The Worcester Bosch Combi Filling Link Key is compatible with Bosch, SBi Series, CDi Xtra Series and CDi Series boilers. It is part of the unit's filling loop. For use in central heating boilers and systems, allowing the user to manually top-up the amount of water in the system if the pressure drops. This product has been discontinued, and the alternative product is 446183. Please note that this is not a direct replacement.