Lennox HP29

Pressure Switch CircuitThe defrost control incorporates a pressure switch circuitthat allows the application of an optional high pressureswitch. See figure 7. During a demand cycle, the defrostcontrol will lock out the unit if the optional high pressureswitch opens. The diagnostic LEDs will display a pattern foran open high pressure switch. See table 5. The unit willremain locked out until the switch resets or is reset.Remove the factory-installed jumper before connecting theoptional high pressure switch to the control board.NOTE − If not using a pressure switch, the factory-installedjumper wire must be connected.Diagnostic LEDsThe defrost board uses two LEDs for diagnostics. The LEDsflash a specific sequence according to the condition.TABLE 5DEFROST CONTROL BOARD DIAGNOSTIC LEDMODE LED 1 LED 2Normal operation /power to boardSynchronizedFlash with LED 2Board failure or no power Off OffBoard failure On OnHigh pressure switch open Flash OnLow pressure switch open* On FlashPressure switch lockout* On OffAnti−short−cycle /5−minute delay**Optional units only.Optional UnitsAlternating Flashwith LED 2SynchronizedFlash with LED 1Alternating Flashwith LED 1Optional units include a defrost control which includes atimed−off delay and a second pressure switch circuit.Time−Delay RelayThe time delay is five minutes long. The delay featureprotects the compressor in cased of an interruption inpower to the unit. The time delay may be bypassed byplacing the temperature select jumper across the TESTpins for 0.5 seconds.Pressure Switch CircuitThe defrost control board used in optional units includes athree−strike lock−out feature and LO PS terminals toaccommodate the addition of a field−provided low pressureor loss of charge pressure switch. See figure 7.During a single demand cycle, the defrost control will lockout the unit after the third time that the circuit is interruptedby any pressure switch that is wired to the control board. Inaddition, the diagnostic LEDs will indicate a locked outpressure switch after the third occurrence of an openpressure switch. See table 5. The unit will remain lockedout until power is broken then remade to the control or untilthe jumper is applied to the TEST pins for 0.5 seconds.NOTE − The defrost control board ignores input from thelow pressure switch terminals during the TEST mode,during the defrost cycle, during a 90−second start−upperiod, and for the first 90 seconds each time the reversingvalve switches heat/cool modes. If the TEST pins arejumpered and the 5−minute delay is being bypassed,the LO PS terminal signal is not ignored during the90−second start−up period.Ambient Thermister & Service Light ConnectionOptional units include a defrost control board whichprovides terminal connections for an ambient thermistorand a service light. The thermistor compensates forchanges in ambient temperature which might causethermostat droop. The service light thermostat provides asignal which activates the room thermostat service lightduring periods of inefficient operation.Page 8