A System Boiler is essentially a regular heat-only boiler that incorporates a pump and an expansion vessel. Unlike a Combi boiler it works on the principle of stored hot water. It creates heat which is circulated by its internal pump and then distributed by a valve or a series of valves. System boilers in general are used on sealed systems as they incorporate an expansion vessel.
The average output ranges between 12KW to 24KW.
A System Boiler can be of condensing or standard efficiency(non-condensing) design. All domestic boilers installed in the U.K. from 2005 onwards are condensing boilers.