Modern Central Heating

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1 Gas and Oil Boilers

This page explains the main types of boiler available. Gas and oil boilers (and the heating installations) are actually not that dissimilar. The page also explains the common flue arrangements and provides a very brief introduction to SEDBUK ratings. Two other important organisations are CORGI (Council for Registered Gas Installers) - the national 'watchdog' for gas safety in the United Kingdom. OFTEC (Oil Firing Technical Association) is almost its oil equivalent. 50plus note: CORGI are no longer the registration body for gas installers. This role is now contracted to Gas Safe.
Until the 1970s most boilers, whether wall mounted, freestanding or back boilers, had open flues (same as a chimney basically). An open flue boiler must sacrifice some efficiency in order to vent unwanted products of combustion from the house. By allowing flue gases to retain a high temperature, the natural force of rising hot air creates the chimney draft. At the same time, air from the room is drawn into the burner to maintain combustion. If there is insufficient air for the combustion process, poisonous carbon monoxide will be produced at the burner - a fixed open vent must therefore provide enough air for combustion. A room sealed flue (sometimes referred to as a balanced flue) consists of two ducts, one inside the other. The fresh air for combustion is taken into the boiler via the outer duct and the exhaust gas is usually discharged outside by the inner duct. The flue terminal may only be fitted on an external wall. The combustion is independent of any air supply within the room which is why these boilers are often called room sealed. There is usually a limit on the length of these flues - normally less than a metre. Most boiler now are fan assisted. They have smaller cross-sectional flue ducts than natural-draught flues. The flue fan provides the fresh air to the burner and also removes the products of combustion from the combustion chamber. The use of a fan allows for more compact boiler design and provides greater flexibility in terms of flue positions and length.
SEDBUK http://www.sedbuk.com is an acronym for 'Seasonal Efficiency of a Domestic Boiler in the UK'. The system was developed under the UK Government's energy efficiency best practice programme with the co-operation of boiler manufacturers and provides a basis for fair comparison of different models of boilers. The SEDBUK rating is the average annual efficiency achieved in typical domestic situations, making sensible assumptions about climate, control, pattern of usage and other similar factors. The rating is calculated from laboratory tests together with other important factors such as boiler type, fuel used, ignition type, UK climate, boiler water content and typical domestic usage patterns. The boiler's performance is then scored which enables the boiler to be placed in a banding system using a scale from "A" to "G." "A" banded boilers being the most efficient and are usually 'condensing' boilers.
	Regular (or vented) boilers. These 'traditional' vented boilers have been around, in one form or another for nearly a hundred years. Modern ones are much more efficient than their predecessors. Regular boilers are designed to heat water in a separate cylinder (usually located in an airing cupboard) and provide hot water for radiators. The boiler is fed by a storage (feed and expansion) cistern usually located in the roof void, and a pipe from the boiler vents back over, and into, the cistern if the boiler controls go wrong and the water boils. The picture on the left was kindly supplied by Worcester Bosch. On the left you can see a free standing boiler from the 1960/1970s. On the right there is an animated sequence showing the removal of a old back boiler - probably installed in the late 1970s. Both these boilers were probably about 70% efficient - modern boilers can reach efficiency levels of 90% plus. Both the models shown here had open flues.
	Nowadays regular or vented boilers can be wall-mounted (most of them) or floor-standing. Until recently a regular or vented wall-mounted boiler typically comprised a fan assembly for the room-sealed flue (right), a cast iron heat exchanger, a burner & combustion chamber, and an electronic control panel. Hot water from the boiler circulates around the radiators; a feed also heats water in the hot water cylinder (usually vented in a traditional installation). The picture on the left shows a Worcester Bosch regular boiler. Nowadays condensing versions of these are also available. In a condensing boiler more energy is extracted from the fuel so the boiler is more efficient and heating costs are therefore lower. Modern condensing boilers usually have aluminium heat exchangers. A typical layout (hot water for washing and heating controls are omitted) can be found here. More information on layouts is on the next page (Gas & Oil Systems).
	A system boiler is designed along similar lines to a regular boiler but is designed for use with an expansion vessel rather than a feed & expansion cistern and an open vent. In some boilers the expanding water is accommodated in an expansion vessel fitted inside the boiler. In others it is not part of the boiler but is usually located near to it. The example shown here is made by Worcester Bosch (this one is a non-condensing type). The boiler is almost a 'half way house' between a regular boiler and a combination boiler. To provide hot water for washing a system boiler heats water in a cylinder. This can be either a vented (traditional) cylinder or a modern pressurised one (see right). The advantage of the system boiler is that there is no feed and expansion tank in the roof - it would make sense to follow the same approach for the hot water cylinder. See the next page for more information on system boiler installations
	Combination boilers. These are very different from regular and system boilers. Combination boilers (so called because they deliver hot water for washing and heating) now account for well over half of all the new domestic boilers installed in the UK every year. They were originally developed for flats to save having lots of tanks in the roof space. The major difference between a combi and regular or system boilers is that a combi eliminates the need to store hot water - a hot water cylinder is not required. As with a system boiler the heating system is topped up directly via a filling ‘loop’ fed directly from the mains. The expanding water is accommodated by an expansion vessel fitted inside the boiler (the orange cylinder shown on the right - again just like a system boiler). Hot water for washing is delivered instantaneously (well, almost) as and when required - not always at very high flow rates but at mains pressure - so very good for feeding showers. In the examples shown here (left and right) a water-to-water 'plate' exchanger heats the hot water for washing. The savings in installation cost (no cylinder and no tank) are probably offset by the slightly higher cost of the boiler. These boilers do have minimum requirements in terms of mains water pressure. They also need a good gas supply - at times these boilers work quite hard.
	Condensing boilers (regular, system or combination). These are available in regular, system and combi versions. In a modern oil or gas fired boiler about 70 – 80% of the fuel is converted into useful heat. The remainder is lost through the flue – the flue gases can be as hot as 250 degrees Celsius. In condensing boilers more heat is extracted from the fuel; their efficiency can exceed 90%. In fact, so much heat is extracted that the flue gases are only about 50-60 degrees. In a typical condensing boiler there are two heat exchangers. After crossing the first one the combustion gases are directed over the second heat exchanger to pre-heat the water returning from the radiators. If the water in the radiator return pipe is cool enough the exhaust gases will condense on the heat exchanger releasing all their latent heat. The secondary heat exchanger must, therefore, be corrosion resistant and a drain pipe must be fitted to remove the condensate. Have a look at the graphic on the right. The image on the left shows a modern Worcester Bosch gas fire condensing boiler (they also make an oil fired version). The exhaust gases of condensing boilers 'plume' - this is quite normal and reflects their lower temperature. Note: some condensing boilers, if fitted in a compartment, may need an air supply to keep them cool. All condensing boilers are room sealed or fanned. The Building Regs now require new boilers to be condensing boilers. In some situations there may be practical reasons why a condensing boiler cannot be fitted - this departure from the Regs needs official approval. This is usually because of problems with siting the flue or condensation drain.
	There are a number of rules regarding the siting of flues for gas and oil boilers. These are quite detailed. The specific requirements can be found in the Building Regulations or in trade information. The graphic on the left shows some of the more common rules for gas terminals. Do not rely on these exact figures and do remember that incorrect siting of flues is not uncommon. The plume from a condensing boiler can be a nuisance. This is why it is sometimes preferable to fix the flue terminal at high level or even site the boiler in another location. The plume should not cross a frequently used access route or patio etc. It should not discharge across a neighbour's dwelling. Nor should it be directed towards a window or door or be sited close to a facing wall. General Regs related to gas and oil flues are shown in the left-hand drawing.
	Most manufacturers supply tables or software to help installers and engineers calculate boiler size. The linked spreadsheet is an adaptation of a manual method provided originally by the Energy Saving Trust. It's included here primarily so you can assess how different U values etc affect boiler size. Don't use it for 'real'. In order to calculate radiator size a similar approach is required. Have a look at the page on radiators for more information.