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What does Delta T mean in HVAC?
Delta T is the temperature difference between two measured points; in AC work, it often means return air minus supply air.
Delta T: The difference between two temperature measurements, usually return air minus supply air in a cooling-system check.
Delta T means “change in temperature.” In residential cooling conversations, it usually means the temperature of air entering the system at the return minus the temperature of air leaving at a nearby supply. If return air is 76°F and supply air is 58°F, the measured split is 18°F.
Why the measurement needs context
The result changes with return-air humidity, airflow, where the probes are placed, system runtime, and equipment operation. The U.S. Department of Energy diagnostic guide uses both return wet-bulb temperature and dry-bulb temperature when evaluating an expected split; one fixed number does not fit every condition (DOE diagnostic guide).
Measure in the air stream, away from a hot attic register surface, after the system has stabilized. Use the AC temperature split guide to calculate and interpret the observation. A technician still needs airflow, electrical, refrigerant, and equipment measurements to diagnose a fault.
Sources
- U.S. Department of Energy — Measure guideline for air-conditioner diagnostics, accessed 8/12/2026
- U.S. Department of Energy — Air conditioning, accessed 8/12/2026