PURPOSE: To suppress the temperature fluctuation of a boiler generated by changing-over steam system by a method wherein a controller to control the change-over hour of the steam systems is interposed in the titled equipment, the system of which comprises changing-over the steam systems to supply steam to an air preheater in response to the load of the boiler.

CONSTITUTION: The preheater 18 is changed-over in response to the load of the heating furnace 17 of the boiler through a change-over valve 19 to either of a steam bleeding system or of an auxiliary system, both of which are different in pressure from each other. When the load of the boiler changes, a monitor 21 is brought into operation in order to output a signal requiring the setting of an external means to a sequence logic 14. After the elapse of T1 seconds since the issue of said signal, the logic 14 outputs the signal corresponding to the forced opening degree set by means of an opening degree setter 15 through the controller 5 to a controlling valve 12 in order to keep the valve 12 open for E seconds at the forced opening degree. Then, after T2 seconds, the logic 14 outputs the signal to command the changing-over of the valve 19 and at the same time the output of an opening degree generator 13 is inputted to the controller 5 in order to change-over the valve 12 in automatic control.