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System Operation
The outdoor unit and indoor blower cycle on demand from
the room thermostat. When the thermostat blower switch
is in the ON position, the indoor blower operates contin-
uously.
HIGH PRESSURE SWITCHES (S4 AND S7)
These units are equipped with an auto-reset high pressure
switch (single-pole, single-throw) which is located on the
discharge line. The switch shuts off the compressor when
discharge pressure rises above the factory setting. High
Pressure (auto reset) – trip at 640 psig; reset at 512 psig.
LOSS OF CHARGE SWITCH (S24)
These units are equipped with a loss-of-charge switch that
is located in the liquid line. The switch is a SPST, auto-re-
set switch that is normally closed. The switch opens at 40
psi and closes at 90 psi.
Defrost System
The defrost system includes 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 on each
coil. When the defrost thermostat senses 42°F (5.5°C) or
cooler, its 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, time
delay, diagnostic LEDs, and a terminal strip for field wiring
connections.
The control provides automatic switching from normal
heating operation to defrost mode and back. During com-
pressor cycle (defrost thermostat is closed, calling for de-
frost), the control accumulates compressor run times at
30, 60, or 90 minute field adjustable intervals. If the de
-
frost thermostat is closed when the selected compressor
run time interval ends, the defrost relay is energized and
defrost begins.
Each timing pin selection provides a different accumulat-
ed compressor run time period for one defrost cycle. This
time period must occur before a defrost cycle is initiated.
The defrost interval can be adjusted to 30 (T1), 60 (T2), or
90 (T3) minutes. The maximum defrost period is 14 min-
utes and cannot be adjusted.
NOTE — Defrost control part number is listed near the P1
timing pins.
• Units with defrost control
100269-02
: The factory de-
fault defrost interval is 60 minutes.
• Units with defrost control
100269-04 or higher
: The fac-
tory default defrost interval is 90 minutes.
If the timing selector jumper is missing, the defrost control
defaults to a 90-minute defrost interval.
Defrost Control Board
DEFROST CONTROL TIMING PINS
24V TERMINAL
STRIP
CONNECTIONS
DIAGNOSTIC
LEDS
HIGH PRESSURE
SWITCH
TEST
PINS
FIELD SELECT
TIMING PINS
REVERSING
VALVE
DEFROST
THERMOSTAT
LOSS-OF-CHARGE
SWITCH
COMPRESSOR
DELAY PINS
S4
S24
SERVICE LIGHT
CONNECTIONS
FIGURE 5. Outdoor Unit Defrost Control Board
A TEST option is provided for troubleshooting. The TEST
mode may be started any time the unit is operating in the
heating mode and the defrost thermostat is closed or jum-
pered. 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 con-
trol will enter the defrost mode. If the jumper is removed
before an additional 5-second period has elapsed (7 sec-
onds total), the unit will remain in defrost mode until the
defrost thermostat opens or 14 minutes have passed. If
the jumper is not removed until after the additional 5-sec-
ond period has elapsed, the defrost will terminate and the
test option will not function again until the jumper is re-
moved and re-applied.
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. When the compressor de-
lay jumper is removed, the compressor will be cycled off
for 30 seconds going in and out of the defrost mode.
NOTE – The 30-second compressor feature is ignored
when the TEST pins have been jumpered.
TIME DELAY
The timed-off delay is five minutes long. The delay helps
protect the compressor from short-cycling in case the
power to the unit is interrupted or a pressure switch opens.
The delay is bypassed by placing the timer select jumper
across the TEST pins for 0.5 seconds.
NOTE – The board must have a thermostat demand for
the bypass function.
PRESSURE SWITCH CIRCUITS
The defrost control includes two pressure switch circuits.
The factory-installed high pressure switch (S4) wires are
connected to the board’s HI PS terminals (figure 5). The
board also includes LO PS terminals to accommodate the
factory installed loss-of-charge switch.
