Page 43
XP21
UNIT SENSORS
Sensors connect to the heat pump control through a field‐
replaceable harness assembly that plugs into the control.
Through the sensors, the control detects outdoor ambient,
coil, and discharge temperature fault conditions. As the de
tected temperature changes, the resistance across the
sensor changes. Tables 21 and 22 shows how the resist
ance varies as the temperature changes for both type of
sensors. Sensor resistance values can be checked by oh
ming across pins shown in table 20.
NOTE — When checking the ohms across a sensor, be
aware that a sensor showing a resistance value that is not
within the range shown in table 20, may be performing as
designed. However, if a shorted or open circuit is detected,
then the sensor may be faulty and the sensor harness will
need to be replaced.
Ambient Temperature Sensor (RT13)
The ambient sensor (location shown in figure 1) considers
outdoor 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 control will not perform demand defrost operation.
The control will revert to time/temperature defrost opera
tion and will display the appropriate alert code. Heating and
cooling operation will be allowed in this fault condition.
Coil Temperature Sensor (RT21)
This sensor (location shown in figure 1) considers coil tem
peratures 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 heat pump control
will not perform demand or time/temperature defrost op
eration and will display the appropriate fault code. Heating
and cooling operation will be allowed in this fault condition.
High Discharge Line Temperature Sensor (RT28)
The high discharge line temperature sensor (location
shown in figure 1 monitors temperature range and open/
short conditions. See figure 35 for the high discharge line
temperature sensor sequence of operation.
Table 20. Sensor Temperature / Resistance Range
Sensor
Temperature
Range °F (°C)
Resistance values
range (ohms)
Pins/Wire
Color
Discharge
(RT28)
24 (-4) to 350
(176)
41,000 to 103 1 and 2
(Yellow)
Outdoor
(Ambient)
(RT13)
-35 (-37) to 120
(48)
280,000 to 3750 3 and 4
(Black)
Coil (RT21) -35 (-37) to 120
(48)
280,000 to 3750 5 and 6
(Brown)
NOTE — Sensor resistance decreases as sensed temperature
increases (see tables 21 and 22).
W Input Fault or Miswire
In case of a W input fault or possible miswire, the system
will function as listed in the sequence of operation in figure
37.
SECOND-STAGE OPERATION
If the control receives a call for second-stage compressor
operation Y2 in heating or cooling mode and the first‐stage
compressor output is active, the second‐stage compressor
solenoid output will be energized by the heat pump control
system operation function.
NOTE — Figure 23 illustrates the correct Y2 field wiring
configuration.
NOTE — The heat pump control system operation monitor
has a five second delay between Y2 being powered and the
solenoid energizing.
Second Stage Lock-in
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 even without a
Y2 room thermostat input.
If the jumper is not connected to one of the temperature
selection pins (40, 45, 50, 55°F), the default lock-in temper
ature of 40°F (4.5°C) will be used.
The heat pump control de-energizes the second‐stage
compressor solenoid output immediately when the Y2 sig
nal is removed or the outdoor ambient temperature is 5°F
above the selected compressor lock-in temperature, or the
first‐stage compressor output is de-energized for any rea
son.
Shift Delay
The heat pump control has a field-selectable function to re
duce occasional sounds that may occur while the unit is
cycling in and out of the defrost mode. Units are shipped
with jumper installed on pins.
* When the jumper is installed: There is a 30-second
compressor shift delay which de-energizes the com
pressor contactor output and ECM fan outputs. After
the delay expires, the compressor contactor and ECM
fan outputs are energized.
* When the jumper is not installed: The reversing valve
is changed by de-energizing the outputs immediately.
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