Unlike a Combi boiler, a Regular Boiler(or just boiler), works on the principle of stored hot water. This means you also need a cylinder to store the hot water, this can be of the unvented or open-vented type. This type of boiler essentially just generates heat which is then circulated via an external pump and distributed by a valve or a series of valves.
In general this type of boiler can be used on an open-vented or sealed system. All modern systems are fully pumped. A lot of the older types of systems like a back-boiler system for example, relied on gravity to heat the cylinder and a pump for the central heating. These older systems and cylinders were open-vented. A modern system is always fully pumped and usually sealed. The cylinder is usually of the un-vented type.
The average output of a regular boiler ranges between 12KW to 24KW.
A Regular 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.