There's a Compressor thermistor and an Outdoor thermistor connecting at lower left on the board diagram.
My guess is the firmware looks at both the compressor (crankcase) and the outdoor (air) temperatures and decides when to turn on heating coil.
That's how I would do it, if I were the firmware coder.
It seems like the easy way would be to cut the AC to the heater.
I can't remember how much power the heater uses.
IIRC it's about 40 watts. Maybe that's not enough to worry about.
I have a spare heater on the spare parts unit. Maybe I could install it on the compressor, and wire it in series to get 20 watts of heating.
It's not like we're in Minnesota or Norway.
