PURPOSE: To enable to completely use nearly the total quantity of hot water in a hot water storage tank without lowering its temperature yet by checking the water supply when a system for supplying hot water by lots is selected by a constitution wherein the selection between the system for supplying hot water by overflowing and the system for supplying hot water by lots can be possible by changing-over a three- way solenoid valve.

CONSTITUTION: A hot water supply pipe 8 for overflowing supply is connected at a position lower than an upper limit hot water level H determined by an upper limit level switch 5 and at the same time a hot water supply pipe 10 for lot supply is connected at a position lower than an lower limit cold water level L determined by a lower limit level switch 6 in a hot water storage tank 2. The other ends of the hot water supply pipes 8 and 10 are connected to the inflowing ports of a solenoid controlled three-way directional control valve 11, the discharge port of which is connected to a hot water supply pipe line C. When a system for supplying hot water by overflowing is selected by the solenoid controlled three-way directional valve 11, a control device 3 sends a "open" signal to a solenoid controlled feed water valve 4. On the hand, when a system for supplying hot water by lots is selected, the control device 3 keeps on sending a "close" signal to the valve 4 in order to act for stopping water supply.