Plumbers, Boilers, Central Heating

020 8099 7576

Introduction to Boilers

According to the dictionary, a boiler is “an enclosed vessel in which water is heated and circulated, either as hot water or as steam, for heating or power”. Boilers are a mainstay of the UK’s heating infrastructure and according to estimates, there are approximately 1.5 million boilers installed in the UK every year. Heating Central is a key player in the boiler replacement market, with an extensive network of heating and plumbing engineers that specialise in boiler service and replacement.


Boilers have gone through a strong evolution over the last few decades improving their performance, their efficiency, their output and their physical design and looks. New boilers are substantially more efficient, offer better value and are simpler to install and service as part of an efficienct central heating system.

The most important advance has been the introduction of the condensing boiler, which in simple terms recover a substantial percentage of the waste heat that is normally expelled into the atmosphere from the flue of a standard (non-condensing) boiler. By using an extra-large heat exchanger (or two heat exchangers in other cases) within the boiler, the system maximises heat transfer from the burner while recovering useful heat that would normally be lost with the flue gases.

Another important system that is proving to be extremely popular is the Combination (Combi) boiler. Combi boilers supply hot water to a sealed heating system as well as a  instant hot water for domestic use (shower, kitchen etc.). Combi boilers are very easy to install, small and save valuable space and are very economic.

In this article we will introduce the main principles of boiler operations, explaining how to improve efficiency in their performance. We will introduce the various types of boilers available in the UK in more detail and the various type of fuels that power them.

Energy Efficiency

The energy efficiency of a boiler is one of the most important performance parameters of the unit. From 1st April 2005 it has been a requirement of the Building Regulations Part L1 (Conservation of Fuel and Power) that any replacement or new gas fired boiler be of the condensing type unless there are exceptional circumstances.

The older your boiler is the more inefficient it becomes. If it is more than 10-15 years old it is recommended to replace it with a new high efficient boiler. The new regulations mean that all new gas boilers installed in England and Wales must be high efficiency condensing boilers with either an "A" or "B" efficiency rating on the SEDBUK table (for details on SEDBUK click here).

During the burning of fossil fuels (oil, gas etc) several gases are produced and released into the air. These greenhouse gases include Sulphur Dioxide (SO2), Oxides of Nitrogen (NOx), and Carbon Dioxide (CO2). Carbon Dioxide is the largest emission in fossil fuel burning process. Each home produces an average of 5 tonnes of CO2 per annum, of which around 16% of the UK’s CO2 emissions comes from domestic heating systems.
According to estimates by The Heating and Hot Water Industry Council there are still around 4 million old, highly inefficient boilers in homes around the country, many of which will still be in operation in 2010. Replacing those with efficient boilers would save 2.5million tonnes of CO2 per annum.

Types of Boilers

Condensing Boiler
Over the last few years boiler manufacturers have been coming up with high efficiency condensing boiler which offer dramatically better output compared with traditional boilers. Traditional boilers typically have efficiency of around 70% (that’s the percentage of useable energy output compared to the energy input). A high efficiency condensing boiler will have efficiency rating of at least 86%. In reality an A rated boiler will have an efficiency of over 90%, with a B rated boiler will have at least 86% (and both rates will qualify for the ‘High Efficiency” band).

The dramatic increase in efficiency between the traditional and the high efficiency condensing boilers is achieved by further extraction of heat from the gasses that would previously been released through the flue into the outside air. Traditional boilers use a single combustion chamber, enclosed by waterways of the heat exchanger where the hot gases travel. The hot gases then push through the flue (normally at the top most point of the boiler) and go into the outer environment at a high temperature of approximately 180c.

This wasted heat is what makes condensing boiler so efficient by actually reclaiming it and converting it back into useable energy. When the heat rises up towards the primary heat exchanger, the condensing boiler then diverts the gases back into a secondary heat exchanger. This way the gases lose much more of their energy and pass it to the water for domestic use. The gases leave the boiler via the flue at a temperature of approximately 55c, significantly lower compared to a traditional boiler.

Such reduction in temperature causes the water inside the gases (which are standard outcome of the burning process) to condensate into small drops, and through gravity to roll down to the base of the flue. The rest of the gases are then expelled from the boiler into the outer environment through a fan assisted flue. The water condensate is drained into a discharge pipe or into an external drain.

Plastic and clay drainage are the most effective in dealing with the water which has an acidic properties and contain traces of nitric and sulphurous acids. Cast iron as well as cement and concrete products show the least resilience to this water and will get damaged over time. This problem can also affect older properties with salt glazed drain pipes with cement joints. However in reality the acidic water gets diluted heavily by water from sanitary uses thus reducing the risk of damage to the pipe and drain systems.

Combination Boiler (Combi Boiler)
Combi boilers are designed to heat water up instantly for domestic use (e.g. shower), as well as provide heat into the central heating system. Because there is no feed tank or storage tanks are required, the installation of Combi boilers is simple and cheap. Since there is no hot water storage cylinder, the Combi boiler eliminates the need to heat up the cylinder in order to keep it at a steady temperature for domestic hot water needs.

However, Combi boilers are only appropriate for relatively small houses or flats, where the hot water demand is not large. The boiler is limited to the amount of hot water it can supply, and in effect when hot water is being drawn for domestic use (e.g. shower, kitchen), there is no heating going into the radiators of the central heating system.

Here is a typical description of the operation of a Combi Boiler:

  1. Upon a signal from the room thermostat for the boiler to fire up the central heating system, the pump inside the boiler fires up. The water starts flowing passing through a narrow gap in the pipe (“venturi effect”) creating pressure differential and causing the gas valve to open.
  2. Gas is ignited in the main burner
  3. The heat exchanger holds a limited amount of water, which is rapidly heated up, circulating around the boiler through the heat exchanger. Any expansion of the water is absorbed by the sealed expansion vessel.
  4. When the water temperature reaches 600c the thermostatic element expands, the hot water system valve closes and the heating system valve opens, which allows water to flow to the central heating system.

A similar operations happens upon request for domestic hot water, but rather the valves operate in opposite directions, letting the water pass through to the domestic hot water circuit rather than the central heating system.

Types of Fuels

There are several types of fuels used in the UK for heating and boiler burning. Of the 20 million heating systems estimated to operate throughout the UK, the most common is powered by mains gas boilers, which accounts for about 75% of the market. Oil boilers accounts for about 10% and the remainder is split between solid fuels boilers, electricity and other sources.

Mains Gas

Mains gas has grown to become the most popular fuel for domestic boilers. At the moment, new gas mains line are laid every year to keep up with demand. Currently gas mains infrastructure is available to over 85% of UK homes.

Boiler running on mains gas are offering the cheapest and most convenient heating solution at present.

Natural gas, like all other fossil fuels emits CO2 into the atmosphere when it is burned. However, it has the lowest CO2 emission per kWh of energy when compared to all other fossil fuels used for domestic boilers. This makes natural gas not only the cheapest and most convenient fuel but also the cleanest one.
CO2 produced, per kilowatt hour of energy, against other fuels.
CO2 emission

Natural Gas   194 g/kWh
LPG              248 g/kWh
Oil               270 g/kWh
Solid Fuel     317 g/kWh
Electricity      511 g/kWh


Oil is used mainly for heating and domestic hot water heating purposes. Current estimates put number of UK homes that use oil at around 0.9 million.

There are two main types of oil: Kerosene (the most common) and gas oil (mostly used for industrial heating). Both of them are the result of a distillation process of crude oil, with Kerosene being the more refined, lighter and containing less sulphur (which produced the greenhouse gas sulphur dioxide upon burning).

Historically oil was the cheapest fuel, and still has a good network of supply around the UK, with a large number of suppliers covering the country. Supply can be based on requirement and ‘top-up’ is a typical way to keep ones supplies.
For further information about oil as a heating fuel visit Oftec (, the Oil Firing Technical Association, representing the oil heating and cooking industry throughout the UK and Republic of Ireland.

LPG (Liquid Petroleum Gas)

LPG is more beneficial than most fuels (except for natural gas) in terms of CO2 emissions. Although it offers many of the advantages of natural gas it suffers from higher cost which makes it less popular.
It is estimated that only around 3% of the heating systems use LPG to fuel their boilers. However, despite this low penetration level (which seems to primarily because of high cost), LPG is highly regarded due to its convenience and ease of control.  

Natural Gas   194 g/kWh
LPG              248 g/kWh
Oil               270 g/kWh
Solid Fuel     317 g/kWh
Electricity      511 g/kWh

As the table above shows, LPG is the second best fuel in terms of low CO2 emission, but is still lagging behind other fuels in terms of market penetration due to high prices.
Although it is possible to get LPG delivered in replaceable cylinders, most of the deliveries are directly into a permanent storage tank, which is the cheapest delivery option. However, there are strict conditions for locating the LPG storage container, which can complicate setting up the LPG delivery system.

The fuel is liquid when delivered, and it is stored in the same form. There is a 15% provision at the top of the tank to allow for the liquid to turn into gas, which is then drawn off. As the gas is drawn off, the pressure drops temporarily converting more liquid into gas and so forth.

Although LPG has many advantages working in its favour, there is not a large variety of LPG boilers to choose from. There are only about half a dozen large boiler manufacturers that are offering LPG boilers at the moment.

Solid Fuel

Historically, solid fuel (wood, coal) was considered to be one of the cheapest fuels available, and even today it still remains competitively priced. Current estimates put the number of UK heating systems using solid fuels boilers at around 4% of the 20 million systems used around the country.

Although it considered ‘dirty’ fuel, there still seems to be strong demand for ‘real fire’, with many seeking to return to more traditional values and are effectively re-opening their fireplaces.


Electric heating is used by about 1.4 million homes in the UK. It is one of the simplest and cheapest heating solutions to install. Yet electricity is the most expensive energy source, even when applying off-peak tariffs.

There are mixed views on the ‘clean’ credentials of electricity heating, as there is no burning of fuels ‘on location’, which grants it green points. However, one should take into account the electricity generation facilities and their respective carbon dioxide emissions during the generation process, the green credentials are less obvious

SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK)

SEDBUK provides the framework for comparison on a fair basis of the energy performance of the various boilers. It was developed under the Government's Energy Efficiency Best Practice Programme with the co-operation of boiler manufacturers

SEDBUK represents the average annual efficiency achieved in typical domestic conditions. The rating makes provisions for usage patterns, climate specific parameters, and several other influences.

The rating is based on standard laboratory tests, combined with other factors, such as the type of the boiler, the internal store size, the ignition arrangement, fuel types and more specific parameters of usage patterns.


This page has been written with contributions from qualified plumbers from Kent and heating engineers from Ealing. Several of the company's Gas Safe engineers from Middlesex have worked on similar issues as well as some of our plumbers from Bromley and our Notting Hill experienced heating engineers.

Article Archive
RSS Feed : RSS Feed