Lennox HP29 Series Unit Information page 8

Pressure Switch Circuit
The defrost control incorporates a pressure switch circuit
that allows the application of an optional high pressure
switch. See figure 7. During a demand cycle, the defrost
control will lock out the unit if the optional high pressure
switch opens. The diagnostic LEDs will display a pattern for
an open high pressure switch. See table 5. The unit will
remain locked out until the switch resets or is reset.
Remove the factory-installed jumper before connecting the
optional high pressure switch to the control board.
NOTE − If not using a pressure switch, the factory-installed
jumper wire must be connected.
Diagnostic LEDs
The defrost board uses two LEDs for diagnostics. The LEDs
flash a specific sequence according to the condition.
TABLE 5
DEFROST CONTROL BOARD DIAGNOSTIC LED
MODE
Normal operation /
power to board
Board failure or no power
Board failure
High pressure switch open
Low pressure switch open*
Pressure switch lockout*
Anti−short−cycle /
5−minute delay*
*Optional units only.
Optional Units
Optional units include a defrost control which includes a
timed−off delay and a second pressure switch circuit.
LED 1 LED 2
Synchronized Synchronized
Flash with LED 1
Flash with LED 2
Off Off
On On
Flash On
On Flash
On Off
Alternating Flash Alternating Flash
with LED 2 with LED 1
Time−Delay Relay
The time delay is five minutes long. The delay feature
protects the compressor in cased of an interruption in
power to the unit. The time delay may be bypassed by
placing the temperature select jumper across the TEST
pins for 0.5 seconds.
Pressure Switch Circuit
The defrost control board used in optional units includes a
three−strike lock−out feature and LO PS terminals to
accommodate the addition of a field−provided low pressure
or loss of charge pressure switch. See figure 7.
During a single demand cycle, the defrost control will lock
out the unit after the third time that the circuit is interrupted
by any pressure switch that is wired to the control board. In
addition, the diagnostic LEDs will indicate a locked out
pressure switch after the third occurrence of an open
pressure switch. See table 5. The unit will remain locked
out until power is broken then remade to the control or until
the jumper is applied to the TEST pins for 0.5 seconds.
NOTE − The defrost control board ignores input from the
low pressure switch terminals during the TEST mode,
during the defrost cycle, during a 90−second start−up
period, and for the first 90 seconds each time the reversing
valve switches heat/cool modes. If the TEST pins are
jumpered and the 5−minute delay is being bypassed,
the LO PS terminal signal is not ignored during the
90−second start−up period.
Ambient Thermister & Service Light Connection
Optional units include a defrost control board which
provides terminal connections for an ambient thermistor
and a service light. The thermistor compensates for
changes in ambient temperature which might cause
thermostat droop. The service light thermostat provides a
signal which activates the room thermostat service light
during periods of inefficient operation.
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