Charging Procedures

Should I check the refrigerant charge with superheat or subcooling?

If the outdoor unit has a single speed or two speed compressor running in second speed, and the unit is not a micro-channel coil, then Total Superheat or Subcooling can be used to check the charge. To determine which one, view the indoor unit rating plate to see the metering device listed as installed.

If the metering device cannot be determined by reading the indoor unit rating plate, verify the metering device by removing the front cover. The metering device listed may have been changed by the technician, so it is best to visually verify the type of metering device.

Which Method?

Fixed orifice — piston or capillary tube: check the charge with Total Superheat.
TXV (Thermostatic Expansion Valve): check the charge with Subcooling, and look for the target subcooling posted at the outdoor unit.

Subcooling Method (TXV Systems)

Measure the liquid line pressure at the small service port and convert it to saturation temperature. Measure the actual liquid line temperature within 6 inches of the service valve and subtract it from the saturation temperature. That is your actual subcooling.

Add refrigerant to raise subcooling, or recover refrigerant to lower subcooling. Match the actual subcooling as close as you can to the target subcooling.

Total Superheat Method (Fixed Orifice Systems)

Measure the indoor wet bulb and the outdoor dry bulb temperatures, then use a superheat chart, calculator, app, or digital manifold to find the target superheat. For example, a 64°F indoor WB with an 85°F outdoor DB.

For the actual superheat, convert the pressure at the large vapor line service valve to saturation temperature, then subtract that from the actual vapor line temperature measured near the valve.

Add refrigerant to lower superheat, or recover refrigerant to raise superheat.

See also the Target Superheat chart, the Target Superheat Calculator, the P/T Chart, and Refrigerant Weights.

If you want to learn more about refrigerant charging methods, procedures, and troubleshooting, read the Refrigerant Charging and Service Procedures book by Craig Migliaccio.

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