Automatic Filling Valves are used on central heating systems to control and maintain the water flow and pressure in the system. The ½” BSP union inlet incorporates a non-return valve and strainer to prevent back siphoning. The black control knob opens and closes and allows to the user to control the flow of water into the system when filling or topping up the system pressure. This valve should be in the closed position after filling the system or when topping up the pressure. The unit includes a pressure reducing valve which protects the system from pressure surges, this is factory set at 1.5 bar but can be adjusted by turning the regulating screw at the top of the valve. The filling valve also incorporates a pressure gauge which measures the downstream system pressure.