Jargon Buster

Fuel Options  [top]

Natural Gas

Natural Gas is a clean fuel, burning with by-products of steam and carbon dioxide, and having the least damaging effect on the environment of all of the other fossil fuels. It is currently the cheapest option in terms of running costs. (see below) Natural gas is mains supplied, thus requiring no storage. The downside is that the supply may not be available to everyone. It is a legal requirement that a gas boiler is fitted by a CORGI registered engineer.


This is the most popular alternative to gas, used, for example, when location means no mains gas supply is available. Oil is not quite the cheapest alternative, but is a competitive option, although prices can change seasonally and vary with location. Oil is not the cleanest fuel and can contribute to acid rain amongst other environmental effects. The use of oil requires a storage tank. OFTEC registered engineers are most suitable for fitting oil boilers, although unlike gas this is not a legal requirement.


Liquid petroleum gas has many of the same advantages of mains gas but is considerably more expensive to use. LPG can also be used with other household equipment such as cookers and fires. As with oil, LPG requires a storage tank, or can be provided in replaceable cylinders. The range of LPG boilers available is considerably less than mains gas. Gas boilers can be converted to LPG with the appropriate conversion kit. It is a legal requirement that an LPG boiler is fitted by a CORGI registered engineer.


Although conveniently available to practically every home, electricity still ranks as the highest costing fuel for heating. However use of electricity for central heating and hot water in this country is still the second most popular choice after natural gas. Electricity appears to the end user to be 100% efficient since no heat is lost through a chimney with fossil fuels. However since much of the country's electricity is provided by fossil fuel burning power stations it is here that the environment is affected.

Mains Gas1.25p

FuelAvailability in the UKConvenience of SupplyOn-site StorageSmell NoiseChoice of AppliancesControllableMaintainance Charge
Mains Gas70%ExcellentNoneLimitedLittleVery Wide FullyYes
Oil100%GoodYesSomeSomeModerateFully Yes
LPG100%GoodYesLimitedLittleModerateFully Yes
Electricity (Peak)100%ExcellentNoneNoneNoneWide FullyNone

Heating Systems  [top]

There are two categories of heating system; Traditional Systems and Combination Systems. Boilers in each system are used to both heat the hot water and provide the heat to the radiators. They differ in the way they heat the hot water, and in the type of radiator system. Traditional boilers can operate either an open vented or a sealed radiator system depending on the boiler type.


Traditional boilers are used to heat the radiators, as well as the domestic hot water that is held in a cylinder, usually located in an airing cupboard. This type of system is best if there is a high demand for hot water since the storage tank can absorb the impact of several hot water outlets being used at one time. Traditional boilers can be an advantage over Combination Systems if the mains pressure is too low to produce a decent flowrate at an outlet in the property.

The central heating circuit is generally an open system, unlike with Combination Systems. As the water in the radiators warms it expands and overflows into an expansion tank in the loft. As the radiators cool again gravity drives this overflow back into the system to keep the pressure up in the radiators.

A Traditional Heating System

A Traditional Heating System

Traditional Systems can sometimes be converted to have sealed radiator systems by adding an expansion chamber to the circuit. System boilers come complete with this expansion chamber, sometimes built into the boiler, eliminating the need for the expansion tank in the loft of the property. They are space saving, less complicated to fit and are more efficient and a fully open vented traditional system.


Combination Systems use Combination, or 'Combi', boilers. Combi boilers heat the hot water on demand by a heat exchanger in the boiler. There is no need for a hot water storage tank. Installation is often quicker and cheaper than a traditional type system.

Combi boilers also have a sealed radiator system, that needs to be pressurised when the heating is off. There is no need for an expansion tank in the loft for the heating. Combination Systems suit a household where hot water is not in great demand. The mains inlet pressure determines the pressure of the hot water at an outlet.

A Combi Heating System

A Combi Heating System

Lack of a hot water storage tank and an expansion tank means that Combi systems occupy less space and can be easier to fit and maintain. Hot water is essentially an endless supply, although demand from several outlets at one time can reduce the flowrate at each. These systems are highly efficient because no heat is lost from water sitting in a storage tank, however in colder times there can be a delay in receiving hot water from a tap due to the initial inflow to the boiler being cooler. Combination systems are best suited for couples or small families where there is a low demand for hot water i.e. less than two bathrooms. Combi boilers are useful space savers because they don't require a hot water tank, so if the boiler is for a flat or a house with a loft conversion then a combi boiler would be the best choice.

Boiler Types  [top]


Condensing boilers, also known as High Efficiency boilers, although more expensive to buy, have very efficient heat exchangers. These are the SEDBUK A rated boilers. In larger properties the initial outlay for the purchase may be greater than with a conventional boiler, but this cost can be recovered in a few years due to the efficiency. Other considerations are a drain to remove the condensate, and a steam from the outlet may be an annoyance or an eyesore. Both Traditional and Combination Systems can use Condensing boilers.

Standard Efficiency

Standard Efficiency type boilers are available for traditional or combination systems. Standard Efficiency boilers operate at lower efficiencies than condensing boilers, but the initial purchase cost is less than High Efficiency (Condensing) models. New leglislation, introduced in April 2005 in England and Wales, strictly controls the number of homes that can be fitted with Standard Efficiency boilers.


Back Boiler Units are fitted into fireplaces that may already have an existing fire front. This type of boiler is a space saver, but higher outputs can be an issue because of the size limitations. Other considerations are a path to the outside for ventilation. BBU are used in traditional systems, and cannot have a sealed radiator system because their power output is not great enough.

System (or Sealed System)

System boilers, or 'Sealed System' boilers, are used in traditional system with a sealed radiator circuit. They have the expansion chamber included in the set or will have the components built in. For this reason some system boilers are larger than other types, and are best for homes where hot water demand is high.

Floor Standing

These can be traditional or combi boilers and are the type that are seated on the floor. Floor Stading boilers can fit in spaces between kitchen units or, in larger homes, in a pantry or a garage.

Wall Mounted

Wall Mounted boilers are the opposite type to the Floor Standing variety, are smaller and generally cheaper. Usually they are more compact and in some instances may even be small enough to be positioned inside a wall mounted kitchen unit.

Flue Types  [top]

A flue is a pipe or chimney that is used to vent the waste gases, from the combustion process in the boiler, to the outside of the property. The waste gases are mostly carbon dioxide and steam. There are several types of flue, some boilers accommodating all types and some just one.

Room Sealed Flues, RS (Fan Flues, FF)

These are by far the most common flues to find on a modern boiler. Boilers with a room sealed fanned flue can only be fitted to an outside wall. A double skinned pipe draws air in from outside and feeds waste gases back out again. In this situation flue gases cannot escape directly back into the room, and is therefore a safer option. Room sealed flues can operate through the natural draught of the air, or may be fan assisted to allow for longer flue runs.

Open Flues, OF (Chimney Flues)

Open flues draw air in from the room in which the boiler is located. This air is used in the combustion of fuel and is then vented to the outside of the property. Open flues can operate through the natural draught of the air, or may be fan assisted to allow for longer flue runs. Use of open flues is most common in BBUs in an existing fireplace.

Balanced Flue, BF (Conventional Flue, CF)

Increasingly less common the balanced flue contains a baffle to control the flow of air in and out of the ducting.

Efficiency  [top]

Under the Government's Energy Efficiency Best Practice Programme a standardised method of defining product efficiency was called for. The result was SEDBUK or Seasonal Efficiency of Domestic Boilers in the UK (see www.sedbuk.com). This is a measure of the average annual efficiency of a product given influences such as the season. The scale goes from A to G, A being the most efficient. You can calculate the size of boiler best suited for your needs at this address http://www.sedbuk.com/whole_house.htm.

BandSEDBUK Range
AAbove 90%
GBelow 70%

The New Legislation - April 2005  [top]

The domestic heating sector is responsible for one third of the United Kingdom's carbon dioxide emissions. The Government has made changes in the Building Regulations in a bid to contribute towards the global move to combat carbon dioxide emissions and reduce global warming.

From the 1st of April 2005 all new gas boilers installed in England and Wales will be Condensing Boilers of SEDBUK rating A or B. From the same date details of all boiler installations must be provided to the local authority building control (LABC).

There are times when it is too inconvenient or too expensive to fit a new Condensing Boiler into a property. In these situations exceptions can be granted and a standard efficiency boiler can fitted legally. The procedure to decide whether an exceptional circumstance has arisen is a points system based on the following:

  • The property type
  • The fuel
  • Whether the boiler needs to be moved
  • The need for an extended flue
  • The need for a condensate pump or soakaway

For more information visit Part L of the Building Regulations (Revised 2006).

Glossary  [top]

A ratedThis is synonymous with "Condensing" or "High Efficiency", although both these terms can refer to A and also B rated boilers. All boilers that are A rated are High Efficiency, or HE.
BBUBack Boiler Unit. A term used to describe a type of boiler that sits inside an existing fireplace. An artificial firefront is fitted to these boilers.
BFBalanced Flue. A flue containing a balanced baffle to control the air flow.
BTUThe British Thermal Unit. A way of measuring heat energy, defined as the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. 10,000 BTUs is equivalent to 2.92997kW.
CombiShort for 'Combination'. This is the name given to a type of boilers which heats the hot water on demand using a heat exchanger.
CORGIThe Council for Registered Gas Installers. The body responsible for keeping track of businesses and individuals registered to fit gas appliances.
FlueThe tubing used to carry the exhaust gases of combustion away from the boiler to the outside.
HEHigh Efficiency. This is the title given to boilers which have a SEDBUK Rating of A or B.
kWEqual to 1000 Watts. 1 Watt is defined as one Joule of energy released per second. 1kW is equivalent to 3413 BTUs.
LPGLiquid Petroleum Gas. An alternative to mains gas that requires a storage tank. LPG conversion kits can be obtained for many boilers, though currently it is a more expensive alternative to natural gas.
OFOpen Flue (or Chimney Flue). The air is drawn in from the room containing the boiler. The flue maybe driven by the air-flow or by a fan. These are most common in Back Boilers placed in an existing fireplace.
OFTECThe Oil Fired Technical Association. The organisation responsible for keeping track of professionals registered to fit oil fired appliances.
RSFRoom Sealed Flue. The flue is made of a double skinned pipe where fresh air from outside a property is drawn in through one section of the pipe and exhausted through the second.
SEDBUK RatingThe annual average efficiency rating of a piece of equipment on the SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) scale, ranging from A (highest) to G.