Defrost System - Lennox XP16 Series Unit Information

Defrost System

DEFROST SYSTEM DESCRIPTION
TEST PINS
DEFROST
TERMINATION
PIN SETTINGS
SENSOR
PLUG IN
(COIL &
AMBIENT
SENSORS)
DELAY
PINS
REVERSING
VALVE
PRESSURE
SWITCH
CIRCUIT
CONNECTIONS
Note − Component locations vary by board manufacturer.
.
Figure 20 Defrost Control Board
The demand defrost controller measures differential
temperatures to detect when the system is performing
poorly because of ice build−up on the outdoor coil. The
controller self−calibrates when the defrost system starts
and after each system defrost cycle. The defrost control
board components are shown in figure 20.
The control monitors ambient temperature, outdoor coil
temperature, and total run time to determine when a defrost
cycle is required. The coil temperature probe 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.
NOTE − The demand defrost board 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 board initiates defrost cycles.
DEFROST BOARD PRESSURE SWITCH
CONNECTIONS
The unit's automatic reset pressure switches (LO PS − S87
and HI PS − S4) are factory−wired into the defrost board on
the LO−PS and HI−PS terminals, respectively.
Low Pressure Switch (LO−PS) When the low pressure
switch trips, the defrost board will cycle off the compressor,
and the strike counter in the board will count one strike. The
low pressure switch is ignored under the following
conditions:
During the defrost cycle and 90 seconds after the
D
termination of defrost
When the average ambient sensor temperature is below
D
15° F (−9°C)
For 90 seconds following the start up of the compressor
D
During test mode
D
XP16
High Pressure Switch (HI−PS) When the high pressure
switch trips, the defrost board will cycle off the compressor,
and the strike counter in the board will count one strike.
DEFROST BOARD PRESSURE SWITCH SETTINGS
High Pressure (auto reset) − trip at 590 psig, reset at 418.
LOW
AMBIENT
THERMOSTAT Low Pressure (auto reset) − trip at 25 psig; reset at 40.
PINS
PRESSURE SWITCH 5−STRIKE LOCKOUT
The internal control logic of the board counts the pressure
DIAGNOSTIC
switch trips only while the Y1 (Input) line is active. If a
LEDS
pressure switch opens and closes four times during a Y1
(Input), the control logic will reset the pressure switch trip
counter to zero at the end of the Y1 (Input). If the pressure
24V TERMINAL
switch opens for a fifth time during the current Y1 (Input), the
STRIP
CONNECTIONS
control will enter a lockout condition.
The 5−strike pressure switch lockout condition can be reset
by cycling OFF the 24−volt power to the control board or by
shorting the TEST pins between 1 to 2 seconds. All timer
functions (run times) will also be reset.
If a pressure switch opens while the Y1 Out line is engaged,
a 5−minute short cycle will occur after the switch closes.
UNIT TEMPERATURE SENSORS
Sensors connect to the defrost board through a
field-replaceable harness assembly that plugs into the board
as illustrated in figure 20. Through the sensors, the board
detects outdoor ambient, coil, and discharge temperature
fault conditions. As the detected temperature changes, the
resistance across the sensor changes. Sensor resistance
values can be checked by ohming across pins shown in table
3.
Table 3. Sensor Temperature /Resistance Range
Sensor
Outdoor
Coil
Discharge (if
applicable)
Note: Sensor resistance increases as sensed temperature decreases.
Figure 21 shows how the resistance varies as the
temperature changes for both type of sensors.
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 Sensor The ambient sensor 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 sensor, 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.
Page 37
Temperature Resistance values
Range °F (°C) range (ohms)
−35 (−37) to 120 (48) 280,000 to 3750
−35 (−37) to 120 (48) 280,000 to 3750
24 (−4) to 350 (176) 41,000 to 103
Pins/W
ire
Color
3 & 4
(Black)
5 & 6
(Brown
)
1 & 2
(Yel-
low)