Relay requirements for Heatpump Thermostat
Don't forget the reversing valve!

While a normal air conditioner outdoor unit only needs a single control signal to it to turn the compressor (and condenser fan) on and off, a heat pump needs a minimum of two control signals: one for turning the compressor on and off, and a second for the reversing valve that selects whether the heat pump is heating or cooling.  Furthermore, there's no set standard for which "way" the reversing valve signal works.  Most heat pumps use an "O" signal, often keyed to an orange wire, that is energized to put the heat pump in cooling mode; however, Rheem/Ruud units use a "B" signal, often keyed to a blue wire (with a different color used for C), that is energized to put the heat pump in heating mode, instead.