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506212−01 11/10
DEFROST SYSTEM SENSORS
Sensors connect to the demand defrost control through a
field-replaceable harness assembly that plugs into the
demand defrost control. Through the sensors, the demand
defrost control detects outdoor ambient and coil
temperature fault conditions. As the detected temperature
change, the resistance across the sensors changes. table
11 shows how the resistance varies as the temperature
changes for both type of sensors.
Table 10. Sensor Temperature / Resistance Range
Sensor
Temperature
Range °F (°C)
Resistance values
range (ohms)
Pins/Wire
Color
Outdoor
(Ambient)
−35 (−37) to 120
(48)
280,000 to 3750
3 and 4
(Black)
Coil
−35 (−37) to 120
(48)
280,000 to 3750
5 and 6
(Brown)
NOTE Sensor resistance decreases as sensed temperature
increases (see table 11).
When a sensor indicates a resistance value that is not
within the range as listed in table 10, then the following
condition may be present:
S
Sensor detects an out−of−range outdoor ambient air
temperature condition and is displaying a
light−emitting diode diagnostic code on the demand
defrost control.
S
The sensor is operating normally. The ambient air
temperature at the sensor is below or above the
demand defrost control’s expected ohm values. The
demand defrost control will indicate the sensor as
faulty. However the sensor has not failed.
S
Once the outdoor ambient air temperature has
returned to within the sensor’s normal operating
range, the demand defrost control’s light−emitting
diodes diagnostic code will stop.
Defrost Coil Sensor (RT21)
This sensor (shown in
figure 2, Detail A) considers outdoor temperatures below
−35°F (−37°C) or above 120°F (48°C) to be a fault. If the
defrost coil sensor is open, shorted or out of the
temperature range of the sensor, the demand defrost
control will not perform demand or time/temperature
defrost operation and will display the appropriate fault
code. Heating and cooling operation will be allowed in this
fault condition.
Ambient Sensor (RT13)
The ambient sensor (shown
in figure 2, Detail B) considers outdoor temperatures below
−35°F (−37°C) or above 120°F (48°C) to be a fault. If the
ambient sensor is shorted or out of the temperature range
of the sensor, the demand defrost control will not perform
demand defrost operation. The demand defrost control 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.
NOTE If 5−strikes occur within a single room thermostat
demand, the demand defrost control will lock out the unit.
demand defrost control 24VAC power
R
must be cycled
OFF, or place a jumper on TEST pins between 1 to 2
seconds to reset the demand defrost control.
DEFROST MODE TERMINATION
1.
Power Defrosts
Once a power defrost mode has
been initiated, the following will occur:,
S
Will terminate and reset the internal timer if the coil
sensor temperature exceeds the selected
Defrost
Termination Temperature Setting
. If the
Defrost
Termination Temperature Setting
(
P1
) jumper is
not installed, the default termination temperature
is 90°F.
S
An internal timer shall count the time that the
defrost mode is engaged and compressor is
energized. After 14 minutes of operation in the
defrost mode, the defrost sequence will terminate
immediately and reset internal timer regardless of
the state of the coil sensor temperature.
EXCEPTIONS
High pressure switch open or
locked out or anti−short cycle delay is active.
2.
Natural Defrosts
Natural defrosts should
terminate after a fixed duration of 150 seconds.
