Delta-T (temperature split)

Delta-T (ΔT) is simply the temperature difference across a cooling coil — for a split/ducted AC it's the return air temperature minus the supply air temperature (the 'temperature split'), and for a chilled-water system it's the difference between the chilled water leaving and returning. It's the single most useful health check because it proves the system is actually removing heat, not just running. For a home AC a healthy air-side split is roughly 16-22°F (about 8-12°C): too low a split usually means a dirty coil, low refrigerant or weak heat transfer, while too high a split (over ~22°F) usually means restricted airflow — the air is crawling across the coil and getting overcooled. (Measure it across the coil, not at distant grilles, or duct gains/losses skew the reading.) On a building chiller the equivalent is the chilled-water delta-T, where a chronically *low* delta-T ('low delta-T syndrome') means the plant is pumping lots of water for little delivered cooling — a costly efficiency red flag. Reading delta-T is a core part of any proper AC or chiller service.