Page 7
9 − Defrost System
Units built April 2002 and later
The HP29 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
defrost thermostat senses 42°F (5.5°C) or cooler, the
thermostat 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
functions of a time/temperature defrost control, defrost
relay, diagnostic LEDs and terminal strip for field wiring
connections. See figure 7.
The control provides automatic switching from normal
heating operation to defrost mode and back. During
compressor cycle (call for defrost), the control
accumulates compressor run times at 30, 60, or 90 minute
field−adjustable intervals. If the defrost thermostat is closed
when the selected compressor run time interval ends, the
defrost relay is energized and defrost begins.
Defrost Control Timing Pins
Each timing pin selection provides a different
accumulated compressor run time period during one
thermostat run cycle. This time period must occur before
a defrost cycle is initiated. The defrost interval can be
adjusted to 30 (T1), 60 (T2), or 90 (T3) minutes. See
figure 7. The defrost timing jumper is factory−installed to
provide a 60−minute defrost interval. If the timing selector
jumper is not in place, the control defaults to a 90−minute
defrost interval.The maximum defrost period is 14
minutes and cannot be adjusted.
A TEST option is provided for troubleshooting.
The TEST
mode may be started any time the unit is in the heating
mode and the defrost thermostat is closed or
jumpered.
If the jumper is in the TEST position at
power-up, the control will ignore the test pins. When the
jumper is placed across the TEST pins for two seconds, the
control will enter the defrost mode. If the jumper is removed
before an additional 5−second period has elapsed (7
seconds total), the unit will remain in defrost mode until the
defrost thermostat opens or 14 minutes have passed. If the
jumper is not removed until after the additional 5−second
period has elapsed, the defrost will terminate and the test
option will not function again until the jumper is removed
and re−applied.
HP29 DEFROST CONTROL BOARD
FIGURE 7
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
DEFROST
INTERVAL
TIMING
PINS
24V
TERMINAL
STRIP
PRESSURE SWITCH
TERMINALS
(Remove factory−
installed jumper
to install
pressure switch.)
AMBIENT
THERMISTER
TERMINALS
SERVICE
LIGHT
TERMINALS
DIAGNOSTIC
LEDs
OPTIONAL UNITS
STANDARD UNITS
CC
Y
CC
Y
