Delta T is a technical measurement of the temperature difference between the air entering the unit and the air exiting it. In cooling mode, a healthy system should typically show a Delta T of 15 to 20 degrees Fahrenheit. If your 18000 BTU unit is blowing air that isn't significantly colder than the room air, it may indicate a refrigerant leak or a dirty filter. Technicians use this measurement to quickly diagnose the performance of one zone wall mount systems. Monitoring this value can help you identify when it’s time to call for service or check your parts. It is a fundamental metric for any heat pump systems efficiency check.