Most traditional central heating systems (regular boilers) store the boiler heated hot water in a copper cylinder in the airing cupboard to provide domestic hot water. Combi boilers do not require this hot water storage saving you space and reducing heat loss from the previously stored hot water.
By using a ‘combi’ the cold water feed tank, often housed in the loft, is also not required this can also save storage space and reduces the number of components in the system simplifying any system installation. Central heating systems incorporating combi boilers have fewer components.
Other factors to be considered
One small disadvantage is that although a continuous flow of hot water is constantly available, the hot water flow rate from some combi boilers is not likely to be as good as with a traditional storage cylinder, particularly in Winter.
Because the boiler is fed directly with mains cold water, it is important to establish that the minimum design water pressure is available otherwise the hot water performance will suffer. For this reason a combi boiler is not always suitable and a site survey is often required.
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Types of combi boilers
There are five ‘varieties’ of combi boilers.
A combi (instantaneous) boiler is the most widely used type and directly heats incoming mains cold water to supply hot water to your taps, showers and other hot water points.
A combi (instantaneous condensing) boiler operates as an ‘instantaneous’ boiler but at somewhat higher efficiencies than the standard combi boiler.
A combi (storage) boiler is a variant of the Instantaneous type above and is designed to give you better hot water performance through the internal storage of hot water. The stored hot water also can give you better initial hot water supply from cold water mains on first turning on the tap. Once the stored heat has been used, this boiler will tend to operate as an ‘instantaneous’ type. Dependent upon the make and model, the improvement in hot water delivery will depend upon the size of hot water store, and this can vary widely.
A combi (storage condensing) boiler operates in principle as a ’storage’ type but at higher efficiencies.
Combined Primary Storage Unit (CPSU)
- This is a special category of storage combi which
will incorporate a very large water store allowing a
high hot water flow rate to taps and other draw off
points and to quickly heat-up to radiators before
the hot water store is used.