Abstract: Control systems (or controllers) for once through, unfired steam generators (or boilers) which control a single parameter--the feedwater flow to the boiler--in accord with an energy or enthalpy balance between the hot gases supplied to the boiler and the steam generated in it. These control systems have a predictive mode of feedwater control and, optionally, an operator selectable, feedback mode to compensate for drifts in the predictive mode. Other corrections may also be made, and predictive and corrective flow splits can be provided to obtain maximum efficiency when the control system is utilized to regulate the operation of a dual pressure boiler.