Types of Boiler


Units that are not combination boilers are commonly referred to as ‘regular’, ‘conventional’ or ‘heat only’ boilers. They can be wall-mounted or floor-standing. Space heating is provided directly but for hot water they need to be connected to a separate hot water storage system i.e. Copper cylinder. A back boiler unit is one designed within a fireplace (condensing back boiler units are not available)

Regular boilers installed after 1st June 2005 must be of the condensing type with limited exceptions.


Combination or combi boilers provide both space & hot water heating directly.  The most common is the instantaneous combi boiler, which heats water on demand & does not maintain a large internal store of already heated water.

These boilers are capable of providing a continuous flow of hot water, but at a lower rate than typical hot water storage systems.  As such, they may be less suitable for dwellings where there may be simultaneous demands for hot water, i.e., multiple bathroom / shower room dwellings.

Combi boilers save space because:

• they are fed directly from the water main, with no need for a hot water storage cylinder or cold water feed cistern.

• they are usually intended for use in a sealed system which does not require a feed-&-expansion cistern.  This allows a ‘dry’ roof space.

Before selecting a combi boiler the following points need to be considered.

• to guard against inadequate hot water performance, checks will need to be carried out to ensure that the dwelling has both satisfactory water pressure & and adequately-sized water supply pipe.

• there may also be the need to increase the size of the gas supply pipe as combi boilers require a greater gas input.

Combination boilers installed after 1st June 2005 must be of the condensing type with limited exceptions.


Combined primary storage units (CPSU’s) are a special category of storage combi boiler. They include a very large store of water (70 litres) designed to buffer both the hot water & space heating requirements.  CPSU’s allow radiators to warm up very quickly & are capable of providing hot tap water at a high flow rate


A condensing boiler has a large heat exchanger which extracts more heat from the flue gases.
In a non-condensing boiler, the flue gases are at a temperature 120 to 200°C. In a condensing boiler more heat is removed & the temperature falls to below 100°C & as low as 50°C for the most efficient boilers operating at reduced boiler return temperature.
The water vapour in the gases condenses (hence the name) & the resulting liquid has to be drained away.  As the heat exchanger gets wet in the process it is more susceptible to corrosion. To avoid this, it has to be constructed from corrosion resistant materials, for example stainless steel.
SEDBUK efficiencies between 86% & 92% (with natural gas as a fuel).
Typically a new condensing boiler will have an efficiency of 90% compared with 79% for a new non-condensing boiler & 55%-65% for older types.
Available in regular & combination models, mostly wall mounted, although some large output units are floor standing.
Only room sealed, fan-flue models are available for domestic applications. Most have extended flue options.
Suitable for replacing most existing boilers, but not back boiler units in the same position.