How to calculate delta T
Delta T = return air temperature − supply air temperature
- Run the system in cooling for 10–15 minutes.
- Measure return air temperature at the return, before the coil.
- Measure supply air temperature in the supply plenum, after the coil and out of sight of the coil.
- Subtract supply from return.
Worked example
Return air is 75°F and supply air is 56°F. Delta T is 75 − 56 = 19°F, which is in range (16–22°F).
What high or low delta T means
Low delta T: common causes are low charge, too much airflow, or a high indoor humidity load (more of the system's capacity goes to removing moisture).
High delta T: usually low airflow, from a dirty filter, dirty coil, or restricted ducts.
Delta T is a quick check, not a charging method. Humid homes often run a lower split even when the system is healthy.
Questions
What is a good delta T for AC?
Most residential systems run a 16–22°F split in cooling with normal humidity and airflow.
Can I charge a system by delta T?
No. Use superheat or subcooling for charge. Delta T helps you spot airflow and humidity problems.