PURPOSE: To improve the operation of a blower in efficiency by controlling the number of revolutions of the blower on the basis of the actual output of the burner when the proportional valve is in a full-open state but the actual output falls short.

CONSTITUTION: When the opening of a proportional valve 8 provided in a fuel- gas line 7 to a burner 6 has reached the maximum, this maximum opening is detected and the actual output of a burner 6 is determined on the basis of signals from temperature-detective means 4, 5 and a signal from a water-flow- detective means 3. This actual output of the burner 6 is compared with a specified value of output; when it is determined that, whereas the opening of the proportional valve 8 is at its maximum, the actual output is the specified value or less, the number of revolution of the blower 9 is subsequently controlled on the basis of the actual output of the burner 6. This method enables reducing the noise from an unnecessarily high level and improving the condition of the combustion as well.