PURPOSE: To make proper work done by a discharging fan, reduce noise and a power consumption in response to a condition of use of each of combustion chambers by a method wherein first and second shape memory alloys are connected through springs to a discharging damper capable of closing a first discharging cylinder and a second discharging cylinder.

CONSTITUTION: When combustion is carried out, for example, only in a first combustion chamber 10, an amount of gas controlled by a fuel control device 16 is supplied to a first burner 11 and then as a combustion is started in the first combustion chamber 10, a discharging gas fan 23 may start to operate and at the same time a heat of discharged gas fed out from a first discharging cylinder 12 is detected by a first shape memory alloy 19 to cause its shape to be bent inwardly, and then a discharging damper 18 may close a second discharging cylinder 15 through a spring 20 and prevent the discharging fan 23 from sucking air through the second discharging cylinder 15. When combustion is carried out in both first and second combustion chambers 12 and 13, the first and second shape memory alloys 19 and 21 are bent inwardly and thus since they are connected to the discharging damper 18 through springs 20 and 22, the discharging damper 18 is placed at a substantial central part and is not moved, and the discharging fan 23 may suck discharging gas from both first and second discharging cylinders 12 and 15.