The Hansen Slo-Flo tank-filling valve is for use when your tank water level falls below a pre set minimum. The valve can draw from an alternative source such as a pump or mains water supply. But the valve only raises the water level a pre-determined amount - either 50mm or 100mm. The purpose of this to just raise your tank level a minimum amount to maintain supply until future rainfall fills it properly. Flow with this valve is restricted to a constant 2 litres/min (at 0.2 - 12bar), consistent with rules of many local water authorities. The valve is self-cleaning, and features a soft open/close action to prevent valve hammer. A valve-lock is also incorporated to stop valve operation if not desired. Supplied complete with tank wall through-fittings as listed.