PURPOSE: To make possible selective supply of hot water to shower and bath-tub by heating one of the two heat exchangers at a prescribed temperature and the other in order to furnish additional heating.

CONSTITUTION: Spiral heat exchangers 100, 99 are provided in heat exchanging cylinders 96, 107. When a sluice valve 84 is opened, a burner 1 at the side of the heat exchanging cylinder 107 begins to burn, the source water running through the heat exchanger 99 is heated to the prescribed temperature. Hot water is obtained from hot water supplying pipe 83 and can correspond with the needs for rest rooms, showers. When a combustion controlling device is manipulated to the hot water supplying part for a bath tub 91, another burner 1 begins to burn and at the same time a selection device 89 connects the heat exchanger 99 with the heat exchanger 100, the source water running through the heat exchanger 99 is instantly heated, flows into the heat exchanger 100 at the prescribed temperature and is supplied through a space 105, a lower circulation pipe 94. If a regulating hole V and a regulating valve X are provided for the said heat exchanger 100 and speed for heating the water is restricted by operating the valve X, the hot water is evaporated. This vapor acts on the heated water in the space 105, etc.