Abstract: Hot water supply temperature is set by a temperature setting device in response to an instantaneous flow rate signal from a water flow rate sensor arranged in a water supply pipe and a feeding water temperature signal from a feeding water temperature sensor which are compared with a predetermined hot water supply temperature and calculated in a control unit. A proportional valve and other devices in a gas supply pipe are controlled in response to the result of the comparison and calculation to define a required volume of gas for ignition and heating. At the same time, a fan damper is controlled by a damper control device so as to adjust the volume of combustion air. A signal representing discharging hot water temperature from a discharging hot water temperature sensor arranged in a hot water feeding pipe is fed back to the control unit and calculated therein, and a valve in the hot water supply pipe is adjusted in response to the result of calculation to attain the desired hot water supply temperature.