8
Units with Indoor Pistons
Units installed with indoor pistons require charging by the
superheat method.
The following procedure is valid when indoor airflow is within
21 percent of its rated CFM.
1. Operate unit a minimum of 10 minutes before checking
charge.
2. Measure suction pressure by attaching an accurate gage to
suction valve service port.
3. Measure suction temperature by attaching an accurate ther-
mistor type or electronic thermometer to suction line at ser-
vice valve.
4. Measure outdoor air dry–bulb temperature with thermomet-
er.
5. Measure indoor air (entering indoor coil) wet–bulb temper-
ature with a sling psychrometer.
6. Refer to Table 8. Find outdoor temperature and evaporator
entering air wet–bulb temperature. At this intersection, note
superheat. Where a dash (––) appears on the table, do not
attempt to charge system under these conditions or refriger-
ant slugging may occur. Charge must be weighted in,
adding or removing 0.6 oz/ft of 3/8 liquid line above or be-
low 15 ft (4.57 m) respectively.
7. Refer to Table 9. Find superheat temperature located in item
6 and suction pressure. At this intersection, note suction line
temperature.
8. If unit has a higher suction line temperature than charted
temperature, add refrigerant until charted temperature is
reached.
9. If unit has a lower suction line temperature than charted
temperature, reclaim refrigerant until charted temperature is
reached.
10. When adding refrigerant, charge in liquid form into suction
service port using a flow–restricting device.
11. If outdoor air temperature or pressure at suction valve
changes, charge to new suction line temperature indicated
on chart.
12. Optimum performance will be achieved when the operating
charge produces 5
_
to 6
_
F suction superheat at suction
service valve with 82
_
F outdoor ambient and 80
_
F
(26.7
_
C) dry bulb (67
_
F / 19.4
_
C) wet bulb) indoor
temperature (DOE “B” test conditions) at rated airflow.
Heating Check Chart Procedure
To check system operation during heating cycle, refer to the
Heating Check Chart on outdoor unit. This chart indicates whether
a correct relationship exists between system operating pressure and
air temperature entering indoor and outdoor units. If pressure and
temperature do not match on chart, system refrigerant charge may
not be correct. Do not use chart to adjust refrigerant charge.
25H
B
B
/25H
B
C
/25H
C
D
