system allow both heat pumps to have independent defrost cycles the accessory transformer and isolation relays are packaged and pre wired in a control box for easy connection in the field application_number US 94481301 A description the following nomenclature is used for the thermostat fan coil and heat pump connection terminals in fig1 4 r 24 volt hot lead from transformer fused c 24 volt common lead from transformer g energizes indoor blower motor y y2 or y energizes outdoor unit contactor which controls compressor and fan motor w w1 or energizes first stage heat on non heat pump w2 systems or second stage heat on heat pump systems o energizes reversing valve on heat pumps only t1 24 volt hot lead from transformer non fused referring to fig1 a single stage control diagram for a twinned heat pump system is shown the system includes two heat pumps one thermostat and two indoor fan coils in single stage operation both heat pumps turn on and off at the same time in one embodiment a control system kit 10 is in kit form with all connections between a transformer 12 and relays r 1 r 2 and r 3 already made wires.