Delta T / Flow and return temperature difference

Delta T means a temperature difference. In heating-system documents it often means the gap between the flow temperature leaving a boiler or heat pump and the return temperature coming back after heat has been given up to radiators, underfloor heating or other heat emitters. That difference helps designers and installers understand how much heat the circulating water is delivering for a given flow rate and whether the emitters and pump settings match the intended design. Radiator datasheets can use Delta T in a related but different way, comparing the radiator’s mean water temperature with the room temperature, so a Delta T 50 rating should not be read as the same thing as a 50 °C flow-and-return drop. The right Delta T is not simply the largest or smallest number; it depends on the heat pump, emitters, pipework and control settings.