Controller, system and method for providing supplementing water

A supplementing water supply controller which provides water from a supplementing water supply to an outlet in preference to a main water supply, wherein if there is insufficient supplementing water to meet demand, the main water supply flows through the controller to the outlet, whereas if there is sufficient supplemental water to meet the demand, a supplementing water pump is activated and the pressure of the supplementing water is raised above a threshold level at which it moves an actuator mounted by two diaphragms to block the main water supply, so that the supplementing water can flow to the outlet. The actuator has a larger effective area at its low pressure end than at its high pressure end, so that the supplementing water threshold pressure level for moving the actuator to block the main water supply is less than the main water pressure.