The boiler is the main component of a central heating system. Its main purpose is to transfer heat energy to water. The heat energy is produced by burning 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. 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. The most common type found in UK households is the combination or combi boiler. These provide hot water for both the central heating and the hot taps. Other types of boiler include heat-only boilers and system boilers. These boilers require a hot water cylinder. Boilers can be supplied with water from a temporary connection to 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. It is illegal for anyone who is not on the Gas Safe register to carry out work on a gas appliance.


Boiler history

The history of boilers in British homes goes hand in hand with the history of central heating. Early systems featured a back boiler, 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. Electric heaters, blankets and hot water bottles would be used everywhere else.

The present

Fifty years later, of course, and things are very 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. It is 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 couple of advantage. It is piped directly into the home and unlike some other fuel types, one cannot run out of it. However, connecting a household to the gas grid can be very expensive.


Oil is the second most common type of boiler fuel in the UK. It’s 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. However, this is more complicated and more expensive to install.

Oil-fuelled boilers are larger than the average boiler in the UK (the gas-fuelled combi). They are usually free-standing and roughly 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. This frees up space in the home and makes any sounds from the 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.

Heating oil

The main disadvantage surrounding oil-fuelled boilers is the cost. Oil boilers 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 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, oil buying clubs are common. Residents 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 is that they are extremely energy efficient. Since the combustion of a fuel is not required, absolutely no heat energy is lost 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. Their simplicity also 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. 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. It’s not so much a boiler type, but an engineering feature which makes the boiler more energy efficient. Condensing boilers are always more energy efficient than non-condensing boilers.

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 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 isn’t heated on demand. The hot water cylinder can be a standard open vented cylinder supplied from a storage cistern, or it can be an unvented cylinder fed 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 that hot water is available for multiple outlets. 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. Consequently, the 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 cost 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 implies, such boilers do nothing other than heat water. There are typically no additional central heating components fitted. The central heating pump is typically located in the airing cupboard. 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 best solution for homes in which 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. This is 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 system boilers and conventional boilers, the boiler heats water for what is known as the primary circuit. The primary circuit is essentially the central heating system. Hot water is pumped through the radiators, emitting its heat and warming the home.

As for the hot water for the taps, this is heated indirectly by the boiler. 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), never come into contact with each other. (In direct cylinders, there is no coil. The water is instead heated by two electric immersion heaters.)

A motorised valve, usually located on the pipework close to the hot water cylinder, controls whether 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. That’s because energy can’t 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, we can simply express this figure 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.


Another figure for boiler and radiator output is the BTU or British Thermal Unit. This measurement was ubiquitous in Britain and many other countries use it, too. It’s also used in other engineering industries, even to this day.

Boiler BTU

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. Namely, 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 heating engineer will need to calculate the heat output needed in every room in the home. There are numerous factors which affect how much energy is required for this. These include – but are not limited to:

  • the dimensions of the room
  • whether or not the windows have double-glazing
  • 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 is also relevant to the calculations. Bedrooms may be one or two degrees cooler than the living room. Bathrooms may be one or two degrees warmer.

If the necessary heat output isn’t calculated correctly and a boiler with insufficient power is selected, the home won’t be warm enough. Plus, it may not produce enough hot water to keep up with demand. On the other hand, a boiler with excessive power will waste energy. This will result in unnecessarily expensive heating bills and damage to the environment.

Common ranges

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. They 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 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 just one person. However, 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. These 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 boiler’s maximum flow rate . 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 depends on the type of boiler, the fuel type, and the capacity of the boiler. It also depends on the cost of any additional labour, and who is actually completing the job. Independent contractors tend to be a lot cheaper than the big energy firms.

Boilers themselves can range from £500 to up to £5,000, excluding 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 usually takes a day at most. 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 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. For example, a condensate pipe must be installed, and the hot water cylinder and loft cisterns must be removed. Such a project will take 2-3 days to complete.

Additional costs

Systems using heat-only boilers and system boilers will always be more expensive to install than those with combi boilers. That’s because they require a hot water cylinder and its associated fittings, such as thermostats and motorised valves. Loft cisterns may also be necessary.

Other costs to consider when replacing or installing a new boiler include pipework changes. For example, the gas supply pipe may need widening. Structural work may also be necessary, e.g. an old flue may need to be bricked up. A powerflush may also be necessary.


Over time, water in the central heating reacts with the metal components of the system. This causes 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. This will protect it from debris for as long as possible. In fact, some boiler manufacturers may actually require a powerflush to be 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 not getting 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. This assumes that no extra work is necessary. Once it’s been completed, a chemical inhibitor is put into the system. This protects it from internal corrosion.

Boiler energy efficiency

The last few years have seen several different scales for grading 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 A being the highest grade. An ‘A’ rated appliance would have an energy efficiency of at least 90%.
  • SEDBUK 2009 – This newer system does away with the A-G rating and simply expresses the figure as a percentage. This is in order to prevent confusion with European energy efficiency ratings.

ErP grading

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.

UK energy efficiency rules state that all boilers installed after April 2005 must be condensing boilers. Plus, as of April 2018 under the new Boiler Plus legislation, all new:

  • Gas boilers must have an energy efficiency of no less than 92%.
  • Gas and oil boilers must have electronic time and temperature controls.
  • Combi boilers must have at least one of several possible energy saving features.

Common boiler problems

It’s easy to take hot water and central heating for granted. However, during the winter months when boiler breakdowns are common, good heating engineers won’t be short of work. If your boiler needs servicing, you should book it in good time.

Low boiler pressure

On a combi boiler system, water for the radiators comes from a temporary connection to the mains. The system must have a water pressure of at least 1 bar, otherwise the boiler won’t work. Low combi boiler pressure is easy to verify by checking the gauge on the front of the boiler.

Low pressure does not necessarily mean a fault with the boiler itself. Bleeding the radiators may cause enough of a drop in pressure to create this problem. It is easily remedied by topping up the system via the filling loop. However, if the boiler keeps losing pressure, the most likely 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 internal parts. The usual suspects are the pressure release valve (PRV) or the expansion vessel. For more on this topic, see: Boiler keeps losing pressure: 7 possible reasons why

Frozen condensate pipe

A frozen condensate pipe is the classic 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 then flows into the sewer via the condensate pipe.

A 24 kW boiler produces around 1.5 – 2 litres of condensate for every hour of operation. If water constantly drips down the condensate pipe, it may be vulnerable to freezing in icy spells. Some boilers try to mitigate this problem by only releasing water in spurts. However, this won’t necessarily prevent it from freezing, especially in harsh winters.

If water in the condensate pipe freezes, pouring hot water on it or wrapping a hot water bottle around it should thaw it. To prevent this problem, the pipe should be insulated with water-proof insulation. An engineer can also widen it or redesign it so that water can’t settle inside.

The condensate pipe usually constitutes part of the flue. For this reason, only Gas Safe engineers should carry out work on it.


While most boilers aren’t silent, a faulty, noisy boiler may produce banging, gurgling or rumbling sounds. This problem is known as boiler kettling. The most common reason for it is a build up of 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. This is a small flame which burns constantly. It’s responsible for lighting the burner when the thermostat demands heat.

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


In the UK, only an engineer who is on the Gas Safe register may legally carry out work on a gas appliance. This is the Health & Safety Executive’s official registration body for gas engineers. It was previously known as the CORGI register.

The equivalent organisation for oil heating is OFTEC, or the Oil Firing Technical Association. Engineers who are registered with organisations such as Gas Safe or OFTEC can self-certify their work. This means that there is no need for 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. For example, a consumer who pays for a new Worcester-Bosch boiler can enjoy an extended warranty of anywhere from 5-10 years, as long as the engineer who installed it is certified by Worcester-Bosch. If the boiler develops a fault, the manufacturer will send one of its own engineers to fix the problem, free of charge. This strategy encourages brand loyalty among engineers, and the benefits for consumers speak for themselves. With an extended warranty, there’s no need to buy boiler breakdown insurance.

Manufacturer conditions

Manufacturers often have a number of conditions in order for them to honour a warranty. If a manufacturer believes the consumer or the engineer who installed the boiler hasn’t met those conditions, it won’t hesitate to wiggle out of the agreement. 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. Afterwards, 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, and its purpose is to reduce fuel poverty. It does this 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. For example, a heat-only boiler for 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. Instead, it may subsidise a boiler. The applicant will then have to cover the rest.