Combination boilers or the "Combi" are a space saving form of heating, because they heat hot water on demand there is no need for the hot water cylinder that is usually found in the airing cupboard or the water tanks found in the loft. All the major parts that are normally found in the airing cupboard, ie pump, valves etc are conveniently housed inside the boiler casing.They are generally suitable for smaller one bathroom households, with moderate demands on hot water. For Higher demands you may want to consider a storage combination boiler, such as the Vaillant Ecotec plus, this has a separate 15 litre hot water store built into it and will provide flow rates of 15.2 litres a minute at 65 degrees centigrade.
Heating requirements are easily met, with some combi boilers, such as the Worcester Bosch Greenstar having an output up to 42kw and are fully modulating. Fully modulating means that the boiler adjusts its burner rate to suit the demand.
The heating is filled with cold water via a temporary connection (filling loop) and pressurised within the system, a gauge on the front panel shows the heating system pressure, though some of the latest boilers have a built in filling loop and a digital pressure gauge. If there is a malfunction and the heating system pressure is too great there is a safety discharge pipe terminating safely outside. This should be tested as part of the annual boiler service
For ease of use most combination boilers are available with a built in programmer to control heating on and off times.Whilst a programmer is not necessary for the hot water, some combination boilers (Worcester Greenstar, Glow worm and Ideal Boilers) have a "pre-heat" facility that heats a small quantity of water ready for your use, this helps to eliminate running the tap for a number of minutes waiting for the hot water to come through.
Some recommended Natural Gas and LPG combination boilers are Worcester Greenstar, Ideal Logic Plus, Glow worm Flexicom, and Ideal Vogue. Storage combination boiler Vaillant Ecotec 937 plus or Viessmann.