PURPOSE: To detect the temperature of feed water and perform the proper changing-over operation in response to the detected result by a structure wherein a detection switch to detect the temperature of the feed water flowing through a feed water passage to a heat exchanger in a plurality of steps and a plurality of pilot lamps to indicate the detected temperature are provided.

CONSTITUTION: The temperature of feed water flowing through a feed water passage 1 is detected in a plurality of steps by a temperature detection switch 14 and the detected water temperature is indicated by a plurality of pilot lamps 15. The temperature of delivery hot water is made proper by controlling the amount of water fed to a heat exchanger 2 in response to the temperature of feed water or concretely by changing-over the operator 16a of a selector valve 16 arranged on the downstream side of a water pressure responding member 12 to the lit-on side of the pilot lamp 15. The lightening of the pilot lamp 15 can be done by connecting either one of the respective pilot lamps 15 to either one of the respective contacts 14a and 14b of the temperature detection switch 14, which connects to a power cell 18.