If you are replacing an older model of boiler, the chances are that you will have a regular (conventional) boiler. 

A typical conventional system incorporates a boiler and extended controls, a feed and expansion cistern, and a hot water cylinder (usually in the airing cupboard) which is often fed by a cold water storage cistern located in the loft.

The high efficiency condensing combi or combination boiler is an ingenious space-saving idea, and an increasingly popular choice in UK homes. In fact, combis now account for well over half of all the new domestic boilers installed in Britain every year. The major difference between a combi and any other type of boiler is that a combi eliminates the need to store hot water – so no hot water cylinder in the airing cupboard. It is both a high-efficiency water heater and central heating boiler, combined (hence the name) within one compact unit.

Unlike a combi, both a system boiler and a regular (conventional) boiler work on the principle of stored hot water – but a system boiler differs from a regular boiler in some important respects. Firstly, many of the major individual components of the heating and hot water system are built in, which means that installation is quicker, neater, easier and more efficient. Secondly, the hot water is pumped from the boiler through the system to the radiators and hot water cylinder, resulting in a fast response and more economical running costs.