Bulletin 30-020.007
Copyright © 2019 Unico Inc. Page 24
After the refrigerant lines and evaporator have been
carefully leak tested and evacuated, release the R410A
operating charge in the condensing unit. The system is
now ready for refrigerant charge adjustment. If the coil
is equipped with a threaded TXV, always verify that
the threads are tight and do not leak.
Start up the system and check line voltage to assure it
is within acceptable limits for the system as dictated by
the condensing unit manufacturer. Run the system for
20 to 30 minutes to get reasonably stabilized
conditions. Do not attempt to adjust charge with
outdoor temperature below 75°F (24°C). An outdoor
temperature of 75 to 85°F (24 to 29°C) is preferred.
If the system charge must be checked when the outdoor
temperature is below 80°F (26.7°C), block the
condenser coil until the head pressure is approximately
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 3 an 8°F (2 to 5°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 10).
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 and refrigerant must be
added.
If it is high, the system is overcharged and some
refrigerant must be removed and collected in an empty
refrigerant container. DO NOT RELEASE
REFRIGERANT INTO THE ATMOSPHERE.
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.
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 it is more than expected please refer to the
Technote 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).
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