PURPOSE: To prevent the combustion operation of a water heater from turning ON and OFF frequency and to improve the durability of the heater by a method wherein when the combustion of the heater on the base of the temperature of water heated by the heater is stopped, the heat loss in the heater is controlled by reducing the quantity of hot water circulating through a hot water circulating device.

CONSTITUTION: When an operating switch 21 turns ON, a contact 25 is closed in response to a signal from a combustion control section 24, a changeover valve 12L in a gas pipe 11 is opened and an ignition device (not shown) is operated to start the combustion of a gas burner. At the same time, a relay 26 is excited and a contact 26a is switched to a constantly open side so that a motor 6M for a circulation pump is operated and a heating operation is started by the circulation of the hot water. Then, when the temperature of the hot water reaches a predetermined value, the changeover valve 12L is closed due to the opening of the contact 25 and the combustion operation of the heater is stopped. In this case, the contact 26a of the relay 26 is opened to energize the motor 6A through a pump controller 22 and the quantity of the circulating hot water is reduced.