This part is based around the diverter valve section. The diverter valve is directional water device that only appears in combi boilers. It allows the hot water to be directed to the central heating (radiators, etc.) or to the domestic hot water system (taps, etc.). This feature is what separates a normal boiler from a combi boiler due to the fact the boiler can provide hot water and heating without the need for a cylinder.
We recommend that all parts are fitted by a fully qualified gas safe certified engineer.