JPH0625606B

PURPOSE:To switch over the number of burners to be used in automatic mode and secure a required combustion volume ranging from a small combustion volume to a large combustion volume by setting both the minimum quantity and the maximum quantity of electric current flowing to a governor proportional valve when electric current does not flow to a solenoid valve so that they may exceed the maximum and minimum electric current when the electric current flows to the solenoid valve. CONSTITUTION:A control device 30 controls a combustion fan 12 and a governor proportional valve 23 based on a predetermined combustion volume. A fuel gas is discharged into burners 11a and 11b from nozzles 16a and 16b. The quantity of gas is proportional to the electric current flowing to the governor proportional valve 23. The electric current flows in a range of diversified current values, depending on the opened and closed status of a switch over solenoid valve 24. When only the burner 11b performs a small amount of combustion, the switch over solenoid valve 24 is in closed status. Under this condition, it is so set that the electric current may be smaller than the minimum and maximum current values flowing to the governor proportional valve 23 when the two burners carry out combustion operation even when the gas pressure supplied to the nozzle 16b is increased. It is, therefore, possible to control the pressure at a low value and reduce the quantity of supply as well. The gas is mixed with the air and burnt by a combustion fan 12, thereby producing a predetermined combustion volume.