energy is transferred between outdoor ambient air being circulated through the outdoor heat exchanger 40 by condenser fan 20 and the indoor heat exchanger 42 having the enclosure air circulated therethrough by supply fan 30 outdoor ambient air may also be drawn into the unit via economizer 50 supply plenum 60 is connected to the air conditioning unit to receive the indoor air being discharged by supply fan 30 discharge plenum thermostat 66 is located to sense the temperature of the air being discharged from the air conditioning unit return plenum 61 is shown connected to direct air from the enclosure to the supply fan of the indoor section of the heat exchange unit return plenum thermostat 64 is mounted in the return air flow stream to sense the temperature of the return air enclosure 100 is labeled to define the space to be conditioned which may be a commercial building store or office building or the like referring now to fig2 there can be seen a partial schematic of a wiring diagram for an air conditioning unit therein it can be seen that power is supplied through lines l 1 and l 2 to a power portion of the circuit wires 101 and 102 are shown connecting the various components l 1 is connected to wire 101 which is connected to compressor relay contacts cr1 1 compressor relay contacts cr2 2 fan relay contacts fr 1 heating relay contacts hr 1 and one winding of transformer t 1 wire 102 is connected to compressor contactor cc 1 compressor contactor cc 2 fan contactor fc and to heating sequencer 110 and one winding of.