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3−Strike Lockout
The demand defrost system control board includes a
3−strike lockout feature. This internal feature registers
protection circuit interruptions while Y1" (input) is ener-
gized. If any protection circuit switch opens three times
during a single Y1" (input) cycle, the control initiates lock-
out. If the circuit is interrupted once or twice during a single
Y1" (input) cycle, the control resets the strike register to
zero at the end of the cycle.
The 3−strike register can be reset by interrupting 24V
power to the control board or by shorting the TEST"
pins.
TEST Mode
The control board includes a test mode to facilitate ser-
vice. During the test mode, the 14−minute defrost period is
shortened. Initiate the test mode by placing the defrost
temperature selection jumper across the TEST" pins.
Normal test mode operation is described below and in fig-
ure 12:
24V power must be applied to the demand defrost con-
trol board BEFORE the jumper is installed on the
TEST" pins.
If test pins are shorted before 24V power is applied to
the board, the test mode will be ignored until the short
is removed and reapplied. Each test pin short will result
in one test event. For each test, the shunt (jumper)
must be removed for at least 1 second then reapplied.
If the Y signal is interrupted, the unit will exit the test
mode.
LO − PS input is ignored during test mode operation.
Test mode operation is limited to 5 minutes. The control
will revert to normal operation and ignore the test mode
after this period of time.
Defrost Temperature Selection Jumper
The control board can be set to terminate defrost at one of
four temperature settings: 70°F, 80°F, 90°F, or 100°F. If the
jumper is removed, the control board will terminate defrost
at the default setting of 100°F.
Communication Port
The demand defrost system control board is equipped
with a communication port. This port has been included for
future use and is not functional at this time.
IMPORTANT
Do not connect an electronic device to the commu-
nication port. Incompatibility between the control
board and the device could result in damage to both.
Defrost Operation (Table 5)
The demand defrost control board has three modes of op-
eration: normal, calibration and defrost.
Normal Operation
During normal operation, the control board continuously
monitors the system operation mode (heating / cooling),
the outdoor ambient temperature, the outdoor coil temper-
ature and compressor run time to determine the need for a
defrost cycle.
Calibration Mode
The demand defrost control board is considered uncali-
brated when one of the following conditions applies: im-
mediately after power has been applied to the control
board, immediately after operation in the cooling mode, or
any time the outdoor coil temperature exceeds the ter-
mination temperature during the heating mode. During the
calibration mode, the demand defrost control board mea-
sures the outdoor coil and outdoor ambient temperatures
to establish an average (calibrated) temperature differen-
tial between the two.
Defrost Mode
The following operation sequence applies when the de-
mand defrost control initiates a defrost cycle:
1 − The control board energizes the reversing valve and
de−energizes the condenser fan motor.
2 − The control energizes the W1" (auxiliary heat) circuit.
The unit will operate in this mode until the outdoor coil
temperature rises above the selected termination
temperature, the 14−minute defrost interval has been
completed, or the room temperature rises to satisfy
the heating demand.
NOTE − If the termination temperature selection jump-
er has been left off, the defrost termination tempera-
ture is set at the 100°F default.
3 − If the defrost cycle is terminated by the room thermo-
stat because the heating demand has been satisfied,
the call for a defrost cycle will be carried over to the
next heating demand cycle. If the outdoor coil temper-
ature is still below the selected termination tempera-
ture when the heating demand is re−initiated, the de-
frost cycle will be continued until it is terminated by
one of the three methods outlined in step 2.
4 − If the defrost cycle is terminated by the 14−minute de-
frost interval and the outdoor coil has not remained
above 35
°
F (2
°
C) for 4 minutes, the control will initiate
a 34−minute time/temperature defrost mode.
Ambient Thermistor & Service Light Connection
The defrost control board provides terminal connections
for the ambient thermistor and a service light. The thermis-
tor compensates for changes in ambient temperature
which might cause thermostat droop. The service light
thermostat provides a signal which activates the room
thermostat service light during periods of inefficient opera-
tion.
