Defrost System Sensors
Ambient Sensor--The ambient sensor considers out-
door temperatures below -35°F (-37°C) or above 120°F
(48°C) as a fault. If the ambient sensor is detected as being
open, shorted or out of the temperature range of the sen-
sor, the board will not perform demand defrost operation.
The board will revert to time/temperature defrost operation
and will display the appropriate fault code. Heating and
cooling operation will be allowed in this fault condition.
Coil Sensor--The coil temperature sensor considers out-
door temperatures below -35°F (-37°C) or above 120°F
(48°C) as a fault. If the coil temperature sensor is detected
as being open, shorted or out of the temperature range of
the sensor, the board will not perform demand or time/tem-
perature defrost operation and will display the appropriate
fault code. Heating and cooling operation will be allowed in
this fault condition.
Discharge Line Sensor--If the discharge line tempera-
ture exceeds a temperature of 300°F (148°C) during com-
pressor operation, the board will de-energize the compres-
sor contactor output (and the defrost output, if active). The
compressor will remain off until the discharge temperature
has dropped below 225°F (107°C) and the 5-minute anti-
short cycle delay has been satisfied. This sensor has two
fault and lockout codes:
1. If the board recognizes five high discharge line tem-
perature faults during a single (Y1) compressor de-
mand, it reverts to a lockout mode and displays the ap-
propriate code. This code detects shorted sensor or
high discharge temperatures. (Code on board is "Dis-
charge Line Temperature Fault and Lockout").
2. If the board recognizes five temperature sensor range
faults during a single (Y1) compressor demand, it re-
verts to a lockout mode and displays the appropriate
code. The board detects open sensor or out-of-tem-
perature sensor range. This fault is detected by allow-
ing the unit to run for 90 seconds before checking sen-
sor resistance. If the sensor resistance is not within
range after 90 seconds, the board will count one fault.
After 5 faults, the board will lockout. (Code on board
is "Discharge Sensor Fault and Lockout").
The discharge line sensor, which covers a range of 150°F
(65°C) to 350°F (176°C), is designed to mount on a V:" re-
frigerant discharge line.
NOTE -Within a single room thermostat demand, if
5-strikes occur, the board will lockout the unit. Defrost
board 24 volt power "R" must be cycled "OFF" or the
"TEST" pins on board must be shorted between 1 to 2sec-
onds to reset the board.
Second-Stage Operation--If the board receives a call for
second-stage compressor operation "Y2" in heating or
cooling mode and the first-stage compressor output is ac-
tive, the second-stage compressor solenoid output will be
NOTE - Figure 8 on page 5 shows the correct Y2 field wir-
If first-stage compressor output is active in heating mode
and the outdoor ambient temperature is below the selected
compressor lock-in temperature, the second-stage com-
pressor solenoid output will be energized without the "Y2"
input. If the jumper is not connected to one of the tempera-
ture selection pins on P3 (40, 45, 50, 55°F), the default
lock-in temperature of 40°F (4.5°C) will be used.
The board de-energizes the second-stage compressor so-
lenoid output immediately when the "Y2" signal is removed
or the outdoor ambient temperature is 5°F above the se-
lected compressor lock-in temperature, or the first-stage
compressor output is de-energized for any reason.
Defrost Temperature Termination Shunt (Jumper)
Pins--The defrost board selections are: 50, 70, 90, and
100°F (10, 21, 32 and 38°C). The shunt termination pin is
factory set at 50°F (10°C). If the temperature shunt is not
installed, the default termination temperature is 90°F
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
jumpering the TEST pins.
The defrost control board has three basic operational
modes: normal, defrost, and calibration.
• Normal Mode--The demand defrost board monitors
the O line, to determine the system operating mode
(heat/cool), outdoor ambient temperature, coil tem-
perature (outdoor coil) and compressor run time to de-
termine when a defrost cycle is required.
• Calibration Mode--The board is considered uncali-
brated when power is applied 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
ensure that there is no ice on the coil. During calibra-
tion, the temperature of both the coil and the ambient
sensor are measured to establish the temperature dif-
ferential which is required to allow a defrost cycle.
•Defrost Mode--The following paragraphs provide a
detailed description of the defrost system operation.
Detailed Defrost System 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 dif-
ference allowed by the control, a defrost cycle will be initi-
IMPQR TANT -The demand defrost control board will allow
a greater accumulation of frost and will initiate fewer de-
frost cycles than a time/temperature defrost system.