Water System - Combi - High - Pressure - Combination

Water Systems - High Pressure - Combination Boiler.

(see also an unvented hot water cylinder).

A combination boiler (also called a combi) provides hot water for central heating
and also heats water as it is required for hot water taps.
There is no stored hot water and the system is can be very efficient.

Cold water at mains pressure (high pressure) runs to all of the cold taps,
showers, toilet cistern and other services. Cold water also runs through a pipe to
the combination boiler which heats it as it flows through the boiler. The hot water
runs through pipes to all of the hot taps and showers.

It is called a high pressure system because all of the pipes contain water at
a high mains pressure. The hot and cold water is at the same high pressure
but the flow rate of hot water through the boiler is not constant
and this can be a problem for showers.

The advantages of a combination boiler are
1. There is no tank of stored cold water in the loft.
This may give a better usage of the loft space (but see below).
2. The combination boiler only heats water as it is needed
and so it can be very energy efficient.

The disadvantages of a combination boiler are
1. There is no tank of stored cold water in the loft.
If the mains water supply stops running due to maintenance
or a fault, then the property has no cold water.
2. The unstable flow rate through the boiler can be a problem for showers.
3. The amount of available hot water is limited by the boiler size.
A combination boiler is only suitable for flats or small houses.

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