Combination boilers are fed directly from the rising main and are the most popular choice for new installations, also a lot of existing installations that require a new boiler are replacing conventional boilers with combination boilers. Combi boilers as they are commonly known provide both hot water for the radiators and a separate supply of instant hot water to the taps and showers.
A combination boiler is connected directly to the rising main, which means it can supply hot water at mains pressure. The only restriction to this is the water needs time to heat up as it passes through the boiler, so the flow from the taps will need to be reduced to obtain a higher temperature. To minimise this problem some combi's contain a small storage heater which allows a greater flow rate and also reduces the time taken for the water to reach the desired temperature.
The reason why combination boilers are so popular is, ease of installation in a new system as there is no cold water storage tank or hot water cylinder to install. This makes them ideal for new style houses and apartments with little or no loft space. In an existing installation, when a conventional central heating boiler is replaced with a combi, the cold water head tank and hot water cylinder become redundant, this space can usually be used as valuable storage space, or sometimes to extend the bathroom.