Appendix R: Adjusting Refrigerant Charge
Adjusting the charge of a system in the field must be
based on determination of liquid sub-cooling and evapo-
rator superheat. On a system with a TXV, liquid sub-cool-
ing is more representative of the charge than evaporator
superheat but both measurements must be taken.
Before Charging
1. Unit being charged must be at or near full load con-
ditions before adjusting the charge.
2. Units equipped with hot gas re-heat must be
charged with the hot gas re-heat valves closed
while the unit is in cooling mode to get the proper
charge. After charging, the unit must be operated in
re-heat (dehumidification) mode to check for correct
operation.
3. Units equipped with heat pump options must be
charged in heating mode to get the proper charge.
After charging, unit must be operated in cooling
mode to check for correct charge. Charge may need
to be adjusted for cooling mode. If adjustments are
made in the cooling mode, heating mode must be
re-run to verify proper operation.
4. After adding or removing charge, the system must
be allowed to stabilize, typically 10–15 minutes, be-
fore making any other adjustments.
5. The type of unit and options determine the ranges
for liquid sub-cooling and evaporator superheat.
Refer to Table R1 when determining the proper sub-
cooling and superheat.
Checking Liquid Sub-cooling
1. Measure the temperature of the liquid line as it leaves
the condenser coil.
2. Read the gauge pressure at the liquid line close to
the point where the temperature was taken. One
must use liquid line pressure as it will vary from
discharge pressure due to condenser coil pressure
drop.
3. Convert the pressure obtained to a saturated tem-
perature using the R410a refrigerant temperature
verses pressure Table R2. Subtract the measured
liquid line temperature from the saturated tempera-
ture to determine the liquid sub-cooling.
4. Compare calculated sub-cooling to Table R1 for the
appropriate unit type and options.
Checking Evaporator Superheat
1. Measure the temperature of the suction line close to
the evaporator.
2. Read gauge pressure at the suction line close to the
evaporator.
3. Convert the pressure obtained to a saturated tem-
perature using the R410a refrigerant temperature
verses pressure Table R2.
4. Subtract the saturated temperature from the mea-
sured suction line temperature to determine the
evaporator superheat.
5. Compare calculated superheat to Table R1 for the
appropriate unit type and options.
CAUTION
The Clean Air Act of 1990 bans the intentional vent-
ing of refrigerant (CFCs and HCFCs) as of July 1, 1992.
Approved methods of recovery, recycling or reclaiming
must be followed. Fines and/or incarceration may be lev-
ied for non-compliance.
CAUTION
It is very important to make certain that the recycle or re-
covery equipment used is designed for R410a. The pres-
sure of R410a refrigerant is approximately 60% greater
than that of R22. Pressure gauges require a range up to
800 psig high side and 200 psig low side. Recovery cyl-
inders require a 400 psig rating.
CAUTION
Thermal expansion valve must be adjusted to ap-
proximately 15°F to 20°F [−9°C to −7°C] of compressor
suction superheat. Compressor suction superheat is
obtained by measuring temperature and pressure at the
suction line closest to the compressor. Failure to have
sufficient superheat will damage the compressor and
void the warranty.
Table R1: Acceptable Refrigerant and Circuit Values
Air cooled Condenser
Liquid sub-cooling
10°F
Evaporator superheat
8–10°F
Compressor discharge gas temperature
<210°F
Water Cooled Condenser or Water Source Heat Pump
Liquid sub-cooling
10°F
Evaporator superheat
8°F
Compressor discharge gas temperature
<210°F
VCES-VHC-IOM-1F – VHC-36, -42 & -50
89
