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CONNECTION FOR
ONE OPTIONAL SWITCH
High
Pressure
Switch
S4
CONNECTIONS FOR
TWO OPTIONAL
SWITCHES
Optional
switch
S4
High
Pressure
Switch
4 − Defrost System
12HPB units built April 2002 and later
The defrost system includes two components: a defrost
thermostat and a defrost control.
Defrost Thermostat
The defrost thermostat is located on the liquid line between
the check/expansion valve and the distributor. When de-
frost thermostat senses 42°F (5.5°C) or cooler, the thermo-
stat contacts close and send a signal to the defrost control
board to start the defrost timing. It also terminates defrost
when the liquid line warms up to 70°F (21°C).
Defrost Control
The defrost control board includes the combined func-
tions of a time/temperature defrost control, defrost relay,
diagnostic LEDs and terminal strip for field wiring connec-
tions. See figure 5.
The control provides automatic switching from normal
heating operation to defrost mode and back. During com-
pressor cycle (call for defrost), the control accumulates
compressor run times at 30, 60, or 90 minute field−adjust-
able intervals. If the defrost thermostat is closed when the
selected compressor run time interval ends, the defrost
relay is energized and defrost begins.
12HPB DEFROST CONTROL BOARD
NOTE − COMPONENT LOCATIONS WILL VARY WITH BOARD MANUFACTURER
PRESSURE SWITCH
SAFETY CIRCUIT
CONNECTIONS
AMBIENT
THERMISTOR
CONNECTION
SERVICE LIGHT
CONNECTION
FIGURE 4
12HPB DEFROST CONTROL BOARD
HIGH PRESSURE SWITCH
SAFETY CIRCUIT
CONNECTIONS
NOTE − Remove factory−
installed jumper to add
pressure switch.
CONNECTION FOR
OPTIONAL
HIGH PRESSURE
SWITCH
High
Pressure
Switch
S4
Page 5
DEFROST
INTERVAL
TIMING PINS
24V TERMINAL STRIP
CONNECTIONS
CC
Y
24V
TERMINAL
STRIP
DEFROST
INTERVAL
TIMING PINS
DIAGNOSTIC
LEDs
FIGURE 5
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