Lennox LGT Series Unit Information page 18

Each compressor is energized by a corresponding com-
pressor contactor.
NOTE-Refer to the wiring diagram section for specific unit
operation. If Interlink compressor replacement is neces-
sary, call 1-800-453-6669.
IMPORTANT
Some scroll compressors have an internal vacuum
protector that will unload scrolls when suction
pressure goes below 20 psig. A hissing sound
will be heard when the compressor is running
unloaded. Protector will reset when low pressure
in system rises above 40 psig. DO NOT REPLACE
COMPRESSOR.
2-Crankcase Heaters HR1, HR2
All LGT units use insertion type heaters. Heater HR1 is in-
stalled around compressor B1 and heater HR2 is installed
around compressor B2. Crankcase heater wattage varies
by compressor size.
3-High Pressure Switches S4, S7
The high pressure switch is an auto-reset SPST N.C.
switch which opens on a pressure rise. All units are
equipped with this switch. On fin/tube outdoor coils, the
switch is located in the compressor discharge line. On al-
laluminum outdoor coils, the switch is located on the liquid
line in the blower section. Switches are wired in series
with the compressor contactor coil.
On standard and high efficiency units, S4 (first circuit) and
S7 (second circuit) are wired in series with the respective
compressor contactor coils. On ultra high efficiency units,
only S4 is used. S4 is located on the common compressor
discharge line and is wired to both compressor contactors
via the A55 Unit Controller.
When discharge pressure rises to 610 ± 15 psig (4206 ±
103 kPa) (indicating a problem in the system) the switch
opens and the respective compressor is de-energized (the
economizer can continue to operate). When discharge
pressure drops to 475 ± 15 psig (3275 ± 103 kPa) the
pressure switch will close.
The A55 Unit Controller has a three-strike counter before
locking out. This means the control allows three high pres-
sure trips per one thermostat demand. The control can be
reset by breaking and remaking the thermostat demand or
manually resetting the control.
4-Filter Drier
LGT units have a filter drier located in the liquid line of
each refrigerant circuit. The drier removes contaminants
and moisture from the system.
5-Low Pressure Switches S87, S88
The low pressure switch is an auto-reset SPST N.O.
switch (held N.C. by refrigerant pressure) which opens on
a pressure drop. All units are equipped with this switch.
The switch is located in the compressor suction line. S87
(compressor one) and S88 (compressor two) are wired to
A55 Unit Controller. A55 governs the low pressure switch-
es by shunting the switches during start up until pressure
is stabilized. After the shunt period, the control has a three-
strike counter, during first thermostat demand, before the
compressor is locked out. The control is reset by breaking
and remaking the thermostat demand or manually reset-
ting the control.
When suction pressure drops to 40 + 5 psig (276 ± 34
kPa), (indicating low pressure), the switch opens and the
compressor( s) is(are) de-energized. The switch automat-
ically resets when pressure in the suction line rises to 90 +
5 psig (620 ± 34 kPa) due to many causes such as refrig-
erant being added.
6-Condenser Fans B4 and B5
See SPECIFICATIONS tables at the front of this manual
for specifications of condenser fans used in all units. All
condenser fans have single-phase motors. The fan as-
sembly may be removed for servicing and cleaning.
7-Temperature Sensors RT46, RT47, RT48 & RT49
Units are equipped with four factory-installed thermistors
(RT46 / RT49) located on different points on the refriger-
ant circuit.
The thermistors provide the Unit Controller with constant
temperature readings of four specific locations on the re-
frigeration circuit. These temperatures are used as feed-
back in certain modes of unit operation.
Each thermistor must be specifically placed for proper unit
operation and to initiate valid alarms. See figure 7 and
figure 8 proper locations.
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