Bulletin 30-015.003
Copyright © 2017 Unico Inc.
Page 13
equal to what its charging chart specifies for an 85°F
(29 °C) day.
For heat pumps always check the charge in cooling
mode. If this is not possible because of low outdoor
temperatures, charge the system in the heating mode,
but return later when the weather is warmer before the
system is switched to cooling.
Sub-cooling Method.
Many condensing unit
manufacturers publish the amount of sub-cooling that
the condenser will produce. Follow their instructions
to charge the unit. Typical sub-cooling values will be
between 8 and 15°F (5 to 9 °C). The unit should
ALWAYS have some amount of sub-cooling. To be
sure there is enough sub-cooling, especially if the unit
is in a hot attic, check the liquid line sight glass near
the evaporator for bubbles or measure the refrigerant
liquid line pressure and temperature AT THE
EVAPORATOR.
To measure sub-cooling use the following procedure:
1.
Measure and record the liquid line pressure using
an accurate refrigerant gauge. Record the
corresponding saturation temperature for this
pressure (see Table 3).
2.
Measure and record the liquid line temperature
using an accurate metal or glass thermometer, or
thermocouple. Tape or strap the sensor firmly
against the surface of the liquid line and cover with
insulation.
3.
Determine the sub-cooling with the following
equation:
SATURATED TEMPERATURE
- LIQUID LINE TEMPERATURE
SUBCOOLING
If the sub-cooling temperature at the condenser is low,
the system is undercharged. If it is high, the system is
overcharged and some refrigerant must be removed
and collected in an empty refrigerant container.
Do not
vent the refrigerant; it is a violation of federal law!
In some cases, such as in a hot attic, the liquid line will
pick up heat and lose its sub-cooling. This will be
apparent if the sub-cooling at the evaporator is low. In
these cases, the liquid line should be insulated or
strapped to the suction line and both insulated. The
same problem can occur for long refrigerant lines; in
this case, increase the size of the liquid line to reduce
the pressure drop.
CAUTION. TO MAINTAIN PROPER
HEAT PUMP OPERATION, DO NOT
STRAP THE LIQUID AND SUCTION
LINES TOGETHER FOR HEAT PUMP
SYSTEMS.
Superheat Method
Do not charge the system based
on superheat. Superheat measurements should only be
used to verify that the expansion valve is working
properly. If is more than expected please refer to
Tech
note 114
on troubleshooting expansion valves.
The superheat should be between 8 to 12°F (4 to 7°C)
at the indoor coil. In some cases, particularly for the
larger capacity match-ups (i.e. 3 ton and 5 ton), a
superheat of 15 to 18°F (8 to 10°C) is satisfactory. It is
not uncommon to measure a superheat above 20 to
25°F (11 to 14°C) at the condensing unit.
Be aware that the superheat value is also dependent on
the outdoor air temperature. At lower air temperatures
the superheat will be higher than at higher air
temperatures. If the condenser ambient temperature is
between 75 and 85°F (24 to 29°C), superheat should
be approximately 10 to 12°F (5 to 7°C). If the outdoor
temperature is between 85 and 105°F (29 to 40°C),
superheat should be approximately 8 to 10°F (4 to
5°C).
To measure the superheat use the following:
1.
Measure and record the suction pressure at the
evaporator outlet using an accurate refrigerant
gauge. If this is not possible, measure the pressure
at the service port on the suction valve fitting at the
condensing unit and add the estimated pressure loss
in the suction line between the condensing unit and
evaporator. Record the corresponding saturation
temperature for this pressure (see Table 5).
2.
Measure the suction line temperature at the
evaporator outlet using an accurate metal or glass
thermometer,
or
thermocouple.
Insert
the
thermometer under the insulation on the suction
line and tape firmly against the surface of the
suction tube.
3.
Determine the superheat with the following
equation:
Suction Line Temperature
— Saturated Temperature
=
Superheat
