Altitude control valve controls the water level in, for example, water reservoirs without the need for using a float valve or other ancillary controls. The highly sensitive pilot valve and the main valve are installed outside the water reservoir and the pilot valve senses the hydrostatic water pressure from the water level in the reservoir. The main valve closes when the maximum set pressure for the pilot valve is reached and reopens when the water level corresponds to the lower set value on the pilot valve. The standard version permits filling of a water storage unit. A special version is also available on request which can be used for both filling and emptying water storage units.