PURPOSE: To obtain high thermal efficiency, as well as to accurately detect the temperature of the water in a bathtub, by connecting one ends of a lower circulation pipe, of a thermister, and of a drum pipe to the lower chamber of a drum 'B', respectively, which is separated from an upper chamber by a bulkhead, and by connecting the other ends of a fin pipe and a drum pipe to a drum 'A'.

CONSTITUTION: A drum 'B8' is divided into the upper and the lower, two chambers by a bulkhead 9. One ends of a hot water feed pipe 14, of an upper circulation pipe 10, and of a fin pipe 12 are respectively connected to the upper chamber 8a of a drum 'B8', and one ends of a lower circulation pipe 11, of a thermister 15, and of a drum pipe 13 are respectively connected to the lower chamber of it. Besides the other ends of a fin pipe 12 and of a drum pipe 13 are respectively connected to a drum 7. With such an arrangement, the fed warm water is prevented from being mixed with cold water before it is heated by a heat exchanger, when feeding of warm water and after-heating are performed at the same time. The temperature of the water in a bathtub can be detected accurately by a thermister, and high thermal efficiency can be obtained in a heat exchanger as well.