Defrost System - Lennox Merit 14HPX Installation Instructions Manual

Coil Sensor (RT21)
The coil temperature sensor (shown in figure 1) considers
outdoor temperatures below −35°F (−37°C) or above 120°F
(48°C) as a fault. If the coil temperature sensor is detected
as being open, shorted or out of the temperature range of
the sensor, the 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 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 sensor, the
defrost control will not perform demand defrost operation.
The 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.

Defrost System

The demand defrost control (A108) 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 demand defrost
control components are shown in figure 22.
TEST PINS
DEFROST
TERMINATION
PIN SETTINGS
SENSOR
PLUG IN
(COIL & AM-
BIENT
SENSORS)
DELAY
PINS
REVERSING
VALVE
PRESSURE
SWITCH
CIRCUIT
CONNECTIONS
.
Figure 22 Demand Defrost Control Control (A108)
The demand defrost
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.
LOW
AMBIENT
THERMOSTAT
PINS
DIAGNOSTIC
LEDS
24V TERMINAL
STRIP
CONNECTIONS
Note − Compo-
nent Locations
Vary by Board
Manufacturer.
control monitors ambient
NOTE − The demand defrost 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 demand defrost control initiates
defrost cycles.
DEMAND DEFROST CONTROL (A108) DIAGNOSTIC
LEDS
The state (Off, On, Flashing) of two LEDs on the defrost
control (DS1 [Red] and DS2 [Green]) indicate diagnostics
conditions that are described in table 7.
5−STRIKE LOCKOUT FEATURE
The internal control logic of the demand defrost control
counts the pressure switch trips only while the Y1 (Input)
line is active. If a 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 switch opens for a fifth time during
the current Y1 (Input), the control will enter a lockout
condition.
If a pressure switch opens while the Y1 Out line is
engaged, a 5−minute short cycle will occur after the switch
closes.
The 5−strike pressure switch lockout condition can be reset
by cycling OFF the 24−volt power to the control control or
by shorting the TEST pins between 1 and 2 seconds. All
timer functions (run times) will also be reset.
DEFROST SYSTEM SENSORS
Sensors connect to the demand defrost control (A108)
through a field-replaceable harness assembly that plugs
into the defrost control. Through the sensors, the defrost
control detects outdoor ambient, coil, and discharge
temperature fault conditions. As the detected temperature
changes, the resistance across the sensor changes. Table
9 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 6.
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 6, may be performing as
designed. However, if a shorted or open circuit is detected,
then the sensor may be faulty and the sensor harness
needs to be replaced.
Table 6. Sensor Temp. / Resistance Range
Temperature
Range °F (°C)
Sensor
Outdoor −35 (−37) to 120
(Ambient) (48)
Coil −35 (−37) to 120
(48)
Note: Sensor resistance decreases as sensed temperature increases
(see Table 9).
Page 29
Resistance values Pins/Wire
range (ohms) Color
280,000 to 3750 3 & 4
(Black)
280,000 to 3750 5 & 6
(Brown)
14HPX SERIES