Superheat & Subcooling Calculator

Determine actual vs target superheat and subcooling for TXV and fixed-orifice piston HVAC systems.

Refrigerant Type

Metering Device

Suction Line (Low Side - Superheat)

Suction Pressure (PSI)

Suction Line Temp (°F)

Liquid Line (High Side - Subcooling)

Liquid Pressure (PSI)

Liquid Line Temp (°F)

Superheat

11.8 °F

Sat Temp: 40.2°F

Subcooling

12.1 °F

Sat Temp: 104.1°F

Diagnostic Assessment

Normal Operating Charge

Both superheat and subcooling are within typical manufacturer specifications (TXV subcooling target: 10°F - 14°F).

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HVAC Charging: Superheat vs Subcooling

Refrigerant charging procedures depend strictly on the evaporator metering device:

Thermal Expansion Valve (TXV): The TXV modulates refrigerant flow to maintain constant superheat. Therefore, charge systems equipped with a TXV using Subcooling (typical target: 10°F to 14°F).
Fixed Orifice / Piston: Fixed orifice pistons cannot adjust flow. Systems must be charged using Target Superheat based on indoor wet bulb and outdoor dry bulb temperatures: Target SH = (3 × Indoor WB - Outdoor DB - 80) / 2.

Frequently Asked Questions

What does low subcooling with high superheat mean?

This is the classic textbook symptom of an undercharged (low refrigerant) system. Add refrigerant until subcooling rises to nameplate specification.

What does high subcooling with high superheat mean?

This indicates a liquid line restriction (e.g. clogged filter drier or defective TXV stuck closed). Liquid refrigerant backs up in the condenser while the evaporator starves.

Why is superheat measured on the suction line?

Superheat ensures that all liquid refrigerant has fully evaporated into vapor before entering the compressor, preventing catastrophic liquid floodback and cylinder slugging.