A system boiler is used to heat your central heating system directly, producing hot water for your cylinder. A system boiler, just like a conventional boiler works on the principle of stored hot water, but differs is some important respects.

They main difference is that 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 and more efficient. For example, the pump, which is normally installed remote from the boiler, is built in. The expansion vessel is also built in, which replaces the feed/expansion tank often installed in the loft. The safety valve, the automatic air vent and even the programmer are also included. As these components are already built in installation time is dramatically reduced, fewer materials are required and the costs are also cut.

This is makes maintenance of the system much simpler. The added bonus of a dry loft removes the worry of leaking or frost damage to the pipes.
The hot water is pumped from the system boiler through the heating system to the radiators and cylinder, resulting in a quicker heating system and more economical running costs.