HPs in North america, like our forced air heating systems, have a W for heat, Y for the compressor, O/B for the the reversing valve (which is dependent on where the unit is destined to be placed. In the American south, the RV is usually set to cooling, power off as it is a cooling dominant climate and in the great white north, it is the other way around. Having both available means it can be tailored to specific needs). Where I am, O will power the RV to cool. So, on a cooling call, the stat will call for a fan, Y and O and in heating (in this case) it will only call for W. This is all outside of the control of the arduino which is just needed to switch over the HP from heat to cool at specific temps and turn on the electric boiler.
The HP does look after all it's own functions including switching the circ pump to the buffer tank.
The HP has 24Vac which runs to the stat so there is 24V at any switched pin.
No extra power is needed other than to provide power to the Arduino.
The reason for regular relays is that I do need some DPDT ones to switch off a heating circ pump on the buffer tank when the back up boiler is running (which has it's own pump). This is one of the outputs from the arduino which reads the outdoor temp at -15C and will shut off the HP in favor of the electric boiler.
The programming is quite house specific and I have determined that the HP will easily heat this house down to -15C but it is not great when colder than that.
At one time I did find some reed relays that would work directly with the max pin current of the arduino but they were not DPDT so i didn't buy them. It may be available.
Drawing will be coming soon of the wiring.
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