Charging, Performance Testing, and Troubleshooting
Charge methods
Weigh-in is the standard for new installations and critically charged systems. Superheat charging applies to fixed-orifice/piston systems; subcooling charging applies to TXV/EEV systems; some manufacturers publish an approach method (condensing temperature minus outdoor ambient). Always verify airflow and coil cleanliness first - airflow faults mimic charge faults and will send you chasing the wrong problem.
Performance checks
Measure evaporator delta T (return minus supply air, typically 16–22 °F in cooling), superheat, subcooling, compressor amperage against nameplate, and static pressure. For heat pumps, charge in cooling mode at a fixed indoor wet bulb when possible; otherwise follow the manufacturer's heating-mode chart exactly, accounting for defrost.
Symptom logic
- Undercharge: low suction and head pressure, high superheat, low subcooling.
- Overcharge: high head pressure and high subcooling, often elevated suction pressure.
- Liquid line restriction: high subcooling, low suction pressure, a temperature drop across the drier, low amp draw.
- Dirty condenser, high ambient, or non-condensables: high head pressure at low or normal load, with subcooling that may look normal or low.
- Low airflow or a restricted evaporator: low suction pressure, low superheat, small delta T, possible icing.
Traps
Condemning the charge when the real fault is airflow or a non-condensable. Reading superheat only at the service port. Charging "until the suction line sweats."
Worked example
Liquid line temperature 95 °F with a condensing saturation temperature of 110 °F gives 15 °F subcooling against a 10 °F target. Check for overcharge, but first compare temperatures before and after the filter drier - a restriction produces the same signature.