How many wires are on a heat pump thermostat?

Usually a minimum of 5: R (24 volt "hot")
G (fan signal)
W (heating signal)
Y (cooling signal)
O (reversing valve: usually energized for cooling)


Many also require a 6th:
W2 (2nd stage of heat--backup heat)


Some thermostats also require
C (24v "common")


More sophisticated systems may also require
Y2 (2nd stage cooling)




The HVAC Veteran