PURPOSE:To reduce the loss in the head of a fluid to increase the maximum hot water feed amount and to facilitate the control of the supply hot water temperature by providing a plurality of burner type heat exchangers each of which has a drum part water pipe and a fin part water pipe, to heat water, the drum part water pipes being connected in parallel and the fin part water pipe 2 in series. CONSTITUTION:Water supplied from a supply water part 5 is supplied to the drum part water pipe 4 of each heat exchanger 1 having each burner 2, and is heated. Heated water is further heated by the fin part water pipe 2 of the heat exchanger 1 on the downmost stream side. Respective fin part water pipes 2 are connected in series, and the water temperature is gradually increased and hot water is supplied from a hot water feed part 7.
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