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3-Reversing Valve L1 and L2
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 en
ergized during cooling demand and during defrost.
Reversing valve L1 and L2 are controlled by the M4 con
troller in response to cooling demand or by defrost.
4-Defrost Pressure Switch S46 and S104
The defrost pressure switch S46 and S104 are auto‐reset
SPST N.C. pressure switches which open on a pressure
rise. All LHT units are equipped with these switches. The
switches are located on the discharge line. S46 and S104
are wired in series with the CMC1 control board.
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 pres
sure 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
switches by shunting the switches during start up until pres
sure is stabilized. After the shunt period, the control has a
three‐strike counter, during first thermostat demand, be
fore the compressor is locked out. The control is reset by
breaking and remaking the thermostat demand or manual
ly resetting the control.
When suction pressure drops to 25 +
5 psig, (indicating low
pressure), the switch opens and the compressor(s) is(are)
de-energized. The switch automatically resets when pres
sure in the suction line rises to 40 +
5 psig due to many
causes such as refrigerant being added.
When discharge pressure reaches 450 + 10 psig (3102 $
69 kPa) in either circuit (indicating defrost is completed) the
appropriate switch opens. The switches automatically re
set when pressure in the suction line drops to 300 +
20 psig
(2068 $ 138 kPa).
6-Defrost Temperature Switch S6 and S9
Defrost thermostat switches S6 and S9 have S.P.S.T. N.O.
contacts which close on a temperature fall (initiating de
frost). The switches are located on the expansion valve dis
tributor assembly at the inlet to the outdoor coil. The switch
monitors the outdoor coil suction temperature to determine
when defrost is needed. When the outdoor coil suction tem
perature falls to 35F +
4F (1.7C + 2.2C) the switch clos
es (initiating defrost after minimum run time of 30, 60, or 90
minutes). When the temperature rises to 60F +
5F
(15.6C +
2.8C) the switch opens.
7-Filter Drier (all units)
LHT units have a filter drier located in the liquid line of each
refrigerant circuit at the exit of each condenser coil (outdoor
coil in LHT units). The drier removes contaminants and
moisture from the system.
8-Condenser Fan Motors B4, B5, B21
See specifications section of this manual for specifications
of condenser fans B4 , B5, and B21 (B21 on 122 and 150
units only). All LHT motors are electrically commutated
condenser fan motors (ECM). The ECM motors are wired
directly to 230VAC power but do not operate until a pulse
width modulated (PWM) control signal is sent from M4 con
troller. All outdoor fans will run at the same speed when the
appropriate PWM signal is received. The fans may be re
moved for servicing and cleaning by removing the fan
grilles.
9-Crankcase Heaters HR1, HR2
Heater HR1 is installed around compressor B1 and heater
HR2 is installed around compressor B2. Crankcase heater
wattage varies by compressor manufacturer.
10-Temperature Sensors RT46, RT47, RT48
& RT49
Units are equipped with four factory-installed thermistors
(RT46 / RT49) located on different points on the refrigerant
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 5 and fig
ure 6 proper locations.
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