PURPOSE: To permit to effect the cut-in of general use hot-water supply during high-temperature hot-water supply simply, safely and in a short period of time by only operating an opening and closing valve by a method wherein the opening operation of a general use hot-water supply opening and closing valve is detected to connect a flow path for general use hot-water supply to the intermediate part of the flow path for heat exchanging automatically based on the detection in a condition preferential to a hot-water supply switching operating equipment.

CONSTITUTION: When the opening and closing valve 10 is opened during pouring or re-heating the hot-water, a pressure switch 24 is put ON, in spite of a condition that the hot-water supply switching operating equipment 28 is positioned at hot-water pouring position P2 or re-heating position P3, to open a water amount regulating valve 6 widely while a switching valve 9 is switched and the general use hot-water supply, in which the hot-water, controlled to a set temperature, from a hot-water outlet path 7 is supplied through the opening and closing valve 10, is effected. However, the switching valve 9 is switched when outlet hot-water temperature is reduced to the set temperature, therefore, the high-temperature hot-water for pouring or re- heating will never be supplied through the valve 10 and the hot-water, comming from branched flow path (a3) is discharged. The hot-water from the branched flow path (a3) may be controlled so as to have a temperature suitable for the general use hot-water supply and, therefore, the general use hot-water supply may be effected before switching the switching valve 9.