OK, here is a simple relay drawing.
What is important is that the control need only turn on the heat pump through the pins on the left.
24vac comes from the HP, to the relays controlled by the arduino and the the arduino needs to determine which pin to apply power to and at what outdoor temp. As noted before, below 17C (for example) the system will be in heat mode and over 25C, it will be in cooling. I think temps need to be sustained for some period of time ( perhaps 12 hrs?) before a switchover to allow for hot days and cool nights.
I hope this helps. Acc on R1 is the arduino output that turns off the HP and turns on the electric boiler when outdoor temps are below -15C.
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