Central Heating and Hot Water using Standard Gas Fired Boilers
Standard (or conventional) boilers heat your radiators and hot water when activated by a programmer or controller. The hot water is then stored in a cylinder (usually in the airing cupboard) until it is needed. There is usually a large cold water cistern (tank) plus small header cistern (tank) in the loft.
This type of system is most suitable for families or large houses where hot water is needed on demand and in significant quantities for heating, baths, washing machines, dish washers and so on.
Combination (Combi) Gas Boilers and Central Heating Systems
Combination boilers (Combis) provide hot water on demand, heated as required with no water storage. They heat your radiators when activated by a programmer or controller similar to standard (or conventional) boiler systems. Combi boilers dispense with the need for the hot water cylinder in the airing cupboard, the large cold water cistern (tank) plus small header cistern (tank) in the loft which are otherwise needed in conventional vented heating systems.
A combi boiler normally also contains the pump and, frequently the hot water and heating timer (controller or programmer).
All the major components are housed in one compact unit which usually sits in the kitchen or utility room, or perhaps the airing cupboard which makes them very convenient to the installer and saving space for the householder.
The further benefits of a Combi boiler is that hot water is delivered through your taps or shower at mains pressure. So you can enjoy powerful showering without the need for a pump.
Combi boiler systems are generally suitable for smaller households, flats and bungalows where there are a smaller number of users with modest heating and hot water demands although some of the latest combination boilers can suit larger households.