PURPOSE: To control the internal part of a water tank at a correct water temperature without being influenced by supply water and supply hot water temperatures by, first, measuring and calculating the supply water and supply hot water temperatures, a water quantity and a flow rate, calculating a remaining water quantity up to target temperature and water quantity from the values, converting them to the opening times of respective electromagnetic valves, and executing the supply of water and hot water.
CONSTITUTION: Water in a small quantity is fed into a water tank 1, the water temperature in the water tank 1 is detected by a temperature sensor 9, and the feed water flow rate is calculated by a water level sensor 8 and a timer. Next, while continuing the hot water, supply the temperature in the water tank 1 is measured, it is measured again after a fixed time, and the supply hot water temperature is inversely calculated by the increase degrees of the temperature and water quantity between a previous time and a present time. Simultaneously, the supply hot water flow rate is calculated from the water quantity of the increase degree and the supply time of the hot water. Next, the remaining supply hot water quantity and supply water quantity made into the target temperature and water quantity are calculated from the water quantity, water temperature, supply hot water temperature and supply water temperature in the water tank 1 at the present time. Next, the remaining supply hot water quantity and supply water quantity are divided by the supply water flow rate and supply hot water flow rate obtained before, respectively, they are converted to the supply hot water and supply water times, and the supply of water and hot water are simultaneously executed as the opening times of a supply water side electromagnetic valve 6 and a supply hot water side electromagnetic valve 7.