To provide a water system capable of detecting intermediate failure of a temperature sensor, and capable of specifying a broken temperature sensor.

The hot water system 100 is connected to a bath 66a, and is equipped with a hot water storage tank 18, a power generation unit 24, and a mixing unit 44. In a path from a water pipe 2 to the bath 66a, three thermistors comprising a water supply thermistor 8, a tapping thermistor 46, and a bath thermistor 58 are provided, detecting temperatures of water flowing through the path. In the hot water system 100, when an opening of a mixer 14 is fully opened to a water side, temperature differences of temperatures detected by the thermistors are compared, and presence or absence of the intermediate failure of the thermistors is determined. If one of the thermistors is broken, by comparing the temperature difference and the temperatures detected by the other two thermistors, the broken thermistor can be specified.

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