The two methods, defined

Charging by weight means adding refrigerant to a system using a calibrated scale, until the mass of refrigerant added equals the manufacturer's specified charge. It is the most accurate method and is used at the factory during equipment assembly. Field technicians use charging by weight for new installations and for major repairs where the system has been fully recovered.

Charging by subcool (or by superheat) means adding refrigerant while monitoring a thermodynamic parameter — typically the subcooling temperature at the condenser outlet or the superheat temperature at the evaporator outlet — and stopping when the measured value matches the manufacturer's target. It is a field method for adjusting charge on an operating system without removing the refrigerant.

When to use weight

Charging by weight is the right choice when:

• The system is fully evacuated (no refrigerant present).
• The manufacturer's nameplate specifies a charge weight in pounds or ounces.
• You have access to a calibrated scale accurate to within ± 0.2 oz (5 g).
• The refrigerant being added is virgin or known-purity reclaimed.

A typical sequence:

1. Connect the refrigerant cylinder to the liquid line service port through a charging manifold.
2. Open the cylinder valve and purge air from the hose.
3. Open the liquid service valve to admit liquid refrigerant.
4. Watch the scale. Stop when the cylinder weight loss equals the manufacturer's specified charge.
5. Close the cylinder valve and the service valve.
6. Run the system, verify operating pressures and temperatures.

Weight charging is exact, but it does not compensate for errors in the nameplate spec, errors in the line set length, or errors in the original factory charge. For an installation with a 25-foot line set versus a 50-foot line set, the additional refrigerant needed may not match the nameplate.

When to use subcool

Charging by subcool is the right choice when:

• The system is already charged and operational, and you are making a small adjustment.
• You do not have a scale or are working on a system where weighing is impractical (large chillers, for example).
• The manufacturer has published a target subcool value for the specific model.

The procedure:

1. Operate the system at design conditions (steady state, indoor blower at rated speed, outdoor temperature above 65 °F).
2. Measure the liquid line temperature at the condenser outlet with a calibrated thermometer or thermistor.
3. Measure the high-side pressure and convert to saturation temperature using a pressure-temperature chart for the specific refrigerant.
4. Calculate subcool = saturation temperature − liquid line temperature.
5. Add refrigerant slowly through the liquid service valve until subcool matches the target.
6. Stop, observe for a few minutes, and recheck.

Target values vary by equipment. Typical residential R-410A air-conditioner targets are 10 to 15 °F of subcool. R-22 systems typically target 12 to 18 °F. R-454B systems typically target 8 to 12 °F. The manufacturer's installation manual is the authoritative source.

When to use superheat

Charging by superheat is used for systems with a fixed-orifice metering device (piston or capillary tube). The procedure:

1. Measure the suction line temperature at the evaporator outlet.
2. Measure the low-side pressure and convert to saturation temperature.
3. Calculate superheat = suction line temperature − saturation temperature.
4. Add refrigerant to lower superheat, recover refrigerant to raise superheat.

Target superheat for R-410A fixed-orifice systems is typically 10 to 15 °F. Adjusting superheat for systems with a TXV (thermostatic expansion valve) is generally unnecessary because the TXV controls superheat automatically.

The exam's favorite comparison question

The Universal exam frequently asks: "Which charging method is most accurate for a new installation?" The answer is charging by weight. Subcool and superheat adjustments are field techniques for fine-tuning an already-charged system.

A related favorite: "When can you add liquid refrigerant directly to the suction service valve?" The answer is never — adding liquid to the suction service port can slug the compressor. Liquid must be added to the liquid service valve (with the cylinder upright and the system not running) or as a vapor to the suction service valve.

Common mistakes in the field

Overcharging by weight. A technician adds 7 pounds when the nameplate calls for 6.5 because the scale was not tared or the cylinder was pre-weighed incorrectly. The system runs high head pressure, low subcool, and may overcurrent the compressor. The fix is recovery to the correct weight.

Charging by subcool on a non-steady system. Adding refrigerant while the system is cycling on the indoor thermostat will give inconsistent subcool readings. Always let the system stabilize for 10 to 15 minutes before charging by subcool.

Charging with the cylinder upside down. Liquid refrigerant in the cylinder flows out as a liquid, which is fine for the liquid service valve, but can overfeed the system quickly. Charge slowly, and verify the cylinder orientation.

The reality in field practice

Most service technicians use a combination approach:

• New installations and full recoveries: charge by weight to the manufacturer's nameplate.
• Warranty top-offs and seasonal adjustments: adjust by subcool or superheat to fine-tune.
• Burnout repairs: charge by weight after triple evacuation and filter-drier installation, then verify subcool at steady state.

The Universal exam rewards the candidate who knows both methods and when to apply each. Master the names, the procedures, and the typical target values — and you will recognize the exam's charging-method questions from a sentence away.