Saunier Duval part is based around the diverter valve section. The diverter valve is directional water mechanism that only appears in combi boilers. It allows the hot water to be guided to the central heating (radiators, etc.) or to the DHW system (taps, etc.). This feature is what makes a combi boiler so popular due to the fact the boiler can provide hot water and heating without the need for a cylinder.