The Worcester Bosch open vent cover plate part is based around the diverter valve section. The diverter valve is a device that only appears in combi boilers. It allows the hot water to be directed to the central heating system (radiators) or to the domestic hot water system (taps). This is what separates a normal boiler from a combi boiler due to the boiler's ability to provide heating and hot water without requiring a cylinder.

Brand Name	Worcester Bosch