Lennox KHA SERIES, Manual

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Page 23
11−Defrost Control CMC1
The KHA defrost system includes two components: a de-
frost thermostat and a defrost control.
Defrost Thermostat (Defrost Switch S6)
The defrost thermostat is located on the liquid line between
the outdoor check/expansion valve and the distributor.
When defrost thermostat senses 42°F (5.5°C) or cooler,
the thermostat contacts close and send a signal to the de-
frost control board to start the defrost timing. It also termi-
nates defrost when the liquid line warms up to 70°F (21°C).
Defrost Control
The defrost control board includes the combined functions
of a time/temperature defrost control, defrost relay and
diagnostic LEDs.
The control provides automatic switching from normal
heating operation to defrost mode and back. During com-
pressor cycle (call for defrost), the control accumulates
compressor run times at 30-, 60-, or 90-minute field−adjust-
able intervals. If the defrost thermostat is closed when the
selected compressor run time interval ends, the defrost
relay is energized and defrost begins.
Defrost Control Board
24V TERMINAL
STRIP
CONNECTIONS
DIAGNOSTIC
LEDS
HIGH PRESSURE
SWITCH
TEST
PINS
FIELD SELECT
TIMING PINS
REVERSING
VALVE
DEFROST
THERMOSTAT
FREEZESTAT
COMPRESSOR
DELAY PINS
S4
S49
FIGURE 4
Defrost Control Timing Pins
Each timing pin selection provides a different accumu-
lated compressor run time period. This time period must
occur before a defrost cycle is initiated. The defrost inter-
val can be adjusted to 30, 60 or 90 minutes (see figure 4).
The defrost timing jumper is factory−installed to provide a
60−minute defrost interval. If the timing selector jumper is
not in place, the control defaults to a 90−minute defrost
interval. The maximum defrost period is 14 minutes and
cannot be adjusted.
A TEST option is provided for troubleshooting.
The TEST
mode may be started any time the unit is in the heating
mode and the defrost thermostat is closed or jump-
ered.
If the jumper is in the TEST position at power-up, the
control will ignore the test pins. When the jumper is placed
across the TEST pins for two seconds, the control will enter
the defrost mode. If the jumper is removed before an addi-
tional 5−second period has elapsed (7 seconds total), the
unit will remain in defrost mode until the defrost thermostat
opens or 14 minutes have passed. If the jumper is not re-
moved until after the additional 5−second period has
elapsed, the defrost will terminate and the test option will
not function again until the jumper is removed and re−ap-
plied.
Compressor Delay
The defrost board has a field−selectable function to reduce
occasional sounds that may occur while the unit is cycling
in and out of the defrost mode. The compressor will be
cycled off for 30 seconds going in and out of the defrost
mode when the compressor delay jumper is removed.
NOTE − The 30-second compressor feature is ignored
when the defrost test pins are jumpered.
Time Delay
The timed-off delay is five minutes long. The delay helps to
protect the compressor from short-cycling in case the pow-
er to the unit is interrupted (demand met for example) or a
pressure switch opens. The delay is bypassed by placing
the timer select jumper across the TEST pins for 0.5 sec-
onds.
Pressure Switch Circuit
The defrost control incorporates two pressure switch cir-
cuits. An optional high pressure switch (S4) is connected to
the board’s HI PS terminals (see figure 4). The board also
includes terminals LO PS for a freezestat (S49) or low pres-
sure switch (S87). Switches are shown in the unit wiring
diagram. During a single demand cycle, the defrost control
will lock out the unit after the fifth time that the circuit is inter-
rupted by any switch wired to the control board. In addition,
the diagnostic LEDs will indicate a locked-out switch after
the fifth occurrence of an open pressure switch (see Table
1). The unit will remain locked out until power to the board is
interrupted, then re-established or until the jumper is ap-
plied to the TEST pins for 0.5 seconds.
NOTE − The defrost control board ignores input from the LO
PS terminals as follows:
during the TEST mode,
during the defrost cycle,
during the 90-second start-up period,
and for the first 90 seconds each time the reversing valve
switches heat/cool modes.
Diagnostic LEDs
The defrost board uses two LEDs for diagnostics. The LEDs
flash a specific sequence according to the condition.
TABLE 1
Defrost Control Board Diagnostic LED
Mode Green LED (DS2) Red LED (DS1)
No power to con-
trol
OFF OFF
Normal operation /
power to control Simultaneous Slow FLASH
Anti-short cycle
lockout
Alternating Slow FLASH
Low pressure
switch, freezestat
fault OFF Slow FLASH
Low pressure
switch, freezestat
lockout OFF ON
High pressure
switch fault Slow FLASH OFF
High pressure
switch lockout ON OFF