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I would like to understand if three 3-relays are enough to control a heatpump. As I understand it, 24V AC is applied to:

  • Green: Fan
  • Yellow: Cool
  • White: Heat

When designing a network connected controller, are there any other circuits that should be controlled by a relay?

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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.

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  • Fully agree, also emergency heat , some newer neat pump can get down into single digits and do not have emergency heat but it is another thing to be aware of+. – Ed Beal Jan 30 at 14:46

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