Abstract: A steam or the like supply system has two parallel legs of supply, a first leg designed for high-demand flows and for the metering thereof, and a second leg designed for low-demand flows and for the metering thereof. Each of these legs has its own control valve, and an electronic controller responds to a sensed drop in demand to control a program of valve in the low-demand line; similarly, the controller responds to a sensed increase in demand to control a program of valve shut down in the low-demand leg while opening the control valve in the high-demand line. Protective features in the controller assure against response to transient changes in demand and against interruption of a valve-changing operation until the programmed change has been fully implemented.