This guide is intended to give a basic introduction to the subject of central heating boilers and flues.

The installation of central heating is a highly technical task and we strongly recommend the use of a fully accredited professional in ALL cases. Gas central heating is installed in over 70% of UK dwellings and has an excellent safey record, but there are inherent dangers associated with highly flammable materials and potentially toxic fumes which demand that installations are carried out by trained and experienced operatives.

We hope that this guide will help you to understand the various options that are available so that you can discuss with your chosen installer the system which is most appropriate to your needs. Further subjects will be featured in the near future and whilst every care has been taken in producing this section it should be noted that the information does not constitute recommendations and is simply a guide.

There are three traditional groups of boiler, Free Standing, Wall Mounted & Back Boilers. There are two other general categories, referred to as Condensing Boilers and Combination Boilers (usually referred to as Combi’s). All these types are described as follows.

Free standing
These floor-standing boilers are usually fairly ‘slim’ for fitting between kitchen units, although they may be installed in any suitable location. Alternatively, in larger homes, free-standing boilers may be sited in garages, out buildings or even purpose-designed boiler rooms. These boilers are available as room sealed, fanned flue or conversional flue versions.




Wall mounted
The vast majority of boilers these days are wall mounted. These boilers have lighter, more compact heat exchangers, constructed from materials such as copper, aluminium, stainless steel or lightweight cast iron. These boilers are available as Room Sealed, or Fanned Flue versions. The Conventional Flue version of this boiler is no longer available.

Most modern boilers of this type, are required to have a ’system by-pass’ fitted to allow sufficient water to circulate through the appliance to prevent ‘kettling’ (the same kind of noise associated with a ‘singing’ kettle)


Back boilers (BBUs)
These boilers are fitted into the opening behind a fireplace surround and are usually hidden behind a gas fire. A back boiler unit will save space elsewhere in the house, but its output is limited to 16kW (55,000 BTU/hr). The noise when operating can at times be intrusive and a ventilation path direct to outside must be maintained at all times.

If an old BBU is replaced for a new unit it will be necessary to renew the existing flue liner as well. It is not possible to renew the fire front only of older, existing BBUs.

Condensing boilers
Condensing boilers are not new, they have been around for many years and users, report very significant reductions in fuel costs, in some cases. This is possible, because this type of boiler has a much more effective 'heat exchanger' allowing more heat to be removed from the flue products.

One of the most significant effects of this is the 'pluming', sometimes mistaken for 'steam' coming out of the flue terminal. This effect is brought about by water droplets held in suspension in the flue products and although non-hazardous, it may be a nuisance if seen constantly passing across windows. This pluming will be evident for most of the time the boiler is operating, so care is needed in positioning the terminal.

Although they have a somewhat higher capital cost than non-condensing boilers, for larger dwellings (with three or more bedrooms) they usually prove to be cost effective within several years.

Typically, a condensing gas boiler might have a seasonal efficiency of around 88% compared to non-condensing boiler of about 75%, and an older type boiler at between 55% -60%. For more information, please visit website

They are as easy to install as any non-condensing boiler, but can only be fitted to 'fully pumped' systems. During it's operation, condensate forms and, although no more acidic than rain fall, a permanent pipe must be run to a drain

This type of boiler is available as a wall mounted or floor standing version.

Combination Boilers
Most wet central heating systems use a boiler to heat a copper cylinder in the airing cupboard to provide domestic hot water. A combination boiler (usually referred to as a 'combi') provides a means of generating hot water for the taps from within the unit and combines this with central heating.

By using a 'combi' neither a cylinder, nor its feed tank, are required. Consequently, hot water running costs will be reduced very slightly since there are no standing losses from the cylinder or boiler pipework. Although a continuous flow of hot water is constantly available, the flow rate is not likely to be as good as with a traditional storage cylinder, particularly in winter.

In dwellings (often larger type) where a number of draw-off points (taps, showers etc) are likely to be used at the same time manufacturers advice should be sought as to the combi's suitability. Showers fitted to this boiler must be a 'mains pressure' type.

Because the boiler is fed directly with mains cold water, it is vital to establish that the minimum water pressure, specified by the manufacturer, is available otherwise the hot water performance will suffer.

Combination boiles are by far the most popular boiler type, accounting for over 50%of all new boiler sales within the UK. There are five types 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 taps, showers and other draw off points.

A combi (instantaneous condensing) boiler operates in principle as an 'instantaneous' type but at somewhat higher efficiencies (see condensing boilers above)

A combi (storage) boiler is a variant of the Instantaneous type and is designed to give better hot water performance. 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 considerably.

In general, the principle of operation is that the stored water will give up it's heat to provide an improved initial hot water delivery. Once the stored heat has been given up, the boiler then operates as an 'instantaneous' type.

A combi (storage condensing) boiler operates in principle as a 'storage' type but at somewhat higher efficiencies (See Condensing Boilers).

Combined Primary Storage Unit (CPSU) - This is a special category of storage combi and incorporate a very large store of water (usually more than 80 litres) allowing a high hot water flow rate to taps and other draw off points and quick heat-up to radiators.


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