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H−Service Light Thermostat HP27
HP27−1 through −7 units are equipped with a service light
thermostat (S54) located on the compressor discharge
line. The switch is electrically connected to the service
light in the indoor thermostat. The service light, when lit,
indicates the compressor is not running. The service light
is powered from W1 (2nd stage heat) terminal of the indoor
thermostat. The service light thermostat will close when
the discharge line falls below 110 + 5
F, indicating a prob-
lem in the system. The service light thermostat opens and
the service light goes off when discharge line reaches 130
+ 5
F indicating the compressor is running.
I−Ambient Compensating Thermistor
HP27−1 through −7 units have an ambient compensating
thermistor (RT3) mounted on the outdoor fan wiring har-
ness. The thermistor is an NTC thermistor (negative tem-
perature coefficient − increase in temperature equals de-
crease in resistance) (see figure 8). The device is con-
nected in series with the heat anticipation resistor inside the
indoor thermostat. This feature helps to prevent abnormal
droop caused by the anticipation resistors. As outdoor tem-
perature increases, the resistance across the thermistor
drops. As the resistance across the thermistor drops, the
current through the heat anticipation resistor increases.
Therefore, heat anticipation increases as outdoor tempera-
ture decreases. Resistance at 77
F = 260 ohms + 5%; at
100
F = 150 ohms; at 32
F = 861 ohms.
FIGURE 8
AMBIENT COMPENSATING
THERMISTOR
OUTDOOR FAN, BRACKET AND
OUTDOOR FAN
FAN MOTOR
BRACKET
J−Dual Capacitor
The compressor and fan in the HP27 series units use per-
manent split capacitor motors. A single dual capacitor
(C12) is used for both the fan motor and the compressor
(see unit wiring diagram). The fan side of the capacitor
and the compressor side of the capacitor have different
mfd ratings. The capacitor is located inside the unit control
box (see figure 6). Table 3 shows the ratings of the dual ca-
pacitor.
TABLE 3
HP27 DUAL CAPACITOR RATING
UNITS
FAN MFD
HERM MFD
VAC
HP27−024
4
40
370
HP27−030
4
40
370
HP27−036
4
45
370
HP27−042
4
55
370
K−Reversing Valve and Solenoid
A refrigerant reversing valve (L1) with electromechanical
solenoid is used to reverse refrigerant flow during unit op-
eration. The reversing valve is energized during cooling
demand and during defrost.
L−Defrost System HP27−1, −2, −3 and −5 units
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
CAUTION
Electrostatic discharge can affect electronic
components. Take precautions during unit
installation and service to protect the unit’s
electronic controls. Precautions will help to
avoid control exposure to electrostatic dis-
charge by putting the unit, the control and the
technician at the same electrostatic potential.
Neutralize electrostatic charge by touching
hand and all tools on an unpainted unit surface
before performing any service procedure.
The defrost system includes two components: a defrost
thermostat (S6) and a defrost control.
Defrost Thermostat
The defrost thermostat is mounted on the liquid line be-
tween the check/expansion valve and the distributor. When
defrost thermostat senses 35
F (2
C) or cooler, its con-
tacts close and send a signal to the defrost control board to
start the defrost timing. It also terminates defrost when the
liquid line warms up to 70
F (21
C).
Defrost Control
The defrost control board has the combined functions
of a time/temperature defrost control, defrost relay,
time delay, diagnostic LEDs and field connection termi-
nal strip. See figure 9.
The control provides automatic switching from normal
heating operation to defrost mode and back. During com-
pressor cycle (room thermostat demand cycle), if the O"
input is not on and the defrost thermostat is closed, the
control accumulates compressor run times at 30, 60 or 90
minute field adjustable intervals. If the defrost thermostat
remains closed when the accumulated compressor run
time ends, the defrost relay is energized and defrost be-
gins.
