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TABLE 6
Defrost Control Board Diagnostic (5 strike)
LED 1
LED 2
Condition
Possible Cause(s)
Solution
OFF
OFF
Power problem
1
No power (24V) to board terminals
R & C.
2
Board failure.
1
Check control transformer power
(24V).
2
If power is available and LED(s) are
unlit, replace board and all sensors.
ON
ON
Coil sensor problem
1
Coil temperature outside of sensor
range.
2
Faulty sensor wiring connections at
board or poor sensor contact on coil.
3
Sensor failure.
1
Sensor function will resume when
coil temperature is between −20°F
and 110°F.
2
Check sensor wiring connections at
board and sensor contact on coil.
3
Replace board and all sensors.
OFF
ON
Ambient sensor problem
1
Ambient temperature outside of
sensor range.
2
Faulty sensor wiring connections at
board or sensor.
3
Sensor failure.
1
Sensor function will resume when
coil temperature is between −20°F
and 110°F.
2
Check sensor wiring connections at
board and sensor.
3
Replace board and all sensors.
FLASH
FLASH
Normal operation
Unit operating normally or in standby
mode.
None required.
ON
OFF
5−Strike pressure lockout
(Short test pins or reset
24V power to board to
override lockout)
1
Restricted air flow over indoor or
outdoor coil.
2
Improper refrigerant charge.
1
Remove any blockages or restric-
tions. Check outdoor fan motor for
proper operation.
2
Check approach, superheat & sub-
li
t
t
ON
FLASH
Low pressure switch circuit
open during Y1 demand
2
Improper refrigerant charge.
3
Improper metering device opera-
ti
pp
,
p
cooling temperatures.
3
Check system pressures. Repair
leaks. Replace metering device.
FLASH
ON
High pressure switch or
discharge thermostat
switch circuit open during
Y1 demand
p p
g
p
tion.
4
Poor contact between coil sensor
and coil.
leaks. Replace metering device.
4
Make sure that sensor is properly
positioned on coil and that firm con-
tact is established. Refer to service
manual for proper placement.
ALTERNATING
FLASH
ALTERNATING
FLASH
5−minute delay
(Jumper test pins to over-
ride delay)
Thermostat demand for cooling or
heat pump operation. Unit operating
in 5−minute anti−short−cycle mode.
None required.
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 difference 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 temperature 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 control
will attempt to self−calibrate after this (and all other) defrost cycle(s). Calibration success depends on stable system 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 demand defrost logic to initiate a defrost cycle. A demand defrost system initiates defrost when the difference be-
tween 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 temperature exceeds the termination temperature or after 14 minutes 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.
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
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 sen-
sor temperature is above the selected termination temper-
ature, 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
termination 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 se-
lected 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 temperature did not remain above 35°F
(2°C) for 4 minutes, the control will go into the 34−minute
Time/Temperature mode.
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 defrost cycle to en-
sure that there is no ice on the coil. During calibration, the
temperature of both the coil and the ambient sensor are
measured to establish the temperature differential which is
required to allow a defrost cycle.
