The two most popular choices of gas boiler are standard or combination boilers and these are both now available as condensing boilers.
Central Heating and Hot Water using Standard Gas Fired Boilers
Standard (or conventional) boilers heat your radiators and hot water when activated by a control. The hot water is then stored in a cylinder until it is needed. 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 control 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. All the major components are housed in one unit which makes them very convenient to the installer and saving space for the householder.
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.
Condensing Gas Boilers
Both of these types of boilers are available as condensing boilers. Condensing boilers are designed to recycle heat with an extra heat exchanger so that the hot exhaust gases are used to pre-heat the water in the boiler system, therefore reducing energy consumption and bills. Condensing boilers cost a little more but the energy savings with repay that extra initial investment in 2 to 4 years of normal use.
See my condensing boilers page or my condensing boilers FAQ for more information or visit the SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) website for some useful facts and figures.
You can find more information on types of gas boiler available from boiler manufacturers websites: