System Operation; Defrost System; Defrost Control Board - Lennox T-Class TPA090S4S Installation Instructions Manual

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
continuously.
HIGH PRESSURE SWITCHES (S4 AND S7)
These units are equipped with a 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 on the liquid line. The switch is a SPST,
auto-reset 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
compressor cycle (defrost thermostat is closed, calling for
defrost), the control accumulates compressor run times at
30, 60, or 90 minute field adjustable intervals. If the defrost
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 accumulated
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 minutes
and cannot be adjusted.
NOTE — Defrost control part number is listed near the P1
timing pins.
S
Units with defrost control 100269-02: The factory
default defrost interval is 60 minutes.
S
Units with defrost control 100269-04 or higher: The
factory 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
FIELD SELECT
TIMING PINS
TEST
PINS
COMPRESSOR
DELAY PINS
REVERSING
VALVE
S24
LOSS-OF-CHARGE
SWITCH
DEFROST
THERMOSTAT
S4
HIGH PRESSURE
SWITCH
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 jumpered. 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 additional 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 removed 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-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
delay 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.
Page 11
DIAGNOSTIC
LEDS
24V TERMINAL
STRIP
CONNECTIONS
SERVICE LIGHT
CONNECTIONS
TP SERIES