A domestic hot water installation (1, 51, 61, 71) comprising: a hot water cylinder; an external heating circuit coupled to the hot water cylinder, the external heating circuit having an inlet (33, 68) fed from the hot water cylinder through which water to be heated is delivered to the external heating circuit and an outlet (35, 69) through which heated water that has passed through the external heating circuit is returned to the hot water cylinder; a pump for circulating water from the hot water cylinder, through the external heating circuit and back to the hot water cylinder; and a control circuit for controlling the amount of hot water that is in the hot water cylinder, the control circuit comprising: a first sensor operable to measure the temperature of hot water being delivered into the hot water cylinder; a second sensor operable to measure a characteristic of the water being delivered into the hot water cylinder for use in the determination of the amount of hot water being delivered into the hot water cylinder; and a programmable controller in communication with the first and second sensors, the programmable controller having: a processor for processing the data received from the first and second sensors; an accessible memory for storage of a domestic hot water profile; and means to operate the pump in accordance with the domestic hot water profile; characterized in that the second sensor is a flow sensor and in which the flow sensor is mounted in a return pipe connecting the outlet of the external heating circuit and an inlet of the hot water cylinder. a hot water cylinder; an external heating circuit coupled to the hot water cylinder, the external heating circuit having an inlet (33, 68) fed from the hot water cylinder through which water to be heated is delivered to the external heating circuit and an outlet (35, 69) through which heated water that has passed through the external heating circuit is returned to the hot water cylinder; a pump for circulating water from the hot water cylinder, through the external heating circuit and back to the hot water cylinder; and a control circuit for controlling the amount of hot water that is in the hot water cylinder, the control circuit comprising: a first sensor operable to measure the temperature of hot water being delivered into the hot water cylinder; a second sensor operable to measure a characteristic of the water being delivered into the hot water cylinder for use in the determination of the amount of hot water being delivered into the hot water cylinder; and a programmable controller in communication with the first and second sensors, the programmable controller having: a processor for processing the data received from the first and second sensors; an accessible memory for storage of a domestic hot water profile; and means to operate the pump in accordance with the domestic hot water profile; characterized in that the second sensor is a flow sensor and in which the flow sensor is mounted in a return pipe connecting the outlet of the external heating circuit and an inlet of the hot water cylinder. a first sensor operable to measure the temperature of hot water being delivered into the hot water cylinder; a second sensor operable to measure a characteristic of the water being delivered into the hot water cylinder for use in the determination of the amount of hot water being delivered into the hot water cylinder; and a programmable controller in communication with the first and second sensors, the programmable controller having: a processor for processing the data received from the first and second sensors; an accessible memory for storage of a domestic hot water profile; and means to operate the pump in accordance with the domestic hot water profile; characterized in that the second sensor is a flow sensor and in which the flow sensor is mounted in a return pipe connecting the outlet of the external heating circuit and an inlet of the hot water cylinder.