Combi Boiler - High Pressure - Hot Water Systems.
(see also other types of boiler and unvented hot water cylinders).

What does a Combi Boiler Do?

A combination boiler (also called a combi) provides hot water for central heating radiators
and also heats water as it is required for hot water taps and showers.
There is no need for a hot water cylinder (compare this with a heat only boiler)
or for water storage tanks in the loft (which frees up loft space for other uses).

A combi boiler is part of a sealed central heating system. The boiler usually contains the
pressure gauge and expansion vessel. It may have a temperature control on the front panel.

In a property with a combi, 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 heated water runs through pipes to all of the hot outlets, taps and showers.

The water system with a combi boiler 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 taps and showers may not be constant
if more than one outlet is used simultaneously. This this can be a problem for showers.

There are two types of combi boiler, the instantaneous combi and the storage combi.
If a new combi boiler of either type is fitted in the UK
by law it must be a condensing boiler because these are more energy efficient.

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