PURPOSE: To obtain a hot water heater which can accurately regulate the outputting hot water temperature with the preferable responsiveness by boiling water in a hot water storage temperature at relatively low temperature, and heating and controlling the hot water from the chamber by an instantaneous heat exchanger at the hot water supplying time.

CONSTITUTION: When supplying hot water, hot water in a hot water storage chamber 32 is heated by an instantaneous heat exchanger 37, and the outputting hot water temperature from an outlet 18 is controlled by an outputting hot water temperature sensor 25. On the other hand, cold water is fed from a cistern 17 through a water feed pump 13 and a passage 11 to the bottom of the chamber 32. The temperature of the inflowed water is inputted from an inflowing water temperature sensor 20, and the flow rate is inputted from a rotary sensor 21 of a feed pump to a microcomputer 23, which calculates heat quantity required for raising the temperature of the chamber. When circulating and heating the water, the water from the pump 13 is circulated from a 3-way valve 14 through a bypass passage 15, energized to a time heater 26 calculated by the microcomputer, and the hot water temperature in the chamber 32 is raised to the set value of a hot water storage chamber sensor 24.