Refrigerant Charge Calculator — Superheat and Subcooling
Determine refrigerant charge status by calculating actual superheat and subcooling values, then comparing them to target ranges. Supports both TXV/EEV and fixed-orifice metering device systems.
System Configuration
Superheat Method (Fixed Orifice / Piston)
Target Superheat Inputs (Fixed Orifice Systems)
Formulas
Actual Superheat (Fixed Orifice Systems):
Superheat (°F) = Tsuction line − Tsat @ suction pressure
Where Tsat @ suction pressure is the saturation (boiling) temperature of the refrigerant at the measured suction line gauge pressure, obtained from refrigerant pressure-temperature tables.
Target Superheat (Fixed Orifice Systems):
Determined by bilinear interpolation of the AHRI/manufacturer superheat charging chart using: outdoor ambient dry-bulb temperature and indoor return air wet-bulb temperature. The chart encodes the relationship between heat load, airflow, and required evaporator performance.
Actual Subcooling (TXV / EEV Systems):
Subcooling (°F) = Tsat @ liquid line pressure − Tliquid line
Where Tsat @ liquid line pressure is the condensing saturation temperature at the measured liquid line gauge pressure. Subcooling confirms all refrigerant has condensed to liquid before the metering device.
Saturation Temperature Interpolation:
Tsat(P) = T0 + (P − P0) × (T1 − T0) / (P1 − P0)
Linear interpolation between adjacent data points in the refrigerant P-T table.
Assumptions & References
- Saturation temperature data sourced from ASHRAE refrigerant property tables for R-22, R-410A, R-404A, R-134a, R-407C, and R-32.
- All pressures are gauge pressure (psig). Absolute pressure = psig + 14.696 psia.
- Target superheat values for fixed-orifice systems are based on the AHRI Standard 210/240 charging chart, applicable to residential split-system air conditioners and heat pumps.
- Target subcooling of 10–15°F applies to TXV/EEV systems per typical manufacturer specifications; always verify against the equipment nameplate or service manual.
- Suction line temperature must be measured at the outdoor unit service valve (not at the evaporator coil) to account for line losses.
- Liquid line temperature must be measured at the outdoor unit liquid service valve.
- R-407C is a zeotropic blend with temperature glide; midpoint values are used as an approximation. Consult manufacturer bubble/dew point tables for precision work.
- This calculator does not account for non-standard conditions such as low airflow, dirty filters, restricted metering devices, or non-condensables in the system.
- Always recover refrigerant per EPA Section 608 regulations. Do not vent refrigerants to atmosphere.
- Readings should be taken after the system has reached steady-state operation (typically 10–15 minutes of runtime).