FieldBench
Procedures
Charge6 min

Charge by subcooling

TXV and EEV systems. Subcooling is the charge knob; superheat is the valve health check.

Readings

  • Liquid line pressure at the liquid service valve. Convert to condenser saturation temperature.
  • Liquid line temperature at the same valve, on bare copper.
  • Subcooling = condenser saturation °F − liquid line °F.
  • Target is on the rating plate or OEM sheet — often 8–14°F on comfort AC, not a universal number.

Checks before you charge

  • Indoor airflow first. A dirty filter will fake a charge problem.
  • Outdoor coil and fan. High head with low SC is heat rejection, not undercharge.
  • Sight glass (if present) can still flash with a correct charge on blends — trust weight and SC.

TXV vs charge

  • If SC is on target and SH is 20°F+ at the evaporator outlet, the TXV may be stuck, starved, or the bulb is lost.
  • If SH is 0–2°F with SC high, you may be overcharged or the TXV is flooding.
  • Charge liquid only on zeotropes (407C, 404A, 454B).