FreeHVACCalculators

HVAC Temperature Split Calculator

Check the temperature difference between return and supply air, and what it suggests about airflow and moisture removal.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system
System mode

Result

Temperature split
18°F

A 18 °F split is within the typical 16–20 °F range for these conditions.

Sensible cooling
23,328BTU/h
Show the working
  1. Temperature split = return − supply
    75 − 57 = 18 °F
  2. Typical range for these conditions
    16–20 °F at 50 % return RH
  3. Sensible capacity = 1.08 × CFM × ΔT
    1.08 × 1,200 × 18 = 23,328 BTU/h

Why there is no single “right” split

The familiar target of 20 °F is a rule of thumb. The split depends on how much of the coil’s work goes into cooling the air versus removing moisture. In humid return air more capacity goes to condensing water, so the dry-bulb split is smaller; in dry air it is larger. Airflow matters just as much: low airflow raises the split, high airflow lowers it.

This calculator narrows its guidance band when you enter return humidity, and says plainly when a split is outside it — as a symptom to investigate, not a diagnosis.

Furnace temperature rise

For furnaces the rating plate gives an allowed temperature-rise range, for example 35–65 °F. A rise above the range usually means too little airflow; a rise below it can mean too much airflow or under-firing. Running above the rated rise can overheat the heat exchanger.

Worked example

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

Inputs

System mode
Cooling
Return-air temperature
75 °F
Supply-air temperature
57 °F
Return-air relative humidity (optional)
50 %
Airflow (optional, for capacity)
1,200 CFM

Result

Temperature split
18 °F
Sensible cooling
23,328 BTU/h

A 18 °F split is within the typical 16–20 °F range for these conditions.

Assumptions

  • Typical cooling split guidance: about 14–22 °F overall, narrowing toward the low end in humid return air and the high end in dry return air.
  • Furnace temperature rise is judged against the range printed on the furnace rating plate.

Limitations

  • Temperature split alone cannot diagnose refrigerant charge or airflow. Combine it with measured airflow, superheat/subcooling and static pressure.
  • Measure in well-mixed air close to the equipment, away from radiant heat from the furnace heat exchanger.

Frequently asked questions

What should the temperature difference be between supply and return?

For cooling, commonly about 14–22 °F overall, lower in humid conditions and higher in dry ones. For a furnace, within the rise range printed on its rating plate.

Does a low split mean low refrigerant?

It can, but so can high airflow or very humid air. Charge is diagnosed with superheat or subcooling measurements against the manufacturer’s charging method, not from temperature split alone.

Sources

  1. 20° Delta T — A Lazy Rule of Thumb — HVAC School. Evaporator temperature split depends on return-air conditions and airflow; roughly 18–22 °F under typical conditions.
  2. Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.

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