Formula
ΔT = Return DB − Supply DB · target shrinks as return wet bulb risesMeasure DB/WB in the return ~6 ft upstream of coil; supply DB ~6 ft downstream. Avoid radiant influence from coil.
Measured ΔT
18.0°F
Target 19°F at 63°F WB
ΔT in normal range. System is moving heat as expected.
Reference
- Dry climate (55°F WB return): expect ~22°F ΔT
- Average (63–67°F WB): 17–20°F
- Humid (75°F WB): 14–16°F
How to use this calculator
Compare return and supply air temperature split against the target for the measured wet bulb.
- Measure return air dry bulb and wet bulb in the return plenum.
- Measure supply air dry bulb past the coil, away from radiant heat from the plenum.
- Read the measured split and the target split for that return wet bulb.
- Investigate airflow, charge or load if the measured split is outside the target.
Frequently asked questions
- What is a normal delta T for air conditioning?
- Typical evaporator split is 16–22 °F, but the correct target depends on return wet bulb: humid return air gives a smaller split, dry return air a larger one.
- What does a low delta T mean?
- Low split points to too much airflow, low charge, a weak compressor, or a heavy latent load.
- What does a high delta T mean?
- High split usually means low airflow — a dirty filter, dirty evaporator coil, undersized duct or slow blower.