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Approach Method of Charge Verification
(Expansion Valve Systems, > 60°F Outdoor Temp.)
The approach method should be used to verify the charge
after the normal operating pressures have been confirmed
to be within the ranges given in table 5. Do not use the ap
proach method if system pressures do not match the pres
sures given in table 5. The approach method is not valid for
grossly over or undercharged systems. After you have
taken the outdoor ambient and liquid line temperature
readings with the unit operating in high speed, subtract
the outdoor ambient temperature from the liquid line tem
perature to determine the Approach Temperature. (Liquid
line °F - Outdoor Ambient °F = Approach temperature)
The resulting difference (Approach Temperature) should
agree with the values given in table 6. If not, add refrigerant
to lower the approach temperature or recover refrigerant
from the system to increase the approach temperature.
TABLE 6
APPROACH METHOD
Model
Liquid Temp. Minus
No.
Ambient Temp. °F (°C)
HP29-090
HP29-120
11.3 + 1 (6.0 + .5)
Note - For best results, the same thermometer should be used to
check both outdoor ambient and liquid temperatures.
System Operation
Heat pump and indoor blower cycle on demand from room
thermostat. When thermostat blower switch is moved to
ON position, indoor blower operates continuously.
HP29 units are equipped with external, belly band crank
case heaters. The crankcase heater should be energized
24 hours before unit start-up to prevent compressor dam
age as a result of slugging.
The scroll compressor manufacturer stipulates that all heat
pumps are to be equipped with an SPST high temperature
discharge line thermostat. If the discharge line ever reach
es or exceeds 275±5°F, the thermostat will open the com
pressor circuit. The switch automatically resets as the line
temperature decreases to 225±5°F.
HP29 units are equipped with a high pressure switch that is
located in the discharge line of the compressor. The switch
(SPST, manual reset, normally closed) removes power
from the compressor when discharge pressure rises above
factory setting at 410 + 10 psi.
HP29 units are equipped with a loss of charge switch that is
located in the liquid line immediately after the condenser
coil. The switch (SPST, auto-reset, normally closed) re
moves power from the compressor when the liquid line
pressure drops below the 25+5 psig setting. The switch au
tomatically resets once the pressure reaches 55+5 psig.
Each outdoor unit is equipped with a bypass relay which
switches control of the outdoor fan(s) to the low ambient
pressure switch (and low ambient thermostat on
9.2 + 1 (5.0 + .5)
HP29-120) during low ambient operation. The bypass
relay is wired parallel to the compressor reversing valve
and is only energized during the cooling cycle.
Each HP29 heat pump is equipped with a low ambient
pressure switch which cycles the outdoor fan during low
ambient operation. The normally closed switch opens
when liquid line pressure drops to 150 + 10 psig. The switch
automatically resets when the liquid line pressure rises to
275 + 10 psig.
HP29-120 units are equipped with a surface mounted low
ambient thermostat (normally closed) which cycles one
outdoor fan during low ambient cooling operation. The
switch opens on temperature fall at 55 + 5°F and automati
cally resets when temperature rises to 65 + 5°F.
The defrost thermostat, defrost pressure switch and
the defrost control work together to ensure that the heat
pump outdoor coil does not ice excessively during the heat
ing mode.
The defrost thermostat is located on the liquid line be
tween the check/expansion valve and the distributor. When
the liquid line temperature drops below 35°F, the switch
closes and signals the defrost control that a defrost cycle
is needed. If the defrost thermostat is still closed after the
field-selected compressor run time (30, 60 or 90 minutes)
has been completed, a defrost cycle begins. The defrost
cycle is limited to a maximum of 14 minutes. The defrost
cycle is terminated by the defrost pressure switch. The de
frost pressure switch is located on the heating cycle va
por line (cooling/defrost cycle discharge line). The normally
closed switch opens on a pressure rise at 275 + 10 psi to
terminate a defrost cycle. The pressure switch automati
cally resets.
The defrost control includes a pressure switch safety circuit
that allows for the application of an additional pressure
switch. The defrost control will lock out unit operation on the
third instance (during one demand cycle) that any auto-re
set switch opens this circuit. The diagnostic LEDs will indi
cate a pressure switch lockout. Refer to typical unit dia
gram for application of an optional loss of charge switch.
The defrost control also provides terminal connections for
an ambient thermistor and a service light. Refer to figure 18
for defrost control parts arrangement and compressor run
timing pin adjustment. Table 7 details defrost control diag
nostic LED codes.
DEFROST CONTROL BOARD DIAGNOSTIC CODES
Mode
Normal Operation/
Power to board
Time Delay to
Protect Compressor
Pressure Switch Open
Pressure Switch
Lockout
Board Malfunction
Page 19
TABLE 7
LED #1
LED #2
Simultaneous
Simultaneous
Flash
Flash
Alternating
Alternating
Flash
Flash
Off
On
On
Off
On
On
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