Measure and interpret

AC temperature split guide

Measure return and supply air correctly, calculate the temperature difference, and compare it with an evidence-based target that accounts for indoor humidity.

Short answer: there is no universal “good” temperature split. Humidity and airflow change the expected number. Measure after the system has run steadily, use matched return and supply locations, and treat the result as a clue—not a refrigerant diagnosis.

Enter the return temperature, supply temperature, and return humidity.

How to get a useful reading

Use the same accurate thermometer for both readings, or two probes you have checked against each other.
Use clean filters, open supply registers, and normal cooling settings. Do not open electrical or refrigerant compartments.
Let the system run at least 15 minutes. Put each probe near the center of the moving air stream and wait for the reading to stabilize.
Record the location, return temperature, supply temperature, indoor humidity, outdoor conditions, and time. A repeatable pattern is more useful than one unexplained number.

What the result means

The Department of Energy diagnostic guide compares measured split with a humidity-adjusted target and uses a tolerance of about 3°F. A result more than 3°F below target can point to reduced cooling capacity. A high split can point to low airflow. Either finding needs more measurements before anyone names a failed part.

A split near target is encouraging, but it does not prove correct refrigerant charge. The DOE guide warns that an overcharged system or a system containing non-condensable gas can still show a proper split.

When to stop measuring and request service

Arrange a diagnosis when supply air is not cooler than return air after you verify cooling mode and measurement locations, when the measured split repeatedly falls well below the humidity-adjusted target, or when the house still cannot cool despite a plausible split. Shut the system off for burning smells, sparking, a repeatedly tripping breaker, active water damage, or heavy ice.

Method

Temperature split is return-air dry-bulb temperature minus supply-air dry-bulb temperature. When return humidity is supplied, this guide estimates wet-bulb temperature with the Stull equation and compares the result with the DOE guide's CEC target table.

What this cannot tell you

A temperature split cannot identify refrigerant charge, airflow, duct leakage, or equipment capacity by itself.
The target comparison applies only when the return conditions fall inside the published table; otherwise the guide reports the measurement without grading it.
Room-grille measurements include duct heat gain, leakage, and room mixing. Professional measurements at the equipment answer a narrower question.