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5 − Discharge Line Thermostat S5
S5 is an automatic reset SPST N.C. switch which opens on
a temperature rise. The switch is located on the discharge
line and wired in series with the CMCI board and S4 pres
sure switch. When discharge line temperature rises to
275° + 5°F the switch opens and the compressor is de−en
ergized through the CMCI. The switch automatically resets
when discharge temperature drops to 225° +
5°F.
6 − Loss of Charge Switch S24
The loss of charge switch is an auto−reset SPST N.O.
switch, which opens on pressure drop. The switch is lo
cated on the liquid line and is wired to the defrost control
board CMC1. When liquid pressure drops to 25 + 5 psig
(172 + 34 kPa), the switch opens and the compressor is de−
energized. The switch automatically resets when pressure
in the liquid line rises to 55 + 5 psig (379 + 34 kPa).
7 − Reversing Valve L1 (all units)
A reversing valve with an electromechanical solenoid is
used to reverse refrigerant flow during unit operation. L1 is
energized during cooling demand and defrost. See figures
12 and 13.
8 − Expansion Valves
The HP29−120−2 uses two expansion valves in the liquid
line adjacent to the left and right refrigerant coil. The
HP29−090−2 units have one expansion valve. Aliquid line
filter/drier and check valve are connected in parallel with
each expansion valve. The check valve allows for reverse
refrigerant flow. The HP29−120−3 uses two internally
checked expansion valveand the HP29−090−3 uses one in
ternally checked expansion valve. The valves are located
in the liquid line adjacent to the left and right refrigerant coil.
In all units expansion valve control is provided by a super
heat sensing bulb which is connected by a capillary tube to
the expansion valve. The sensing bulb is strapped to the
vapor line where it exits the coil. If the bulb senses low su
perheat, the expansion valve throttles down and restricts
refrigerant flow through the coil. When excessive super
heat is sensed, the valve opens to allow more refrigerant
flow through the coil. See figures 12 and 13.
9 − Condenser Fan B4 (both units)
B5 (HP29−120)
See page 2 for the specifications on the condenser fans
used in the HP29 units. All condenser fans have single−
phase motors. The HP29−090 units are equipped with a
single condenser fan. The HP29−120 is equipped with two
fans. The fan assembly may be removed for servicing by
removing the motor mounts nuts.
C−Defrost System
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
CAUTION
Electrostatic discharge can affect electronic
components. Take precautions during unit instal
lation and service to protect the unit’s electronic
controls. Precautions will help to avoid control
exposure to electrostatic discharge by putting
the unit, the control and the technician at the
same electrostatic potential. Neutralize electro
static charge by touching hand and all tools on an
unpainted unit surface before performing any
service procedure.
The defrost system includes four components: a defrost
thermostat, defrost pressure switch, defrost relay and de
frost control.
Defrost Thermostat Switch S6, S124
Defrost thermostat switches S6 (refrigeration circuit one)
and S124 (refrigeration circuit two) are S.P.S.T. N.O. con
tacts which close on temperature fall (initiating defrost after
minimum run time of 30, 60, or 90 minutes). The switches
are located on each of the expansion valve distributor as
semblies. The switches monitor the outdoor coil saturation
temperature to determine when defrost is needed. When
the outdoor coil temperature falls to 35_ F+ 4_F (1.7_C +
2.2_C), the switch closes (initiating defrost after minimum
run time of 30, 60, or 90 minutes). When the temperature
rises to 60_F + 5_F (15.6_C + 2.8_C), the switch opens.
Defrost Pressure Switch S46
Defrost switch S46 is an auto−reset SPST N.C. pressure
switch which opens on pressure rise of 275 + 10 psi (1896
+ 69 kPa). When S46 opens, defrost operation ends. The
switch will reset when the unit receives a heat call and
pressure falls to 195 + 10 psi (1344 + 69 kPa). All HP29
units are equipped with this switch located on the dis
charge line. See figures 1 and 2. S46 is wired through the
K8 transfer relay to the defrost board CMC1.
Defrost Relay K4
Defrost relay K4 controls defrost in the HP29 units. K4 is
controlled by defrost board CMC1 and defrost pressure
switch S46. When K4 is energized, contacts close and de
frost is initiated.
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