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Condenser Approach and Evaporator TD Calculator

Turn high-side and suction gauge readings into saturation temperatures, then see how far the condenser runs above outdoor air and how far the evaporator runs below the return air.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Only refrigerants whose generated tables agree with at least two independent published tables are offered. R-454B is not offered yet.

Measured at the liquid service valve.

Dry-bulb temperature of the air entering the evaporator coil.

Gauges read pressure above the local atmosphere, which is lower at altitude. Leave at 0 for sea level.

Result

Condenser approach (condensing − outdoor air)
11.2°F
Evaporator TD (return air − evaporating)
35.2°F

Condensing 11.2 °F above outdoor air; evaporating 35.2 °F below return air.

  • Compare both values with the equipment manufacturer’s data for the same indoor and outdoor conditions; they depend on the equipment design and the load.
Condensing (bubble-point) temperature
86.2°F
Evaporating (dew-point) temperature
39.8°F
Show the working
  1. Absolute pressures = gauge + atmospheric
    high 274.7 psia, suction 132.7 psia (atmosphere 14.696 psia)
  2. Saturation temperatures from the validated PT table
    condensing 86.18 °F, evaporating 39.84 °F
  3. Condenser approach = condensing − outdoor air
    86.18 − 75 = 11.18 °F
  4. Evaporator TD = return air − evaporating
    75 − 39.84 = 35.16 °F

Two temperature differences that describe the coils

The condenser approach (sometimes called the condenser split) is the saturated condensing temperature minus the outdoor air temperature entering the coil. It grows when the coil is dirty, airflow is blocked or the system is overcharged.

The evaporator temperature difference is the return air entering the indoor coil minus the saturated evaporating temperature. It grows with low indoor airflow and shrinks when the coil is flooded or the load is low.

How the saturation temperatures are found

Both temperatures come from the same validated pressure–temperature tables as the refrigerant PT calculator, after the gauge readings are corrected to absolute pressure for your elevation. For R-410A, the condensing temperature is the bubble point and the evaporating temperature is the dew point; for single-component refrigerants they are the same.

There are no universal pass/fail values: normal numbers depend on the equipment design and the operating conditions, so compare with the manufacturer’s data.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Refrigerant
R-410A
High-side (liquid line) gauge pressure
260 psi
Outdoor air temperature entering the condenser
75 °F
Suction gauge pressure
118 psi
Return air temperature entering the indoor coil
75 °F
Site elevation
0 ft

Result

Condenser approach (condensing − outdoor air)
11.2 °F
Evaporator TD (return air − evaporating)
35.2 °F
Condensing (bubble-point) temperature
86.2 °F
Evaporating (dew-point) temperature
39.8 °F

Condensing 11.2 °F above outdoor air; evaporating 35.2 °F below return air.

Assumptions

  • Condensing temperature is the bubble point at the high-side pressure; evaporating temperature is the dew point at the suction pressure (the same for single-component refrigerants).
  • Gauge readings are corrected to absolute pressure with the standard atmosphere at the site elevation.

Limitations

  • Pressure drop between the compressor and the service valves shifts the saturation temperatures slightly.
  • No pass/fail limits are applied: normal values depend on the equipment design, so compare with the manufacturer’s data.

Frequently asked questions

What is a normal condenser approach?

It depends on the equipment: higher-efficiency units with larger coils run closer to outdoor air. Compare with the manufacturer’s data for the same conditions.

Is condenser approach the same as subcooling?

No. Subcooling compares the condensing temperature with the liquid-line temperature; the approach compares it with the outdoor air.

Which pressure should I use for the high side?

The liquid-line service valve reading, the same one used for subcooling.

Sources

  1. CoolProp — open-source thermophysical property library (version 8.0.0) — CoolProp developers (MIT licence). Equations of state for pure fluids and mixtures, incompressible fluids and brines, and IAPWS-IF97 water/steam; free for commercial use under its licence. Used here to generate the refrigerant saturation tables.
  2. Refrigerant PT charts and how they were validated — FreeHVACCalculators.com. Each published refrigerant’s generated table agrees with at least two independent published tables (worst differences and tolerances listed); R-454B is withheld because it does not.

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