26Sequence of operationWhen power is supplied to unit, the transformer (TRAN) isenergized.On units with crankcase heater, heater is also energized.Cooling — With the thermostat subbase in the coolingposition, the thermostat makes circuit R-O. This energizesthe reversing valve solenoid (RVS) and places the unit instandby condition for cooling.As the space temperature rises, the thermostat makes,closing circuit R-Y. When compressor time delay (5±2minutes) is completed, a circuit is made to contactor (C),starting the compressor (COMP) and outdoor-fan motor(OFM). Circuit R-G is made at the same time, energizing theindoor-fan relay (IFR) and starting the indoor-fan motor(IFM) after a 1-second delay.When the thermostat is satisfied, contacts open, deenergizingC. The COMP and OFM stop, and the IFM stops after a 60sec time delay.Heating — On a call for heat, thermostat makes circuits R-Y and R-G.When compressor time delay (5±2 minutes) is completed, acircuit is made to C, starting COMP and OFM. Circuit R-Galso is completed, energizing IFR and starting IFM after a 1-sec delay.Should room temperature continue to fall, circuit R-W ismade through second-stage thermostat bulb. If optionalelectric heat package is used, a relay is energized, bringingon first bank of supplemental electric heat. When thermostatis satisfied, contacts open, deenergizing contactor and relay;motors and heaters deenergize. The IFM is controlled by atime-delay relay that keeps the fan on for 60 sec.Defrost — Defrost board (DB) is a time and temperaturecontrol, which includes a field-selectable time period (Dipswitch 1 and 2 on the board) between checks for defrost (30,60, 90, or 120 minutes). Electronic timer and defrost cyclestart only when contactor is energized and defrost thermostat(DFT) is closed.The defrost board is also equipped with a 3rd dip switch forselecting Quiet Shift operation. The Quiet Shift operationturns compressor off at defrost initiation and termination.Defrost mode is identical to cooling mode, except outdoorfan motor stops and a bank of optional electric heat turns onto warm air supplying the conditioned space.Controls