Page 5
FIGURE 4
CONNECTION FOR
ONE OPTIONAL SWITCH
CONNECTIONS FOR
TWO OPTIONAL
SWITCHES
NOTE − COMPONENT LOCATIONS WILL VARY WITH BOARD MANUFACTURER
S4
S4
High
Pressure
Switch
Optional
switch
12HPB DEFROST CONTROL BOARD
High
Pressure
Switch
24V TERMINAL STRIP
CONNECTIONS
PRESSURE SWITCH
SAFETY CIRCUIT
CONNECTIONS
DEFROST
INTERVAL
TIMING PINS
AMBIENT
THERMISTOR
CONNECTION
SERVICE LIGHT
CONNECTION
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.
FIGURE 5
24V
TERMINAL
STRIP
HIGH PRESSURE SWITCH
SAFETY CIRCUIT
CONNECTIONS
NOTE − Remove factory−
installed jumper to add
pressure switch.
S4
CONNECTION FOR
OPTIONAL
HIGH PRESSURE
SWITCH
DEFROST
INTERVAL
TIMING PINS
High
Pressure
Switch
DIAGNOSTIC
LEDs
12HPB DEFROST CONTROL BOARD
CC
Y
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