Abstract: A method, including: receiving an instruction from a user of a hot water tank (24) indicating a target temperature and a duration of a supply of water. In response to receiving the instruction, water at multiple different temperatures within the hot water tank is mixed so as to form mixed water in the tank. The method includes measuring an actual temperature of the mixed water in order to make a determination of whether the mixed water will satisfy the instruction. The method also includes activating a heater (120) to heat the mixed water responsively to the determination.