A system boiler is similar in some respects to a regular boiler in that they both work on the principle of stored hot water. However, they differ in one important respect in that a system boiler does not require a feed and expansion tank. This frees up valuable space in the loft and results in a cheaper, neater and more efficient installation. Unlike a combi boiler you still require a hot water tank, usually within the airing cupboard so you can still benefit from a full tank of hot water and a useful area for drying towels. Another advantage of a system boiler over a regular type is that hot water is pumped directly from the boiler around the radiator system and hot water tank, this results in a faster response time and lower running costs.
A system boiler tends to be larger than a regular boiler as it contains the expansion vessel, filling loop and pump, however, many advantages including simple installation as most of it is pre-plumbed, pre-wired and tested prior to delivery.