PURPOSE: To prevent any abnormal combustion by a method wherein the amount of gas is controlled in a stepwise manner in a variation range of a specified amount of gas up to an amount of gas required for getting a required amount of hot water fed and then the number of revolution of a fan is controlled under the number of revolution corresponding to a variation range of the amount of gas.

CONSTITUTION: A gas pressure determined by a degree of opening of a proportional valve 8 is detected by a gas pressure sensor 24 as a current value of the proportional valve 8 and then the detected value of gas pressure is fed back by a required number calculation part 20. A control part 27 for controlling the number of revolution of a fan controls the number of revolution of a fan 11 in response to an energized current under a control of gas pressure calculated by a gas pressure calculating part 21. A gas pressure and the number of revolution of the fan 11 are fixed for a specified period of time by a timer 22 and controlled in a stepwise manner. That is, it is controlled in a stepwise manner with a specified variation range of gas amount required for getting a required hot water feeding volume and the number of revolution of the fan 11 is controlled in a stepwise manner at the number of revolution corresponding to the variation range of the gas amount. With such an arrangement, it is possible to continue combustion under a superior combustion state even if a flow rate of the fed hot water or a hot water feeding set temperature is rapidly changed.