Abstract: This invention relates to a domestic hot water installation (1, 51, 61, 71) comprising a hot water cylinder (3), an external heating circuit (4) such as a boiler (5) in combination with a heat exchanger (7), a pump (9), and a control circuit (11). The control circuit comprises a pair of sensors (15, 17), at least one of which is a temperature sensor, and a programmable controller (13) in communication with the sensors. The programmable controller has a processor (19), an accessible memory (21), and means (23) to operate the pump. The programmable controller monitors the data from the sensors and accurately calculates the amount of hot water that has been delivered to the hot water cylinder. When the correct amount of water, according to a domestic hot water profile, has been delivered to the hot water cylinder, the programmable controller stops hot water from being delivered into the tank by shutting off the pump.