Central Heating Boilers

This guide will help you to understand the options available so that you can discuss with your chosen installer the central heating system most meets your needs. Every care has been taken in producing this section but the information does not constitute recommendations and should be used as guide only

Installing central heating is a highly technical task and we strongly recommend that you employ the services of an accredited professional in ALL cases. Over 70% of UK dwellings depend on gas central heating and has an excellent safey record, but there are inherent dangers associated with potentially toxic fumes and highly flammable materials. Therefore ensure that installations are carried out by experienced and trained operatives.

Traditionally there are three groups of boiler - Free Standing, Wall Mounted & Back Boilers. The other general categories are referred to as Condensing Boilers and Combination Boilers (Combi’s). These Boiler types are described as follows.

Free Standing floor standing boilers are designed to be quite slim for installation between the units in a kitchen, although installation can be in any suitable location. In larger homes, free standing boilers may be sited in out buildings, garages or even purpose built boiler rooms. Free Standing Boilers are available as convensional flue, fanned flue or room sealed versions

Most boilers these days are wall mounted. Wall mounted boilers are lighter and have compact heat exchangers. They are usually constructed from materials such as stainless steel, aluminium, copper 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.

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 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 only the fire front of older, existing BBUs.

Condensing boilers have been around for many years and have significant savings in fuel costs. Condensing Boilers achieve this by having a much more efficient 'heat exchanger' which allows more heat to be removed from flue products.

The significant effect of a condensing boiler is the pluming, which looks like steam coming out of the flue terminal. The Pluming 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.

A condensing boiler is normally more expensive to purchase at the outset but the savings will normally make up for this within 2-3 years.

A Typical 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%.

Condensing Boilers are as easy to install but can only be fitted to a fully pumped system. When operating, condensate forms and, although no more acidic than rain fall, a permanent pipe must be run to a drain

Condensing Boilers are available as a wall mounted or floor standing version.

Alot of central heating systems use a boiler to heat a copper hot water cylinder in an airing cupboard to provide domestic hot water. Combination boilers (usually referred to as a 'combis') generate hot water for the taps and showers in your home from within the the boiler itself and combines (Combi) this with central heating.

When using a combination boiler, hot water cylinders and feed tanks are not required. This normally results in hot water running costs being reduced very slightly because 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 2 types of combi boilers:

• A combi 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 condensing combi boiler operates in the same principle as a combi boiler but is much more efficient

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.

All boilers require a flue to remove their products of combustion. In the past traditional chimney flues placed severe restrictions on where a gas appliance could be used. Today those restrictions no longer apply since modern boilers, with far more flexible flue systems allow greater siting options.

Open Flue (Natural Draught)

With a boiler having this type of flue, air for combustion is taken from the room in which the boiler is located. Flue products are removed by natural draught vertically to atmosphere, through a suitable terminal. The room must have a route, for combustion air, direct from outside.

Open Flue Boiler

If an extract fan is located in the same room as the boiler, then manufacturer's advice should be sought, since additional ventilation will probably be required to ensure that the boiler flue products do not 'spill' into the room.

All new open flue boilers must now be fitted with a safety device which will turn the boiler off in the event of flue products spilling back into the room because of blockage or for other reasons.

Open Flue Boilers (Fanned)

Recently, open flue boilers have been introduced incorporating a small 'flue fan' to give assistance in clearing flue products. Exactly the same conditions apply to this type as with natural draught, and it must not be assumed that the requirements for running the flues are any different. The introduction of a fan does NOT turn this boiler into a 'fanned flue type.

Room Sealed (Natural Draught)

This boiler draws it's air for combustion directly from outside through the same rectangular flue assembly used to discharge the flue products. This boiler is inherently safer than an 'open flue' type, since there is no direct route for flue products to spill back into the room.

This type of boiler can only be fitted to an outside wall and the siting of the terminals in relation to doors and windows is expected to become more difficult with new regulations soon to be introduced.

Room Sealed Boilers(Fanned Flue)

With the use of a fan, the flue products are discharged directly to outside through the central core of a circular, concentric flue, around which air is drawn in and passed to the combustion chamber of the boiler.

Fanned flue boilers are generally more efficient than other types, since less heat is lost from the small circular flue, whilst at the same time allowing the option of extended flue runs if required.

Room Sealed Fan boiler

Some fanned flue boilers are available with a 'twin tube' arrangement, which provides separate connections for the flue pipe and combustion air pipe. Such an arrangement allows considerably longer flue runs than would otherwise be possible.

Twin Flue Arrangement Boilers

Some earlier fanned flue boilers were able to adopt a 'Vertex' flue arrangement, in which the flue products are discharged vertically through the central core of the concentric flue assembly to discharge through a terminal in the roof, whilst air for combustion is drawn in from the ventilated roof space.