Lennox 13HPX Series Unit Information page 5

3 − Defrost System
The 13HPX defrost system includes two components: a
defrost thermostat and a defrost control.
Defrost Thermostat
The defrost thermostat is located on the liquid line between
the check/expansion valve and the distributor. When de-
frost thermostat senses 42°F (5.5°C) or cooler, the thermo-
stat contacts close and send a signal to the defrost control
board to start the defrost timing. It also terminates 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, diag-
nostic LEDs and terminal strip for field wiring connections.
See figure 4.
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.
13HPX Outdoor Unit Defrost Control Board
Timing
Pins
Test Pins
Compressor
Delay Pins
Reversing
Valve
Low Pressure
Switch (S87)
Defrost
Thermostat
High Pressure
Switch (S4)
FIGURE 4
Defrost Control Timing Pins
Each timing pin selection provides a different accumu-
lated compressor run time period for one defrost cycle.
This time period must occur before a defrost cycle is initi-
ated. The defrost interval can be adjusted to 30 (T1), 60
(T2), or 90 (T3) minutes (see figure 4). The defrost timing
jumper is factory−installed to provide a 60−minute defrost
interval on the 5 ton unit but a 90−minute defrost interval
on all other 13HPX size units. If the timing selector jump-
er is not in place, the control defaults to a 90−minute de-
frost 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 jumper-
ed. 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 off" cycle is not functional when
jumpering the TEST pins.
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 or a pressure switch opens. The
LED's
delay is bypassed by placing the timer select jumper across
the TEST pins for 0.5 seconds.
Pressure Switch Circuit
The defrost control incorporates two pressure switch cir-
cuits. The high pressure switch (S4) connects to the
board's HI PS terminals. The board also includes terminals
24v terminal
LO−PS for a low pressure switch (S87). See figure 4 for
strip
switch terminal location.
During a single demand cycle, the defrost control will lock
out the unit after the fifth time that the circuit is interrupted
by any pressure switch wired to the control board. In addi-
tion, the diagnostic LEDs will indicate a locked-out pres-
sure 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 applied to the TEST pins for 0.5 seconds.
NOTE − The defrost control board ignores input from the
low-pressure switch terminals as follows:
S
during the TEST mode,
S
during the defrost cycle,
S
during the 90-second start-up period,
S
and for the first 90 seconds each time the reversing
valve switches heat/cool modes. If the TEST pins are
jumpered and the 5-minute delay is being by-
passed, the LO PS terminal signal is not ignored
during the 90-second start-up period.
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