What is a normal Delta T for air conditioning?

A healthy cooling split is typically 16–22°F (about 9–12°C) on a properly charged system with correct airflow. Indoor humidity matters too: on humid days the split moves toward the low end of the range even when the system is fine.

Delta T Calculator

Check your temperature split across the indoor coil or your furnace temperature rise. Enter return-air and supply-air temperatures to see whether the system is performing in a healthy range.

Return Air Temperature (°F)

Supply Air Temperature (°F)

Indoor Relative Humidity (optional)

Temperature Split (Delta T)

0.0°

Enter values above and click calculate.

How It Works

Delta T (also called the temperature split) is the difference between the return air entering the system and the supply air leaving it. It is one of the fastest checks a technician can make to judge whether a system is moving heat the way it should.

Cooling Split

Delta T = Return Air Temp − Supply Air Temp

Measure return air a few feet before the indoor coil and supply air a few feet after it, so you are only reading air that has actually crossed the coil. A healthy cooling split is typically 16–22°F (about 9–12°C) on a properly charged system with correct airflow.

The split moves with airflow: faster air spends less time on the coil and gives a lower split, while slower air sits longer and gives a higher split. Indoor humidity matters too — on humid days the coil spends energy pulling moisture out of the air, which pushes the split toward the low end of the range even when the system is fine.

What a reading tells you

Low split (below ~16°F): often low charge, a dirty coil, or too much airflow.
High split (above ~22°F): often low airflow — a dirty filter, blower set too slow, or restricted ductwork.

Furnace Rise

Rise = Supply Air Temp − Return Air Temp

On the heating side the same two probes give you the furnace temperature rise. Unlike the cooling split there is no universal target — every furnace has an acceptable rise range printed on its rating plate (often something like 35–65°F). Enter your unit's range and the calculator checks your reading against it.

Pair this with the Superheat & Subcooling calculator for a fuller charge-and-airflow diagnosis on the same service call.