WATER HEATER

PROBLEM TO BE SOLVED: To provide a water heater to effectively utilize remaining hot water in a bathtub. SOLUTION: This water heater comprises a circulation flow passage 2 to circulate remaining hot water in a bathtub 1 ; and a heat-exchanger 13 for hot water supply situated on a flow passage 12 for hot water supply. A temperature sensor 5 of a remaining hot water temperature detecting means and a pump 7 for circulation are situated on the circulation flow passage 2. Water flowing through the heat-exchanger 13 for hot water supply is heated by a gas burner 14. Further, a temperature sensor 15 of a supply water temperature detecting means to detect the temperature of supply water to the heat-exchanger 13 for hot water supply is situated on the flow passage 12 for hot water supply. A liquid-liquid heat-exchanger 10 is provided to perform heat-exchange between remaining hot water flowing through the circulation flow passage 2 and supply water for the heat-exchanger 13 for hot water supply. A bypass flow passage 2B running through the liquid-liquid heat-exchanger 10 is situated on the circulation flow passage 2 and a bypass valve 9 is situated on the bypass flow passage 2B. The bypass valve 9 is controlled in a manner to be opened when remaining hot water in the bathtub 1 is utilized.