A system boiler is designed for properties that require larger amounts of hot water, combine the energy efficiency benefits of the combi with the big-performance capacity of the conventional boiler. They can work either as heating-only boilers or with a storage cylinder to provide hot water, most of the manufacturers make their boilers as a two pipe system which then goes to external controls (diverter valve, programmer, room and hot water thermostats), some make their boilers with four pipes, flow and return for the hot water and flow and return for the central heating the diverter valve and controls are integrated in the boiler giving a wide range of usage, ideal when replacing an old gravity system which has a separate flow and return for heating and hot water. Also the four pipe system is ideal for outside weather compensation as the flows are separated at source enabling different temperatures for the heating and hot water.
Most system boilers come with integrated circulating pumps, expansion vessel and programmer. When fitting a system boiler the primary (the water that is heated by the boiler) is converted to a sealed system which makes the small header tank in the loft area redundant as the expansion and contraction is taken up within the expansion vessel. As a safety measure a pressure release valve is built into the boiler which releases outside if there is a problem. There is a filling loop which is attached between cold mains and the primary return which is used to fill and top up the system.