What delta T range across the evaporator coil indicates an air conditioner is cooling efficiently?

A properly operating residential air conditioning system should maintain a temperature drop, or delta T, between 16°F and 22°F across the evaporator coil under standard indoor conditions of roughly 50% relative humidity.

Measuring the evaporator coil split verifies heat transfer and system balance before attaching invasive refrigerant gauges. With residential equipment standards requiring higher SEER2 efficiencies across the United States since 2023, verifying sensible versus latent heat exchange prevents technicians from misdiagnosing airflow restrictions as refrigerant leaks.

If you only do one thing: Measure dry-bulb and wet-bulb temperatures simultaneously at the supply plenum and return drop after the system runs for at least 15 continuous minutes.

• Baseline operating target: Standard direct-expansion cooling systems operating at 350 to 400 CFM (cubic feet per minute) of airflow per ton of capacity produce a 16°F to 22°F dry-bulb split across the indoor coil.
• High indoor humidity conditions: When indoor relative humidity exceeds 60%, the evaporator spends capacity condensing water vapor into liquid (latent cooling), dropping the expected sensible delta T to 14°F to 17°F.
• Low indoor humidity conditions: In dry climates with indoor relative humidity below 40%, the system performs mostly sensible cooling (dropping dry air temperature), driving normal temperature splits up to 20°F to 24°F.
• Blower speed correlation: Systems delivering 350 CFM per ton maximize latent moisture removal in humid coastal climates with a higher temperature split, whereas 450 CFM per ton increases sensible capacity with a lower split.
• Target chart validation: Field diagnostics require comparing return wet-bulb and dry-bulb temperatures against manufacturer target evaporator drop charts to determine whether the exact measured split matches engineered performance.

• Watch out for: Readings taken immediately after compressor startup, because transient pull-down states yield inaccurate temperature splits during the first 10 to 15 minutes of operation.
• Watch out for: Delta T values exceeding 23°F in moderate humidity, which frequently indicate choked airflow from high-MERV filters, undersized ductwork, or low blower speed rather than peak performance.
• Watch out for: Delta T values below 14°F under normal load, which typically point to attic duct leakage, dirty evaporator coil fins, slipping blower belts, or low refrigerant charge.

Insert calibrated digital psychrometer probes into the return drop before the filter and in the supply plenum out of line-of-sight from the coil, then evaluate the dry-bulb difference. If the split falls outside the target range, verify external static pressure across the air handler before adjusting refrigerant charge.