Lennox LHT Series Service Manual page 21

2-Crankcase Heaters HR1 and HR2
All LHT units use insertion type heaters. Heater HR1 is
installed around compressor B1 and HR2 compressor B2.
3-High Pressure Switches S4 and S7
The high pressure switches is an auto-reset SPST N.C.
switch which opens on a pressure rise. All units are
equipped with this switch. The switch is located in the
compressor discharge line and is wired in series with the
compressor contactor coil through A55 unit controller or
A178 compressor 3 and 4 controller.
S4 and S7 are is wired in series with B1 and B2 compres-
sor contactors.
When discharge pressure rises to 640 ± 10 psig (indicat-
ing a problem in the system) the switch opens and the re-
spective compressor(s) is de-energized (the economizer
can continue to operate). When discharge pressure drops
to 475 ± 20 psig the pressure switch will close re-energiz-
ing the compressor(s).
Main control A55 has a three-strike counter before lock-
ing out. This means the control allows three high pressure
trips per one thermostat demand. The control can be reset
by breaking and remaking the thermostat demand or man-
ually resetting the control
4-Low Pressure Switches S87 and 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. See
figure 7.
S87 and S88 (compressor one and two) are wired in series
with the contactor coils through the A55 Unit Controller
The 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 a single thermostat demand,
before the compressor(s) is locked out. The control is re-
set by breaking and remaking the thermostat demand or
manually resetting the control.
When suction pressure drops to 40 + 5 psig (indicating
low pressure), the switch opens and the compressor(s) is
de-energized. The switch automatically resets when pres-
sure in the suction line rises to 90 + 5 psig.
5-Defrost Control
The defrost control ensures that the heat pump outdoor
coil does not ice excessively during the heating mode. The
defrost control uses input from the coil and ambient sen-
sor to initiate demand defrost cycles from the M4 Board. If
the system fails to calibrate or obtain readings for demand
defrost, defrost will run-time at field setting. Low gas heat
(LHT) or electric heat (LHT) is energized during defrost.
6-Filter Drier (all units)
Units have a filter drier located in the liquid line of each re-
frigerant circuit at the exit of each condenser coil. The dri-
er removes contaminants and moisture from the system.
7-Condenser Fans
B4, B5, B21, B22, B23 and B24 (All Units)
See SPECIFICATIONS tables at the front of this manual
for specifications of condenser fans used in all units. All
condenser fans used have single-phase motors. The fan
assembly may be removed for servicing and cleaning.
8-Reversing Valve
A refrigerant reversing valve with a 24 volt solenoid coil
is used to reverse refrigerant flow during unit operation
in all LHT units. The reversing valve is connected in the
vapor line of the refrigerant circuit. The reversing valve coil
is energized during cooling demand and during defrost.
Reversing valve L1 & L2 are controlled by the A55 Control
board in response to a cooling demand or by defrost.
9-Temperature Thermistor
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. In addition, the
Unit Controller uses these temperatures to initiate alarms
such as loss of outdoor or indoor coil airflow and loss of
charge.
Each thermistor must be specifically placed for proper unit
operation and to initiate valid alarms. See figure 9 for in-
door coil location and figure 10 for outdoor coil location.
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