Lennox XP17-024-230-01 Installation And Service Procedure page 11

UNIT SENSORS
Sensors connect to the heat pump control through a
field‐replaceable harness assembly that plugs into the
control. Through the sensors, the heat pump control
detects outdoor ambient and coil temperature fault
conditions. As the detected temperature changes, the
resistance across the sensor changes. table 4 shows how
the resistance varies as the temperature changes for both
type of sensors. Sensor resistance values can be checked
by ohming across pins shown in table 3.
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 3, 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)
See table 3 for sensor range. If the ambient sensor is
detected as being open, shorted or out of the temperature
range of the sensor, the heat pump control will not perform
demand defrost operation. The heat pump control will
revert to time/temperature defrost operation and will
display the appropriate alert code. Heating and cooling
operation will be allowed in this fault condition.
Coil Temperature Sensor (RT21)
See table 3 for sensor range. 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 operation and will display the
appropriate fault code. Heating and cooling operation will
be allowed in this fault condition.
High Discharge Line Temperature Sensor
This model does not use a high discharge line temperature
sensor. The cable assembly attached to the heat pump
control's E30 connection has a 10K resister installed
between pins 1 and 2 as illustrated in figure 5. No alerts or
alarms would be generated if resistor is damage.
COIL TEMPERA­
TURE SENSOR
AMBIENT AIR
TEMPERATURE SENSOR
Figure 5. 10k Resistor Location
XP17
10K RESISTOR
Page 11
Table 3. Sensor Temperature / Resistance Range
Temperature
Sensor
Range °F (°C)
Outdoor
(Ambient)
-40 (-40) to 140
(60)
Coil
NOTE — Sensor resistance decreases as sensed temperature
increases (see table 4).
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
33.
Shift Delay (E37)
The heat pump control 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 (E37), 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.
*30
SECOND DELAY
30
0
SECOND DELAY
0
Figure 6. Shift Delay Settings
THERMAL PROTECTION SWITCH (S173) -
COMPRESSOR MOUNTED
Some units are equipped with a compressor mounted
normally closed temperature switch that prevents
compressor damage due to overheating caused by internal
friction. The switch is located on top of the compressor
casing. This switch senses the compressor casing
temperature and opens at 239-257°F (115°C-125°C) to
shut off compressor operation. The auto-reset switch
closes when the compressor casing temperature falls to
151-187°F (66°C-86°C),
re-energized. This single-pole, single-throw (SPST)
bi-metallic switch is wired in series with the 24V Y
input signal to control compressor operation.
Resistance values Pins/Wire
range (ohms) Color
3 and 4
(Black)
280,000 to 3750
5 and 6
(Brown)
FACTORY DEFAULT OR WHEN
JUMPER IS MISSING
and the compressor is