The boiler is the key component of a central heating system. Its main purpose is to transfer heat energy to water. The heat energy is produced by the combustion of a fuel, such as natural gas, oil, or a solid fuel such as coal or wood. Hot water is then distributed to the radiators in the household, which then heat the home. As well as for central heating, boilers are also used to heat the water for the taps – either directly on demand, or indirectly, usually by heating a body of water in a hot water cylinder.

There are several different types of boiler available. The most common type found in UK households is the combination or combi boiler. These are so called because they directly provide hot water for both the central heating and the hot taps. Other types of boiler include heat-only boilers and system boilers. Depending on the type, boilers can be supplied with water directly from the mains, or they may draw their water supply from a feed and expansion cistern in the loft. Despite the name, household boilers do not actually bring water to boiling point as part of their general operation.

Boiler history

The history of boilers in British homes goes hand in hand with the history of central heating. Early installations featured a back boiler, which consisted of a device fitted to the rear of an open fireplace. Cold water from the feed and expansion tank would enter the boiler at the bottom, before rising to the top of the circuit via gravity. This hot water would flow through the radiators and through a coil of pipe in the hot water cylinder, heating the hot water for the taps. Even so, only 30% of households in Britain had central heating. The main way of keeping warm was an open fireplace in the living room, and electric heaters, blankets and hot water bottles everywhere else.

Fifty years later, of course, and things are drastically different. As of 2017, 95% of British households now have a central heating system. Not only has the average household temperature risen by four degrees from 18 to 22, but it’s fair to say that boilers have radically changed how we live our lives. Richard Sober, a lecturer in interior design at the University of Teesside, explained to the BBC that families tend not to huddle around the fireplace any more. When it gets cold, we turn the heating up, instead of putting on more clothes and eventually going to bed when it’s cold. We don’t need to change our lifestyle or sleeping pattern. People can stay up all night – in any room, instead of congregating around the fireplace. Dr. Sober remarks that it can be argued that “…central heating played a part in the disintegration of the family.”

Boiler fuel types

Natural gas

Boilers are available in several different fuel types. By far the most common is natural gas – this accounts for the heating fuel in approximately 85% of homes in the UK. According to the Energy Networks Association, gas offers up to 90% energy efficiency, and the UK gas grid is almost entirely underground, protecting it from cold weather. Gas itself offers the homeowner a number of advantages – it is piped directly into the home and one cannot run out of it, unlike other fuel types. However, getting a household connected to the gas grid can be very expensive, and any work on a gas appliance can only legally be carried out by a Gas Safe-registered engineer.


Oil is the second most common type of boiler fuel in the UK, and is used in approximately 1.5 million of the 4 million British households which aren’t connected to the gas network. The oil is usually stored on the property itself. This generally requires a large unsightly plastic tank, which can take up a fair amount of room in the garden. It is possible for the tank to go underground, although this makes installing it more complicated and more expensive.

Oil-fuelled boilers are larger than the average boiler in the UK (the gas-fuelled combi). They are usually free-standing and are approximately the size of a large washing machine. In other words, they take up more space because they cannot be mounted on a wall. However, they do have their own advantages. Some oil-fuelled boilers can actually be installed outside, freeing up space in the home and making the sounds of a boiler much less of an annoyance. Another significant advantage is that there is no risk of a potentially deadly carbon monoxide leak inside the home. If the boiler is installed in a garage or outbuilding, then any ambient heat from the boiler may actually useful, helping to keep mould away.

The main disadvantage surrounding oil-fuelled boilers is the cost – they can be several hundred pounds more expensive than equivalent gas boilers. The annual cost of the oil may also be a few hundred pounds more expensive than the same amount of gas. Like most things, it is better to buy heating oil in bulk in order to obtain better prices. In small villages and parishes, it is quite common to find an oil buying club, where residents all pitch in together to get the best possible price per litre.

There are actually two different types of heating oil: 28sec and 35sec. The measurement of seconds is actually a measurement of viscosity or thickness, i.e., how many seconds it takes for ball bearing to sink through a column of the oil. 28sec is raw kerosene and is the most popular type of heating oil. It burns much more cleanly than 35sec and creates less smoke and soot. 28sec is also used in oil-fuelled cookers, such as Agas and Rayburns. It is easily possible to distinguish one oil from the other visually: 28sec oil is yellow, 35sec oil is red.


Electric boilers do exist, although they are few and far between. They should not be confused with storage heaters, like those typically found in flats. These are individually controlled and do not constitute ‘central’ heating.

However, some electric boilers do work on a storage principle and offer wet central heating. A classic example is the old range of GEC Nightstor boilers. These used cheap Economy 7 night electricity to heat up a core which consisted of dozens of feolite bricks. Air heated by these bricks would then be fanned onto a heat exchanger, heating the water for the radiators.

Electric boilers have a number of advantages – the main one being that they are extremely energy efficient. Since the combustion of a fuel is not required, absolutely no heat energy is expelled out of the building via a flue. Every penny of electricity is used as heat for the home. The absence of a flue also means that electric boilers are generally cheaper to install, and their simplicity means cheaper maintenance and servicing costs. However, the main disadvantage is that, as a fuel, electricity costs more than gas. Even if the boiler is more energy efficient, it may not necessarily be cheaper to run.

Types of boiler

There are several different types of boiler available. Getting the right boiler depends on several factors, including the size of the home and the source of fuel.

Condensing boiler

This term is frequently mentioned, and is more akin to a boiler attribute rather than a boiler type. A condensing boiler is a boiler with extra energy efficiency.

How do condensing boilers work?

One of the products of the combustion process is water vapour. Instead of simply expelling this hot vapour out of the flue and wasting the latent heat energy it carries, a condensing boiler draws so much heat out of it, that it cools down and turns into water. This water – condensate, as it is known – is collected in a trap inside the boiler before then being discharged into the sewer via the condensate pipe.

Due to environmental regulations, all boilers installed in the UK after April 2005 must be condensing boilers.

Combi boiler

As mentioned, the most popular type of boiler in the UK is the combination or combi boiler. These are so called because they combine both domestic hot water services and a central heating system in one unit. Combi boilers are supplied directly from the mains. Hot water for the taps is heated on demand, and water for the radiators is provided by a temporary connection to the mains. An internal expansion vessel accommodates the expansion of the central heating water when it is heated.

As a result, there is no need for a cold water storage cistern and a feed and expansion cistern in the loft – an ideal solution for a loft conversion, or in households where there is little to no loft space. There is also no need for a bulky hot water cylinder, exchanging an airing cupboard for extra living space.

However, the main disadvantage of combi boilers that they are unsuitable for larger properties, such as those with two bathrooms or indeed any household where there may be simultaneous demands for hot water at different outlets.

System boiler

Like combi boilers, system boilers integrate all of the central heating components – such as the pump and the expansion vessel – into one system, hence the name. They do, however require a separate hot water cylinder, as the hot water for the taps is not heated on demand. The hot water cylinder can be a standard open vented cylinder supplied from a cold water storage cistern, or it can be an unvented cylinder fed directly from the mains.

The main advantage of a system boiler is that it has the main advantage of a combi boiler while also compensating for its disadvantage. There is no need for tanks and pipework in the loft, and having a hot water cylinder means water means there is plenty of hot water immediately available, even if requested by multiple outlets at once. Of course, this does mean that extra space is required for the hot water cylinder.

Other disadvantages of a system boiler include the energy wastage from heating the water and then storing it – the hot water cylinder must be insulated to protect it from heat loss. A heating system with a system boiler will also be more expensive due to the extra expense of installing the cylinder.

Conventional boiler

Conventional boilers, also known as regular boilers or heat-only boilers, are typically found on older, family-size properties. As the name heat-only implies, such boilers do nothing other than heat water – there are no additional central heating components fitted. The central heating pump is typically located in the airing cupboard, and the expansion of the water is accommodated by a feed and expansion cistern in the loft. Hot water for the taps is stored in a hot water cylinder, which is fed by a cold water storage cistern.

Conventional boilers are the tried and tested solution for homes in which there may be a high demand for water which would outstrip the capacity of the mains alone. The presence of a feed and expansion cistern also means there is no need to top up the boiler – the system does this itself automatically. Plumbing systems with a conventional boiler are the most expensive and the most complicated, due to the cisterns, the extra pipework, and the hot water cylinder. However, they can be complimented with solar energy systems, helping to cut fuel bills.

How do boilers work?

While boilers may operate in slightly different ways, they all operate on the same principle with roughly the same key components. A fuel is burned in a combustion chamber. The heat from this process is transferred to water in a heat exchanger. However, what happens next depends on the type of boiler.

In a system boiler and a conventional boiler, the boiler heats water for what is known as the primary circuit. The primary circuit is essentially the central heating system – this hot water is pumped through the radiators, emitting its heat and warming the home.

As for the water for the taps in the hot water cylinder, this is heated indirectly by the boiler. As well as through the radiators, water in the primary circuit also flows through a coil of copper pipe inside the hot water cylinder. Heat is then transferred from the boiler to the water in the cylinder, ready for the taps. The two bodies of water, the primary circuit and the domestic hot water (DHW), should never come into contact with each other. (In some cylinders, there is no coil – the water is heated by two electric immersion heaters, one of them usually being on an Economy 7 tariff.)

A motorised valve, usually located on the pipework close to the hot water cylinder, controls whether or not water flows through the radiators or through the cylinder coil.

Boiler sizes

Boiler size or to be more accurate, boiler output, is generally measured in kW or kilowatts. Watts are a measurement of power because they are a measurement of how much energy is transferred per second – one watt equals one joule of energy per second. A kilowatt – one thousand watts – therefore equates to one thousand joules of energy per second. A 24 kW boiler therefore delivers 24,000 joules of energy every second. (Note that energy “used” is a bit of a misnomer – energy cannot be used, but simply converted from one form into another.)

Another important measurement is the kilowatt-hour or kW/h. This is not a reference to power, but refers to the total sum of energy delivered, expressed as kilowatt hours. There are 3600 seconds in one hour, and so per the above example, if the boiler delivers 24,000 joules of energy per second, then it delivers 86,400,000 joules of energy per hour. (3,600 x 24,000 = 86,400,000.) Instead of writing 86,400,000 joules, this figure can simply be expressed as 24 kW/h.

  • The same 24 kW boiler operating for 2 hours would therefore deliver 48 kW/h of energy or 172,800,000 joules.
  • The same 24 kW boiler operating for half an hour would therefore deliver 12 kW/h of energy or 43,200,000 joules.


Boiler and radiator output was previously – and sometimes still is – given in BTUs. This stands for British Thermal Unit. As implied by the name, this measurement was ubiquitous in Britain and is also used in many other countries. It is also used in other engineering industries, even to this day.

However, the use of BTU within the field of plumbing and heating is slightly different. This is because a BTU is a measurement of energy (like kW/h) rather than power – one BTU does not refer to the rate of energy consumption, i.e. one joule per second, but refers to the amount of heat energy needed to raise the temperature of one pound (0.454 kg) of water by one degree Fahrenheit (1055.06 joules.) Consequently, it cannot be converted directly into watts, and must instead be expressed as a quantity over time – BTU per hour or BTU/h. To clarify:

  • BTU/h and kW are measurements of power – the rate at which energy can be used.
  • BTU and kW/h are measurements of energy – the amount of energy available or expended.

1 BTU/h is equivalent to 0.000293 kW or 0.293 watts, and 1 kW is equivalent to 3,413 BTU/h.

  • In order to convert BTU/h to kW, multiply by 0.000293
  • In order to convert kW to BTU/h, multiply by 3412

These terms may sound confusing, but all they are is two different ways of quantifying the same thing: the rate at which heat can be produced (power), and how much of it can be produced over time (energy).

Heat input

This refers to the power required. For example, a 24 kW boiler may take 27 kW of heat input. What happens to that 3 kW for every 24 kW of heat produced? Energy can’t be destroyed or consumed, rather simply converted from one form into another, so where did it go? The answer is simple – it is lost through the flue, or from heating up the boiler casing and internal components, and so on.

By dividing the heat output by the heat input, we can calculate the energy efficiency of the boiler. 24 divided by 27 equals 0.88888888888. Thus, we can round this figure up to say that in the above example, the boiler has an energy efficiency of 89%.

What size boiler do I need?

In order to decide the appropriate size of boiler for a household, a competent heating engineer will need to calculate the appropriate heat output needed in every room in the home. There are numerous factors which affect how much energy is required to heat a room to the suitable temperature. These include – but are not limited to – the dimensions of the room, whether or not the windows are double glazed, whether or not the room faces north, and if the room is upstairs or on the ground floor. (A room on the first floor will benefit from heat rising from the room below it). The purpose of the room must also be taken into consideration – bedrooms may be one or two degrees cooler than the living room; bathrooms may be one or two degrees higher.

If the necessary heat output is not calculated correctly and a boiler with insufficient power is selected, the home won’t be warm enough and won’t have enough hot water to keep up with demand. A boiler with excessive power will waste energy, resulting in unnecessarily expensive heating bills and damage to the environment.

While the necessary boiler capacity must be calculated on a case-by-case basis, most properties fall into one of three ranges. A 24-27 kW boiler should be suitable for small to medium-sized properties such as flats and terraced houses with 1-2 bedrooms, one bathroom or up to 10 radiators. For medium to large properties with 3-4 bedrooms, one bathroom, or up to 15 radiators, a 28-34 kW boiler should be suitable. For large houses with 4+ bedrooms, two bathrooms and up to 20 radiators, a 35-42 kW will be necessary.

However, these ranges are a guideline rather than a rule, and assume average-sized rooms and therefore average-sized radiators. A 3 bedroom property with a one bathroom may still require 35 kW or 38 kW boiler if the room calculations deem it necessary.

Flow rates

As well as boiler power, the size of the property will also dictate the type of boiler required. Combi boilers should always be avoided on properties with more than one bathroom. This is because they can only provide a single stream of hot water, heated on demand from the mains. There is no reserve of hot water in a tank or cylinder. This may not be a problem in households occupied by a single person, but dividing a single mains supply between two outlets will result in two equally disappointing showers.

It is therefore important to take flow rates into consideration, which are measured in litres per minute. For example, a 24 kW boiler will typically deliver 9.5 – 10 litres of hot water per minute. While it may be possible to connect a shower to a combi boiler, the required flow rate for some mixer showers and digital showers may exceed the maximum flow rate of the boiler. In such cases, a hot water cylinder would be necessary – and consequently a system boiler or a heat-only boiler.

Arguably the most important flow rate to consider is the mains. Upgrading a 24 kW to a 34 kW boiler with a flow rate of 15 litres per minute still won’t fill the bath any quicker or be suitable for a certain type of mixer shower, if the mains flow rate is e.g. 9.5 litres per minute.

How much does a new boiler cost?

This not only depends on the type of boiler, the fuel type, and the capacity of boiler required, but also the cost of any additional labour required and who is actually completing the job. Independent contractors tend to be substantially cheaper than the big energy firms.

Boilers themselves can range from £500 to up to £5,000, without including installation costs. Generally speaking, the more powerful the boiler, the more expensive it will be. As a rough guideline, replacing a gas combi boiler in the same location as the previous one is likely to cost around £2,000, including parts and labour. This job can usually be completed within a day. Installing the boiler in a new location will probably cost about £500-£600 extra, and take twice as much time.

Replacing a regular or heat-only boiler in the same location will cost slightly less: around £1,750. Installing a the boiler in a new location will probably tip the balance over the £2,000 mark.

Replacing a gas heat-only boiler for a combi boiler will cost £2,500 – £3,000. The extra expense is due to the additional work required: the removal of the hot water cylinder and the cisterns in the loft, and the installation of a condensate pipe. Such a project will take 2-3 days to complete.

Where installing central heating, systems using heat-only boilers and system boilers will always be more expensive than those using combi boilers. This is due to the requirement of a hot water cylinder and its associated fittings, such as the thermostat and motorised valves. Loft cisterns may also be required.

Other costs to consider when replacing or installing a new boiler include pipework changes – the gas supply pipe may need widening – and structural work on a property, e.g. an old flue may need to be bricked up. A powerflush may also be required.


Over time, water in the central heating reacts with the metal components of the system, causing the formation of rust and other oxides. The most common oxide is magnetite, and it tends to form a black sludge-like substance which can solidify. These unwanted substances will not only reduce the efficiency of the central heating, but can cause serious damage to the boiler and other parts of the system if left unchecked.

That’s where powerflushing comes in. Powerflushing works by pumping water through the central heating system, dislodging the sludge and debris. The sludge is then collected in the powerflush unit before being disposed of.

A powerflush is often advisable when installing and commissioning a new boiler in order to protect the appliance from debris for as long as possible. In fact, some boiler manufacturers may actually require that a powerflush is carried out, in order for any guarantees or extended warranties to be valid.

Signs that the central heating may benefit from powerflushing include:

  • Grey, metallic-coloured water when bleeding the radiators
  • Radiators do not get warm at the bottom
  • Unusual sounds from the boiler or the pump
  • The central heating is slow to warm up in general

A powerflush generally costs between £250 and £500, and takes a full day to complete, assuming no extra unanticipated work is required during the process. Once it has been completed, a chemical inhibitor is put in the central heating circuit to prevent rust and oxides from forming.

Boiler energy efficiency

Calculating the energy efficiency of a boiler in terms of heat input vs heat output has already been discussed in the above section. The last few years have seen several different scales used to grade boiler energy efficiency.

The main system used in Britain is the SEDBUK rating – this stands for Seasonal Efficiency of Domestic Boilers in the UK. There are actually two different classes of SEDBUK rating.

  • SEDBUK 2005 – This system used the letters A-G to rate boiler efficiency, with G being the worst performing, and A being the highest. An ‘A’ rated appliance would have an energy efficiency of at least 90%.
  • SEDBUK 2009 – In order to prevent confusion with European energy efficiency ratings, this newer system does away with the A-G rating and simply expresses the figure as a percentage.

In September 2015, the European Union introduced the ErP (Energy-related products) rating. This system uses a letter grading system from A++ to G. Out of these three systems, the SEDBUK 2009 scale therefore gives the most precise figure as to a boiler’s efficiency.

As well as the requirement that all boilers installed after April 2005 must be condensing boilers, as of April 2018 under the new Boiler Plus legislation, all gas boilers must have an energy efficiency of no less than 92%. All gas and oil boilers must have electronic time and temperature controls, and all combi boilers must have at least one of a range of different energy saving features.

Common boiler problems

It is easy to take hot water and central heating for granted, but their absences are sorely felt. Needless to say, boiler breakdowns and call outs for emergency plumbers are extremely common during the winter months.

Low boiler pressure

On a system with a combi boiler, the system is supplied with water via a temporary connection to the mains. The system must have a water pressure of at least 1 bar, otherwise the boiler will not operate at all. Low combi boiler pressure is easy to verify by checking the gauge on the front of the boiler.

Low pressure does not necessarily indicate a fault with the boiler itself. Bleeding the radiators may cause enough of a drop in pressure to create this problem, and it is easily remedied by topping up the system via the filling loop. However, if this does not solve the issue, and the boiler must be frequently topped up, then the only other possible explanation is a leak. The leak may be on a central heating fitting (radiator, pipe, joint), or it may be one of the boiler’s components. The usual suspects are a faulty pressure release valve (PRV) or a broken expansion vessel. Click here to read more about this fault.

Frozen condensate pipe

A frozen condensate pipe is the quintessential winter boiler breakdown. As previously mentioned, condensing boilers draw so much heat out from steam in the flue gases, it condenses and turns into water. This slightly acidic water is then expelled into the sewer via the condensate pipe. The condensate pipe is made out of plastic and often runs outside the house towards the drain, much like the kitchen sink drainpipe.

A 24 kW boiler produces around 1.5 – 2 litres of condensate for every hour of operation. The constant dripping of water down this pipe is vulnerable to freezing in a cold snap and blocking the pipe, especially if it is exposed to an icy wind. Some boilers try to mitigate this problem by only releasing water in spurts, however this is not guaranteed to prevent it.

If the condensate pipe is frozen, pouring hot water on it, or wrapping a hot water bottle on it will shift the blockage and get the boiler up and running again. To prevent this problem from happening, the pipe should be insulated with water-proof insulation, and can be widened or re-fitted so that the water cannot settle inside.


While boilers do not operate in silence, a faulty, noisy boiler may produce banging, gurgling or rumbling sounds. This problem is known as “kettling”, and it may be due to a number of reasons – a frozen condensate pipe can be one of them. However, the most common reason is a build of up limescale within the heat exchanger. Mechanical causes are also possible, causing the boiler to overheat.

Pilot light keeps going out

Older boilers feature a pilot light, a small flame which must be constantly lit in order for the boiler to operate normally. When heat from the boiler is demanded, the pilot light is responsible for igniting the burner.

The most common reason for a pilot light which won’t stay lit is a faulty thermocouple. If the pilot light is extinguished, this component turns off the boiler’s internal gas supply as a safety precaution.


In the UK, any work on a gas boiler or indeed any gas appliance may only legally be carried out by an engineer who is registered with Gas Safe. Previously known as CORGI registration, this is the Health & Safety Executive’s official registration body for gas engineers.

The equivalent organisation for oil heating is OFTEC, or the Oil Firing Technical Association. Unlike Gas Safe, it is not a legal requirement to be registered with OFTEC in order to work on oil-fuelled appliances. Registering with an organisation such as Gas Safe or OFTEC as a ‘competent individual’ means that the engineer can self-certify their work, and does not need to arrange an expensive Building Control Notice and inspection from the local authority to certify that the work is safe and satisfies Building Regulations. For peace of mind, you should always use a Gas Safe or OFTEC-registered engineer.


Most boilers come with a 1-2 year guarantee as standard. Many boiler manufacturers, however, offer an engineer accreditation scheme. As an example, this means that a consumer who pays for a new Worcester-Bosch can enjoy an extended warranty of anywhere from 5-10 years, as long as the plumber is certified by Worcester-Bosch. If the boiler develops a fault, then the manufacturer will send one of its own engineers to fix the problem free of charge. For boiler manufacturers, this strategy encourages brand loyalty among engineers, and the benefits for consumers speak for themselves. As an added bonus for consumers, these extended warranties make boiler breakdown insurance – typically offered by energy firms – a completely unnecessary expense.

However, there are a number of conditions which must be met in order for the manufacturer to honour the warranty, and the manufacturer will not hesitate to wiggle out of the agreement if it believes that its terms and conditions have not been met by either the consumer or the engineer who installed the boiler. Common requirements include:

  • The boiler must be installed in accordance with the manufacturer’s instructions.
  • The engineer must register the boiler with the manufacturer within 30 days of installation. Once this is done, the homeowner or landlord will usually receive a letter confirming this.
  • The system must be powerflushed when the boiler is installed and commissioned.
  • The homeowner or landlord must have the boiler serviced annually by one of the manufacturer’s accredited installers.

Boiler grants

The UK government offers a boiler grant scheme under the Energy Company Obligation’s ‘Affordable Warmth scheme. The ECO was introduced in January 2013. Its purpose is to reduce fuel poverty by helping people on lower incomes to heat their home efficiently, and to reduce the country’s carbon emissions. The ECO also offers grants for wall cavity and loft insulation.

Only homeowners and landlords may apply for the scheme; private tenants must ask their landlord to apply on their behalf. The scheme is not open to everyone – only those receiving certain state benefits are eligible. If the applicant does meet the eligibility criteria, then the boiler will be replaced like-for-like, e.g. a heat-only boiler with a heat-only boiler, a combi for a combi, and so on. Upgrading to an A-rated, modern condensing combi boiler from e.g. a G-rated, non-condensing appliance will typically cut fuel bills by £200 – £300 per year.

Due to government cuts, the ECO may not be able to cover 100% of the costs, and may instead subsidise a boiler with the applicant expected to contribute the rest. The current ECO scheme will come to an end on 30 September 2018, and a new one will subsequently introduced under the same name.

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