The Vokera domestic diaphragm black part is based around the diverter valve section. The diverter valve is an instrument that only appears in combi boilers. It allows the hot water to be directed to the heating system (radiators, etc.) or to the hot water system (taps, etc.).
Replacement diaphragm for the domestic valve
Technical specification
TypeDiaphragmToilet Cistern CapacityFTW--MaterialRubberColourBlackSupplier Part Number6882Brand NameVokera