Lennox LDT Series Information page 19

B-Cooling Components
Enlight units use independent cooling circuits consisting of
one compressor, one condenser coil, and one evaporator
coil per circuit.
Six draw-through type condenser fans are used in LDT156,
180 & 240.
Cooling may be supplemented by a factory‐ or
field‐installed economizer.
evaporators. Each evaporator uses a thermostatic
expansion valve as the primary expansion device. Each
evaporator is also equipped with enhanced fins and rifled
tubing. In all units each compressor is protected by a
crankcase heater, high pressure switch and low pressure
switch.
1-Compressors B1 and B2
All units use scroll compressors. All units use 2
compressors, the first stage compressor is a 2-speed and
the second stage compressor is fixed speed. Compressor
capacity may vary from stage to stage. In all cases, the
capacity of each compressor is added to reach the total
capacity of the unit. See "SPECIFICATIONS" and
"ELECTRICAL DATA" (table of contents) or compressor
nameplate for compressor specifications.
WARNING
Electrical shock hazard. Compressor must be
grounded. Do not operate without protective cov­
er over terminals. Disconnect power before re­
moving protective cover. Discharge capacitors
before servicing unit. Failure to follow these pre­
cautions could cause electrical shock resulting in
injury or death.
Each compressor is energized by a corresponding
compressor contactor.
NOTE-Refer to the wiring diagram section for specific
unit operation.
If a compressor replacement
1-800-453-6669.
IMPORTANT
Some scroll compressors have an internal vacuum
protector that will unload scrolls when suction pres­
sure 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 COMPRES­
SOR.
All units intertwined
is necessary, call
Page 19
2-Crankcase Heaters HR1 and HR2
All LDT units use insertion type heaters. Heater HR1 is
installed around compressor B1 and heater 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.
S4 and S7 are is wired in series with B1 and B2 compressor
contactors.
When discharge pressure rises to 640 ± 10 psig (4413 ± 69
kPa) (indicating a problem in the system) the switch opens
and the respective compressor(s) is de-energized (the
economizer can continue to operate). When discharge
pressure drops to 475 ± 20 psig (3275 ± 138 kPa) the
pressure switch will close re-energizing the compressor(s).
Main control A55 has a three-strike counter before locking
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 manually
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 figures
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 switches
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 reset
by breaking and remaking the thermostat demand or
manually resetting the control.
When suction pressure drops to 40 + 5 psig (276 ± 34 kPa),
(indicating low pressure), the switch opens and the
compressor(s) is de-energized. The switch automatically
resets when pressure in the suction line rises to 90 + 5 psig
(620 ± 34 kPa).
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 sensor
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
(LDT) or electric heat (LHT) is energized during defrost.