Diagnostic Sensors - Lennox LHT/LDT024 Installation Instructions Manual

C-Compressor Controls
See unit wiring diagram to determine which controls are
used on each unit. Optional controls are identified on
wiring diagrams by arrows at junction points.
1- High Pressure Switch (S4)
The compressor circuit is protected by a high
pressure switch which opens at 640 psig + 10 psig
(4413 kPa + 70 kPa) and automatically resets at 475
psig + 20 psig (3275kPa + 138 kPa).
2- Low Pressure Switch (S87)
The compressor circuit is protected by a loss of
charge switch. Switch opens at 25 psig + 5 psig (172
+ 34 kPa) and automatically resets at 40 psig + 5
psig (246 kPa + 34 kPa).
3- Diagnostics Sensors (RT46, RT48)
Two thermistors are located on specific points in the
refrigeration circuit. The thermistors provide constant
temperature feedback to the Unit Controller to protect
the compressor. Thermistors take the place of the
freezestat and low ambient pressure switch.
4- Defrost Controls (RT48, RT17)
Both sensors provide input to the defrost control
which cycles defrost. The ambient sensor is located
on the inside of the corner mullion on the back of the
outdoor coil section. The coil sensor is located on a
return bend on the front of the outdoor coil.
5- Compressor Crankcase Heater (HR1)
Crankcase heater must be energized at all times to
prevent compressor damage due to refrigerant
migration. Energize crankcase heater 24 hours before
unit start-up by setting thermostat so that there is no
cooling demand (to prevent compressor from cycling)
and apply power to unit.
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 issue demand defrost controls from the Unit
Controller. 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 (optional) is energized
during defrost.
Defrost Test or Forced Defrost Option
A TEST option is provided for troubleshooting. The TEST
mode may be started at any time using the mobile service
app. Defrost mode may be started by entering the Defrost
Mode in the Component Test Menu. When defrost is
started, unit will run in Defrost Mode for a maximum of 5
minutes or when the outdoor coil reaches 100°F,
whichever occurs first.

Diagnostic Sensors

Units are equipped with two factory-installed thermistors
(RT46 and RT48) located on different points on the
refrigerant circuit.
The thermistors provide the Unit Controller with constant
temperature readings of two specific locations on the
refrigeration circuit. These temperatures are used as
feedback in certain modes of unit operation. In addition,
the Unit Controller uses these temperatures to initiate
alarms such as loss of condenser or evaporator airflow
and loss of charge.
Each thermistor must be specifically placed for proper
unit operation and to initiate valid alarms. See table 10 for
proper locations.
THERMISTOR LOCATION
Unit
024, 036, 048 Indoor Coil
024, 036 Outdoor Coil
048 Outdoor Coil
060 Indoor Coil
060 Outdoor Coil
Page 26
TABLE 10
Sensor
Yel­
low
RT46
RT48
RT48
RT46
RT48
LHT/LDT024, 036, 048, 060
Figure
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