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HI−PS/LO−PS Terminals
High pressure switch (S4) is factory wired into the defrost
board HI−PS terminals. When (S4) trips, the defrost board
will cycle off the compressor and the strike counter in the
board will count one strike.
Low pressure switch (S87) is factory wired into the defrost
board LO−PS terminals. When (S87) trips, the defrost
board will cycle off the compressor and the strike counter
in the board will count one strike.
(S87) is ignored during certain conditions:
During the defrost cycle and 90 seconds after the
termination of defrost
When the average ambient sensor temperature is
below 15 F (−9)
For 90 seconds following the start up of the
compressor
During "Test" mode
3−Strike Lockout Feature
(Board 60L3901)
The internal control logic of the board counts the pres-
sure switch trips only while the Y1 (Input) line is active. If
a pressure switch opens and closes twice during a Y1
(Input), the control logic will reset the pressure switch
trip counter to zero at the end of the Y1 (Input). If the
pressure switch opens for a third time during the current
Y1 (Input), the control will enter a lockout condition.
The 3−strike pressure switch lockout condition can be re-
set by cycling OFF the 24−volt power supply to the con-
trol board or by shorting the TEST pins. All timer func-
tions (run times) will also be reset.
If a pressure switch becomes open while the Y1 Out line
is engaged, a 5 minute short cycle will occur after the
switch closes.
5−Strike Lockout Feature
(Boards 46M8201, 56M8501)
The internal control logic of the board counts the pres-
sure switch trips only while the Y1 (Input) line is active. If
a pressure switch opens and closes 4 times during a Y1
(Input), the control logic will reset the pressure switch
trip counter to zero at the end of the Y1 (Input). If the
pressure switch opens for a fifth time during the current
Y1 (Input), the control will enter a lockout condition.
The 5−strike pressure switch lockout condition can be re-
set by cycling OFF the 24−volt power supply to the con-
trol board or by shorting the TEST pins. All timer func-
tions (run times) will also be reset.
If a pressure switch becomes open while the Y1 Out line
is engaged, a 5 minute short cycle will occur after the
switch closes.
Operational Description
The demand defrost board has three basic operational
modes: Normal, Defrost, and Calibration.
Normal Mode
The demand defrost board monitors the O" line, to deter-
mine the system operating mode (heat/cool), outdoor ambi-
ent temperature, coil temperature (outdoor coil) and com-
pressor run time to determine when a defrost cycle is re-
quired.
Defrost Mode
When a defrost cycle is initiated, the control energizes the
reversing valve solenoid and turns off the condenser fan.
The control will also put 24VAC on the W1" (auxiliary heat)
line. The unit will stay in this mode until either the coil sensor
temperature is above the selected termination tempera-
ture, the defrost time of 14 minutes has been completed, or
the room thermostat demand cycle has been satisfied. (If
the temperature select shunt is not installed, the default ter-
mination temperature will be 100°F.) If the room thermostat
demand cycle terminates the cycle, the defrost cycle will be
held until the next room thermostat demand cycle. If the coil
sensor temperature is still below the selected termination
temperature, the control will continue the defrost cycle until
the cycle is terminated in one of the methods mentioned
above. If a defrost is terminated by time and the coil temper-
ature did not remain above 35°F (2°C) for 4 minutes, the
control will go to the 34−minute Time/Temperature mode.
DELAY"
PINS
The 56M8501 defrost board has a field selectable function
to reduce occasional noise that may occur while the unit is
cycling in and out of defrost mode. When a jumper is
installed on the DELAY" pins, the compressor will cycle off
for 30 seconds going in and out of defrost mode. Units are
shipped with jumper installed on DELAY" pins.
NOTE − 30 second off cycle is not functional when
jumpering TEST" pins.
Calibration Mode
The board is considered uncalibrated when power is ap-
plied to the board, after cool mode operation, or if the coil
temperature exceeds the termination temperature when it
is in heat mode. Calibration of the board occurs after a de-
frost cycle to ensure that there is no ice on the coil. During
calibration, the temperature of both the coil and the ambient
sensor are measured to establish a temperature differential
required to allow a defrost cycle.
