is using the compressor it will defrost either according to a time schedule or, on more efficient models, when it senses the outdoor coil is too cold and needs to be defrosted. When it does this, it turns off the outdoor fan and reverses the flow of refrigerant into cooling mode. It activates the heat strips to keep the air inside running warm. When it reaches a set temperature or after a set period of time, it will change back into heating mode.
They showed us this in my Residential HVAC Technician Class last year. Should do everything you want it to. All in one universal replacement heat pump defrost controller with included sensors can be set up for demand or timed defrosts. https://climate.emerson.com/documents/47d01u-843-universal-heat-pump-defrost-control-specs-en-us-1569854.pdf About $81 on Amazon.