To provide a water heater that is formed by combining a water heating device and an adsorption heat pump, can utilize heat recovered from the atmosphere further efficiently, and has a further simple constitution.

The water heater is formed by combining the combustion type water heating device 1 and the adsorption heat pump 2 for auxiliary heating. The adsorption heat pump 2 comprises an integrated adsorption/desorption unit of an adsorption apparatus for repeating an adsorption operation and a desorption operation of an adsorbate and an evaporating/condensing apparatus for evaporating the adsorbate in the adsorption operation and condensing adsorbate vapor in the desorption operation, a heat collector 31 for recovering the heat of the atmosphere into a liquid heating medium circulating between it and the evaporating/condensing apparatus of the adsorption/desorption unit 2A, and a radiator 32 for discharging heat of the liquid heating medium circulating between it and the evaporating/condensing apparatus of the adsorption/desorption unit 2B. The adsorption apparatus for performing the adsorption operation and the radiator 32 are constituted so as to pass water that is not heated by a heat exchanger 11 of the water heating device 1.

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