Measure the air entering and leaving the system while it cools. The drop between them, the delta T, should land between 19 and 23 °F.
Temperature units
Air entering the system, measured at the return
°F
Air leaving the system, measured at the supply plenum or nearest register
°F
–
01019233040
UndercoolingTarget rangeOvercooling
Enter both temperatures
Your result appears here as soon as you fill in both readings.
Common things to check
For best accuracy, let the system run for 10–15 minutes first, use a probe thermometer, and take both readings at the same time. Very humid days can pull delta T toward the low end. This tool is a quick screening aid, not a diagnosis; have a qualified HVAC technician confirm any problem.
