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Demand Defrost System
The demand defrost controller uses basic differential tem-
perature means to detect when the system performs poor-
ly because of ice build−up on the outdoor coil. The control-
ler also uses self−calibrating" principles to calibrate itself
when the system starts and after every time the system
defrosts. The control board has the following compo-
nents: defrost relays, anti−short cycle timed−off control,
pressure switch/safety control, 5−trip lockout circuit,
manufacturing test mode, ambient and coil temperature
sensors, field selectable termination temperature pins, and
a field low voltage connection terminal strip. See figure 17.
The control monitors ambient temperature, outdoor coil
temperature and total run time to determine when a de-
frost cycle is required. Two temperature probes are per-
manently attached to the control. The coil temperature
probe is designed with a spring clip to allow mounting to
the outside coil tubing. The location of the coil sensor is
important for proper defrost operation.
NOTE − The logic of the demand defrost board accurately
measures the performance of the system as frost accumu-
lates on the outdoor coil. This typically will translate into
longer running time between defrost cycles as more frost
accumulates on the outdoor coil before the board initiates
defrost cycles.
The temperature probes cannot be removed from the con-
trol. The control and the attached probes MUST be re-
placed as a unit. Do not attempt to cut or splice probe wires.
Diagnostic LEDs
The defrost board uses two LEDs for diagnostics. The
LEDs flash a specific sequence according to the diagnos-
tic condition. See table 9.
HI−PS/LO−PS Terminals
The unit’s automatic reset pressure switches are wired to
the PS terminals. The high−pressure switch is factory
wired to the HI−PS terminal, and the low−pressure switch
is factory−wired to LO−PS terminal.
The path between Y1 (input) and Y1 Out is interlocked
through the pressure switches. When one of the pressure
switches trip, the board will cycle off the compressor and
the 5−strike counter in the board will count one strike.
5−Strike Lockout Feature
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 fifth time during the current
Y1 (Input), the control will enter a lockout condition.
The 5−strike pressure switch lockout condition can be
reset by cycling OFF the 24−volt power to the control
board or by shorting the TEST pins. All timer functions
(run times) will also be reset.
If a pressure switch opens while the Y1 Out line is en-
gaged, a 5−minute short cycle will occur after the switch
closes.
Delay Mode
The defrost board has a field−selectable function to reduce
occasional sounds that may occur while the unit is cycling
in and out of the defrost mode. When a jumper is installed
on the DELAY pins, the compressor will be cycled off for 30
seconds going in and out of the defrost mode. Units are
shipped with jumper installed on DELAY pins.
NOTE − The 30 second off cycle is not functional when jum-
pering the TEST pins.
Operational Description
The defrost control 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 am-
bient temperature, coil temperature (outdoor coil) and
compressor run time to determine when a defrost cycle is
required.
Defrost Mode
Refer to table 9 for demand defrost operation.
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 ambi-
ent sensor are measured to establish the temperature dif-
ferential which is required to allow a defrost cycle.
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