What is a Condensing Boiler?

A condensing boiler is a high efficiency central heating boiler.
All new boilers fitted in the UK must be condensing boilers by law.

A condensing boiler works by reclaiming the heat (see below) from the hot waste
gases which usually leave the boiler through a flue into the air outside.
The reclaimed heat is used as extra heat to make the boiler more energy efficient.
A condensing boiler is usually more than 90% efficient. This compares to around
80% for a modern non-condensing boiler or even down to 60% for older boilers.

When a boiler burn gas (or oil) the heat is released from the fuel along with waste gases.
These waste gases are carbon dioxide and water. The water is in the form of steam.
In a condensing boiler, the steam is quickly cooled back into liquid water.
This process is called condensing. When the steam condenses it gives up some
of its heat. It is this extra heat that is used by the boiler to make it more efficient.
The condensed water is acidic and is called the condensate. The condensate needs
to be able to drain safely away from the boiler to a gully or a soil pipe or a soak away.
This needs to be done in such a way that the condensate will not freeze in the winter
because this will stop the boiler working.

Any type of boiler, whether it is a combi or heat only, is available as a condensing boiler.