Firstly, many of the major individual components of the heating and hot water system are built into a system boiler, which means that installation is quicker, neater, easier and more efficient.
Secondly, the hot water is pumped from the system boiler through the heating system to the radiators and hot water cylinder, resulting in a fast response and more economical running costs. The system boiler removes the need for a feed and expansion cistern.

Perfect Heating And Plumbing - System Boiler with Open Vented Hot Water Cylinder
An example of a central heating and hot water system layout using a system boiler with an open-vented hot water cylinder:


