Charge6 min
Charge by superheat
Fixed orifice and piston systems. Indoor wet bulb plus outdoor dry bulb set the target.
When to use this
- Piston, capillary, or other fixed metering. Not TXV/EEV — those are charged by subcooling.
- System must be running in cool, filters clean, blower on the correct speed, outdoor coil clean.
Readings
- Suction pressure at the service valve. Convert to saturation temperature on the PT chart for that refrigerant.
- Suction line temperature 6–8 inches from the compressor, insulated well, on a bare copper spot.
- Superheat = suction line °F − evaporator saturation °F.
- Indoor WB at the return and outdoor DB in the shade. Use the OEM superheat chart — 8–12°F is only a rough band.
What the number means
- High superheat: undercharge, restriction, or low indoor load. Confirm piston size and liquid line temp before adding gas.
- Low / zero superheat: overcharge, indoor airflow problem, or oversized piston. Floodback risk — do not leave it.
- Weigh in a known recovery, then trim. Do not ‘top off’ a leak.