Unlike a combi boiler, both a system boiler and a regular (conventional) boiler work on the principle of storing hot water in a copper cylinder (usually in the airing cupboard). Notwithstanding this system boilers differ from regular boilers in some fundamental respects.
Many of the major individual components of the heating and hot water system are built in to the system boiler which means that installation is quicker, neater, easier and more efficient. You will see that system boilers include an expansion vessel so there is no requirement for a feed and expansion tank in the loft, which allows the system to save space in your home.
Loft space not required
The header or feeder tank required by a regular boiler to continually feed water to your central heating system is not required. System boilers take a direct feed from the cold water mains and store water in a small tank built into the boiler. Accordingly space is freed in your loft which may enable you to use it for alternative purposes.
The storage of cold water means that the boiler is less dependant upon a flow of water from the cold water mains and can be used in areas where the water pressure is too low for a combi boiler to be used.