Defrost System - Lennox XP21 Series Installation Instructions Manual

HIGH (S4) AND LOW (S87) PRESSURE SWITCHES
The unit's pressure switches (LO PS − S87 and HI PS − S4)
are factory−wired into the control on the LO−PS and HI−PS
terminals, respectively.
Low Pressure Switch (LO−PS)
pressure switch sequence of operation.
High Pressure Switch (HI−PS)
pressure switch sequence of operation.
Pressure Switch Event Settings
The following pressures are the auto reset event value
triggers for low and high pressure thresholds:
S
High Pressure (auto reset) − trip at 590 psig; reset at
418.
S
Low Pressure (auto reset) − trip at 25 psig; reset at 40.
COMPRESSOR PROTECTION
LOCKOUT SAFETY FUNCTION
The five−strike lockout safety function is designed to
protect the unit's compressor from damage. The five−strike
feature is used for high pressure (S4) and low (S87)
pressure switch trips, high discharge temperature (RT28)
sensor input and W input fault or miswire.
Resetting Five−Strike Lockout
Once the condition has been rectified, power to the heat
pump control's R terminal must be cycled OFF, or a jumper
placed on the FIELD TEST pins between 1− to 2−seconds
to reset the heat pump control.

Defrost System

The heat pump control (A175) measures differential
temperatures to detect when the system is performing
poorly because of ice build−up on the outdoor coil. The heat
pump control self−calibrates (see figure 27) when the
defrost system starts and after each system defrost cycle.
The heat pump control monitors ambient temperature,
outdoor coil temperature, and total run−time to determine
when a defrost cycle is required. The coil temperature
sensor is designed with a spring clip to allow mounting to
the outside coil tubing. The location of the coil sensor is
important for proper defrost operation (see figure 1 for
location of coil sensor).
NOTE − The heat pump control accurately measures the
performance of the system as frost accumulates on the
outdoor coil. This typically will translate into longer running
time between defrost cycles as more frost accumulates on
the outdoor coil before the heat pump control initiates
defrost cycles.
DEFROST OPERATING MODES
The heat pump control has three operational modes which
are:
S
Defrost calibration and operation (see figure 27)
S
Defrost test (see figure 28)
See figure 29 for low
See figure 30 for high
FIVE−STRIKE
Page 33
DEFROST TERMINATION TEMPERATURES (E47)
The heat pump control selections are: 50, 70, 90, and
100°F (10, 21, 32 and 38°C). The jumper termination pin is
factory set at 50°F (10°C).
If the temperature jumper is not installed, the default
termination temperature is 90°F (32°C). See figure 27 for
on how this settings affects defrost calibration and defrost
modes.
NOTE − Colder climates could require a high discharge
termination temperature setting to maintain a clear coil.
100
90
DEGREE
DEGREE
TARGET
TARGET
70
50
DEGREE
DEGREE
TARGET
TARGET
Figure 24. Defrost Termination Temperature
Settings
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 detected temperature changes, the resistance
across the sensor changes. Tables 11 and 12 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 10.
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 10, 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 10 for sensor range. If the ambient sensor is
detected as being open, shorted or out of the temperature
range of the sensor, the control will not perform demand
defrost operation. The
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 10 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.
IF JUMPER IS
NOT INSTALLED
(90ºF)
FACTORY DEFAULT
(50ºF)
control will revert
XP21 SERIES
to