Lennox HP29

9 − Defrost SystemUnits built April 2002 and laterThe HP29 defrost system includes two components: adefrost thermostat and a defrost control.Defrost ThermostatThe defrost thermostat is located on the liquid line betweenthe check/expansion valve and the distributor. Whendefrost thermostat senses 42°F (5.5°C) or cooler, thethermostat contacts close and send a signal to the defrostcontrol board to start the defrost timing. It also terminatesdefrost when the liquid line warms up to 70°F (21°C).Defrost ControlThe defrost control board includes the combinedfunctions of a time/temperature defrost control, defrostrelay, diagnostic LEDs and terminal strip for field wiringconnections. See figure 7.The control provides automatic switching from normalheating operation to defrost mode and back. Duringcompressor cycle (call for defrost), the controlaccumulates compressor run times at 30, 60, or 90 minutefield−adjustable intervals. If the defrost thermostat is closedwhen the selected compressor run time interval ends, thedefrost relay is energized and defrost begins.Defrost Control Timing PinsEach timing pin selection provides a differentaccumulated compressor run time period during onethermostat run cycle. This time period must occur beforea defrost cycle is initiated. The defrost interval can beadjusted to 30 (T1), 60 (T2), or 90 (T3) minutes. Seefigure 7. The defrost timing jumper is factory−installed toprovide a 60−minute defrost interval. If the timing selectorjumper is not in place, the control defaults to a 90−minutedefrost interval.The maximum defrost period is 14minutes and cannot be adjusted.A TEST option is provided for troubleshooting. The TESTmode may be started any time the unit is in the heatingmode and the defrost thermostat is closed orjumpered. If the jumper is in the TEST position atpower-up, the control will ignore the test pins. When thejumper is placed across the TEST pins for two seconds, thecontrol will enter the defrost mode. If the jumper is removedbefore an additional 5−second period has elapsed (7seconds total), the unit will remain in defrost mode until thedefrost thermostat opens or 14 minutes have passed. If thejumper is not removed until after the additional 5−secondperiod has elapsed, the defrost will terminate and the testoption will not function again until the jumper is removedand re−applied.HP29 DEFROST CONTROL BOARDSTANDARD UNITSOPTIONAL UNITSHIGH PRESSURE SWITCHSAFETY CIRCUITCONNECTIONSNOTE − Remove factory−installed jumper to addpressure switch.CONNECTION FOROPTIONALHIGH PRESSURESWITCHCCYDEFROSTINTERVALTIMING PINSDIAGNOSTICLEDsPRESSURE SWITCHTERMINALS(Remove factory−installed jumperto installpressure switch.)DEFROSTINTERVALTIMINGPINSCCYDIAGNOSTICLEDsSERVICELIGHTTERMINALSAMBIENTTHERMISTERTERMINALS24VTERMINALSTRIPHighPressureSwitch24VTERMINALSTRIPS4FIGURE 7Page 7