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Demand Defrost Operation
The demand defrost control board initiates a defrost cycle
based on either frost detection or time.
Frost Detection
− If the compressor runs longer than 34
minutes and the actual difference between the clear coil
and frosted coil temperatures exceeds the maximum differ-
ence allowed by the control, a defrost cycle will be initiated.
IMPORTANT − The demand defrost control board will allow
a greater accumulation of frost and will initiate fewer defrost
cycles than a time/temperature defrost system.
Time
− If 6 hours of heating mode compressor run time has
elapsed since the last defrost cycle while the coil tempera-
ture remains below 35°F (2°C), the demand defrost control
will initiate a defrost cycle.
Actuation
− When the reversing valve is de−energized, the
Y1 circuit is energized, and the coil temperature is below
35°F (2°C), the board logs the compressor run time. If the
board is not calibrated, a defrost cycle will be initiated after
34 minutes of heating mode compressor run time. The con-
trol will attempt to self−calibrate after this (and all other) de-
frost cycle(s). Calibration success depends on stable sys-
tem temperatures during the 20−minute calibration period.
If the board fails to calibrate, another defrost cycle will be
initiated after 90 minutes of heating mode compressor run
time. Once the defrost board is calibrated, it will use de-
mand defrost logic to initiate a defrost cycle. A demand de-
frost system initiates defrost when the difference between
the clear coil and frosted coil temperatures exceeds the
maximum difference allowed by the control OR after 6
hours of heating mode compressor run time has been
logged since the last defrost cycle.
Termination
− The defrost cycle ends when the coil temper-
ature exceeds the termination temperature or after 14 min-
utes of defrost operation. If the defrost is terminated by the
14−minute timer, another defrost cycle will be initiated after
34 minutes of run time.
Test Mode
− When Y1 is energized and 24V power is being
applied to the board, a test cycle can be initiated by placing
the termination temperature jumper across the Test" pins
for 2 to 5 seconds. If the jumper remains across the Test"
pins longer than 5 seconds, the control will ignore the test
pins and revert to normal operation. The jumper will initiate
one cycle per test.
HPXA12 DEFROST CONTROL BOARD
Figure 6
24V TERMINAL STRIP
CONNECTIONS
DIAGNOSTIC LEDs
PRESSURE SWITCH
CIRCUIT CONNECTIONS
DEFROST INTERVAL
TIMING PINS
NOTE − COMPONENT LOCATIONS WILL VARY WITH BOARD MANUFACTURER
PRESSURE SWITCH
CIRCUIT CONNECTIONS
Y2
Y2
Y1
