PURPOSE: To reduce fluctuating width of hot-water temperature upon supplying hot-water and prevent local boiling due to remaining heat after stopping the supply of hot-water by a method wherein high-temperature hot-water in a boiler cylinder is guided from the upper part of a mixing tank into the lower part thereof to mix the high-temperature hot-water with low-temperature hot-water while the supply of hot-water is effected from the upper part of the tank.

CONSTITUTION: Water, which flows into the boiler through a feed water pipe 5 and a boiler body 4, is heated by the combustion in a burner 3, is guided to the lower part of the mixing tank 7 through a hot-water outlet pipe 6 and is mixed with the hot-water in the mixing tank 7. According to this method, the temperature of water is averaged and, thereafter, the hot-water is supplied through a hot-water supplying pipe 8. During supplying hot-water, the non-return valve 11 of a bypass pipe 10 is closed while the hot-water is circulated between the mixing tank 7 and the boiler body 4 through the non-return valve 11 during stopping the supply of the hot-water to restrict boiling in the boiler body 4 due to remaining heat. According to this method, the fluctuating width of hot-water supplying temperature may be reduced and the local heating due to additional heating after stopping the supply of hot-water may be prevented.