Page 17
12HPB Defrost Control Board
24V
terminal strip
High Pressure Switch
Safety Circuit
Connections
S4
connection for
optional
high pressure
switch
defrost
interval
timing pins
high
pressure
switch
diagnostic
LEDs
Figure 16
NOTE − To add pressure
switch, remove factory−
installed jumper
Defrost Control Timing Pins
Each timing pin selection provides a different accumu-
lated 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 16. 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 jump-
ered.
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 addi-
tional 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 re-
moved 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−ap-
plied.
Pressure Switch Circuit
The defrost control incorporates a pressure switch circuit
that allows the application of an optional high pressure
switch. See figure 16. 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 8. The unit will re-
main 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 8
Defrost Control Board Diagnostic LED
Mode
LED 1
LED 2
Normal operation /
power to board
Synchronized
Flash with LED 2
Synchronized
Flash with LED 1
Board failure or no power
Off
Off
Board failure
On
On
Pressure switch open
Flash
On
