Customer says her AC 'isn't cold enough.' What temperature split across the evaporator should I expect and what does it depend on?

A healthy temperature split (ΔT) across the evaporator coil should be 15–22°F (8–12°C) under normal operating conditions.

What it depends on:

1. Airflow – Low CFM across the coil reduces heat transfer, lowering the split. Check filter, blower wheel, and duct restrictions.
2. Refrigerant charge – Undercharge or overcharge shifts the coil’s effective surface area, altering the split. Verify superheat/subcooling.
3. Outdoor conditions – Higher outdoor temps increase head pressure, slightly raising the split.
4. Humidity – High humidity increases latent load, which can reduce sensible ΔT.
5. Evaporator coil condition – Dirt or ice reduces heat transfer efficiency.

Quick check: Measure return air and supply air temps at the furnace with a calibrated digital thermometer. If ΔT is below 15°F, investigate airflow or charge first. Above 22°F may indicate overcharge or restricted airflow.

If the split is normal but the customer still complains, check for duct leakage, poor insulation, or oversized equipment short-cycling.

Target Split: 16°F–22°F (Delta T) across the evaporator coil (Return Air DB – Supply Air DB).

What it depends on:

1. Airflow (CFM/ton): This is the #1 variable. Low airflow (<350 CFM/ton) raises split (risk of freeze-up); high airflow (>450 CFM/ton) lowers split (poor latent removal). Verify airflow first (static pressure + fan table or TrueFlow grid).
2. Entering Air Conditions: Wet-bulb (WB) temp: Higher WB (more latent load) = lower sensible split. At 75°F DB/63°F WB (50% RH), expect ~18°F–20°F. At 75°F DB/67°F WB (65% RH), split drops to ~14°F–16°F. Dry-bulb (DB) temp: Minimal direct effect on split if WB is constant, but changes total capacity.
3. Refrigerant Charge & Metering Device: Undercharge/overcharge or stuck TXV/EEV alters superheat/subcooling, reducing capacity and split.
4. Coil Cleanliness: Dirty evaporator = restricted airflow = falsely high split (but low capacity). Dirty condenser = high head pressure = reduced mass flow = low split.
5. Compressor Health: Bad valves or scroll = low compression ratio = low split.
6. Duct Leakage: Return leaks (attic air) raise entering WB/DB, lowering split. Supply leaks lose capacity.

Field Procedure:

1. Confirm clean filter/coil/blower wheel.
2. Measure Return DB/WB and Supply DB (probe in plenum, not register).
3. Measure External Static Pressure (ESP). Plot on fan curve to verify CFM.
4. Check Superheat (fixed orifice) or Subcooling (TXV/EEV) per manufacturer charging chart.
5. Compare measured Delta T to manufacturer’s performance chart for actual entering WB, OD ambient, and verified CFM. Generic 20°F rule fails at high humidity.

Code/Std: ACCA Manual S (sizing), Manual D (ducts), ANSI/ACCA 5 QI (verification). EPA 608 for refrigerant handling.

*If split is low with correct airflow & charge:* Check compressor amp draw vs. RLA, condenser fan operation, and liquid line restriction (filter drier temp drop).