Boiler Types Explained: Combi, Heat-Only, System, and Back Boiler Functionality, Prices, and More

Boiler types provide a classification system for residential and commercial boilers based on their functions and interaction with the central heating and domestic hot water systems. Understanding boiler types helps you determine which boiler models are most suitable for your domestic hot water and central heating systems and your budget.

Below are the four boiler types that are commonly used in the UK’s residential and commercial heating and hot water applications.

• Combi boiler: A combi boiler (short for “combination boiler”) supplies both heat and domestic hot water on demand, without requiring hot or cold water storage tanks. Combis have a small footprint and are highly energy efficient because they do not burn fuel to maintain high temperatures inside a hot water cylinder. That said, combi boilers rely on main water pressure and are thus unable to supply large quantities of water to multiple taps simultaneously. Combis are ideally suited for smaller homes with low-to-moderate hot water demand. A new combi boiler costs £1,700 – £2,200 without installation. A storage combi functions much like a standard combi boiler, but comes with an integral hot water reservoir. A storage combi is able to supply larger volumes of hot water thanks to its integral hot water cylinder. Storage combis cost £1,700 – £4,200 before installation.
• System boiler: A system boiler heats water for the home’s radiators. This water then cycles through a coil inside the domestic water cylinder, thus indirectly heating the tap water inside the cylinder. System boilers are able to satisfy a higher demand for hot water than combis and are thus more suitable for large families. The total cost of a new system boiler ranges between £800 and £1,500.
• Heat-only boiler: A heat-only boiler (also known as a “regular” and “conventional” boiler) functions much like a system boiler, by keeping a supply of tap water hot inside an insulated cylinder. However, a regular boiler gets its cold water supply from two cold water storage tanks, which are generally located in the home’s loft. One of the cold water tanks supplies the central heating system, whilst the other provides tap water. The tanks get refilled from the mains when needed. Heat-only boilers cost £1,500 – £3,000 (boiler-only).
• Back boiler: A back boiler is a small boiler fitted behind a fireplace hearth. Back boilers have become obsolete since the 2005 Building Regulations came into force, but many older UK homes still use these boilers for heating and hot water.

Figuring out which boiler type you currently have entails establishing whether your home has cold water storage tanks and a hot water cylinder, as the different boiler types come with different combinations of these components.

All boiler types (apart from back boilers, which are inefficient and possibly dangerous) have their pros and cons, and each type is suitable under different circumstances. Deciding which boiler type is best for your home comes down to your existing heating setup, domestic hot water consumption levels, and budget.

Combi boilers are the most popular in the UK, largely thanks to their small footprint, energy efficiency, and relatively low costs. Durability is an important factor in boiler selection, but the type of a boiler has little impact on its lifespan. The longest-serving boilers are manufactured with quality components and typically come with long warranty terms.

The article below thoroughly analyses the UK’s common boiler types, explaining and comparing their function, suitability, and prices.

What is a combi boiler?

A combi boiler is a type of boiler that operates by heating tap water directly from the cold mains whilst separately keeping the central heating water hot. Combis get their full name (“combination boiler”) from their ability to supply both the domestic and heating water on their own, without requiring a hot water cylinder. The lack of a hot water cylinder means that combis have a smaller footprint and a higher level of energy efficiency than their system and heat-only boiler counterparts. Combi boilers are the most common type of boiler in the UK because of their small size, fuel efficiency, and comparatively low costs. Combi boilers are well-suited to small homes and apartments with a limited number of occupants who will not place undue demands on domestic hot water.

How does a combi boiler work?

A combi boiler works by supplying both the home’s heat and domestic hot water on demand. A typical combi boiler relies on two heat exchangers (called “primary” and “secondary”) to meet a home’s heating and hot water needs. The heat exchangers are made with a highly conductive metal, like aluminium or stainless steel, that heats up when your combi boiler burns fuel. A combi boiler’s primary heat exchanger heats water for the radiators and towel rails. This central heating water flows through the primary heat exchangers in a closed circuit as the heat exchanger maintains its temperatures. A combi boiler’s secondary heat exchanger heats the domestic hot water that you get from your taps and showers.

Is a combi the best boiler type for my property?

A combi may be the best boiler type for your property if it’s got a small floor area and you don’t use large volumes of hot water. Below is an overview of how property size and hot water consumption influence help determine whether a combi is a suitable type of boiler for your home.

• Property size: Combi boilers are the ideal type of boiler for smaller properties because they do not need external hot and cold water tanks to operate the way system and heat-only boilers do. These tanks take up additional floor space, so combis have a smaller footprint than other boiler types.
• Hot water demand: Combi boilers are perfect for smaller families with low demand for hot tap water, as they’re able to heat mains water as needed without burning fuel to keep an entire water storage cylinder hot. On the other hand, combi boilers are limited to mains pressure, so they struggle to keep up with domestic hot water demand when multiple occupants use taps and showers at the same time.

What are the advantages of combi boilers?

The four advantages of combi boilers are listed below.

• Instant hot water: Combi boilers heat water when you need it, so you never have to wait for hot water because your boiler “ran out.” In contrast, depleting a domestic hot water cylinder is common in a conventional or system boiler setup, and it often takes up to 30 minutes for the cylinder to regain the desired temperature of over 60°C.
• Energy efficiency: Combi boilers consume only as much energy as is needed to heat the water you’re using at any given moment. A combi does not need to keep a tank of water hot at all times, so it consumes less energy than system and heat-only boilers, which burn gas to maintain the hot water cylinder at or above 60°C.
• Small footprint: Combi boilers are much smaller than system or regular boilers because they have no storage tanks. Combi boilers’ small footprint is perfectly suited for small apartments for this reason.
• Low cost: Combi boilers are cheaper than system or regular boilers. Combis are more affordable partly because you don’t need to purchase a separate hot water cylinder to work with your boiler.

What are the disadvantages of combi boilers?

The two disadvantages of combi boilers are listed below.

• Immersion heaters not possible: A combi boiler is unable to accommodate an immersion heater, so there’s no backup way of heating tap water if the combi breaks down.
• Reliance on mains pressure: Combi boilers are only able to heat as much tap water as the mains pressure allows, so they struggle to meet hot water demand when several taps or showers run at the same time. In contrast, system and regular boilers are able to supply higher volumes of hot water because they’re connected to a heated water storage cylinder.

How much does a combi boiler cost?

A combi boiler costs £1,700 – £2,200, excluding installation. Combi boiler prices are influenced by the following three factors.

• The combi manufacturer: A combi boiler’s manufacturer plays a big role in its sticker price. Premium brands like Viessmann and Worcester charge more for their reputation and for the peace of mind you get from owning one of their combis.
• The combi boiler’s features: Some combi boilers have additional features, like WiFi connectivity or weather compensation that add a lot to its price.
• The combi boiler’s capacity: Combi boiler prices generally increase with their heating capacity. The price difference may be marginal between a 24kW and a 30kW model, but combis with an output of 35kW and higher tend to be notably more expensive than their lower-capacity peers. The normal capacity range for combination boilers is roughly 24kW-45kW, and the cheaper, low-output models are not well-suited for large households with high hot water consumption.

The aforementioned cost factors sometimes affect the fitting costs, as some brands’ combis cost more to fit than others, whilst larger, heavier combi boilers generally require more time and effort to install.

What are the combi boiler prices fitted?

Combi boiler prices fitted vary depending on your installer, but installation typically adds between £800 and £1,500 to the overall boiler cost. National heating companies generally charge on the higher end of the aforementioned price range. In contrast, local heating trades and online companies offer more reasonable combi boiler installation prices.  Online boiler fitters such as Heatable offer some of the lowest rates on combi boiler installations because of their low overhead and lean online business model.

Which is the best combi boiler?

What the best combi boiler is depends on the fuel you use to power your boiler. Below are the best combi selections for gas, electric, oil, and LPG-fired heating and hot water systems.

• Best gas combi boiler: The best gas combi boiler is the Worcester Greenstar 4000. The Greenstar 4000 is the best combi you’re able to get in the UK because it’s reliable, fuel-efficient, compact, and comes at a reasonable price compared to equivalents from Worcester themselves or other industry-leading brands.
• Best electric combi boiler: The best electric combi boiler is the Comet Combi Boiler from the Electric Boiler Company. The Comet Combi is 100% energy efficient and comes with components that improve its durability, such as the stainless steel heat exchanger and magnetic sludge filter.
• Best oil combi boilers: The best oil combi boilers come from Warmflow, Grant UK, and Worcester Bosch. The three boiler brands offer a diverse array of efficient and reliable oil combis for various property sizes and hot water consumption levels.
• Best LPG combi boiler: The best LPG combi boiler is the Worcester Greenstar 4000. The LPG version of the Greenstar 4000 has all the same features, benefits, and performance metrics as its gas-fired counterpart.

What is a storage combi boiler?

A storage combi boiler is a type of combination boiler that comes with an internal hot water storage cylinder. A storage combi boiler is able to produce higher volumes of hot water on demand than a standard combi thanks to its integral water reservoir. At the same time, storage combis take up less space than conventional or system boilers. Storage combi boilers are far less common than regular combis but are well-suited for small properties with higher hot water demand.

How does a storage combi boiler work?

A combi storage boiler works by heating water from the mains and keeping a reserve of its internal hot water cylinder. A storage combi heats water from the cold mains by burning fuel (like gas or oil) and transferring the heat from that flame to the cold water via a heat exchanger. It then deploys hot water to your taps when you turn the hot water on or to your central heating system when your home’s thermostat registers a temperature lower than its set point. Excess hot water is stored in the boiler’s internal storage tank and supplied when your demand for heated water exceeds the boiler’s on-demand heating capacity.

Do I have a storage combi boiler?

Check your boiler and heating system setup to determine whether you have a storage combi or another boiler type. Below are the five indicators that your combi boiler has an integral storage tank.

• Large boiler dimensions: Storage combis are larger than standard combis, system, and heat-only boilers because their housing contains the water storage cylinder. For example, the Viessmann Vitodens 111-W storage combination boiler is 15-20cm larger in height, width, and length than a standard Vitodens combi.
• Lack of an external pump: Storage combi boilers have internal circulation pumps. You likely have a system or conventional boiler if you see an external central heating pump.
• Lack of a separate hot water cylinder: Storage combi boilers do not use separate hot water cylinders, since they’ve got an integral one inside the boiler housing. Hot water cylinders are large tanks of water typically located in an airing cupboard. You have a conventional or system boiler if you can find one.
• No cold water storage tank: Storage combi boilers do not use cold water storage tanks because they get their water supply straight from the mains. Cold water storage tanks are usually located in the attic and indicate that you have a heat-only boiler.
• Five or more pipes coming out of your boiler: Storage combis typically have seven pipes emerging from the unit. These include the gas supply, mains cold water supply, hot water outlet, pressure relief, condensate, and central heating flow pipe and return pipes.

Should I get a combi boiler with a storage tank?

Whether you should get a combi boiler with a storage tank depends on how much hot water you consume, how much floor space you have to spare, and what your budget is like for the new boiler. Below is an overview of the aforementioned criteria for deciding whether or not to get a storage combi.

• Hot water consumption: Storage combis are able to satisfy a higher demand for hot water than combis that lack an internal storage tank. Combis with an integral hot water cylinder are thus more appropriate for busy families that use lots of hot water.
• Property size: Storage combi boilers do not require external tanks for cold and hot water, and are thus able to conserve floor space. Storage combis’ small footprint means that they’re a suitable alternative to system and heat-only boilers for thirsty families with limited floor space.
• You are budget-conscious: A storage boiler is cheaper than a system or regular boiler because it does not require additional infrastructure to keep a supply of water. Going with a storage combi is a budget-friendly solution if you need a higher hot water supply than a standard combi is able to provide, but do not want to spend money on cold and hot water tanks.

Are storage combis any good?

Storage combis are good as long as they’re a suitable match for your property, as discussed in the section above. Smaller properties with high hot water consumption stand to benefit from storage combis for four main reasons. Firstly, a storage combi boiler is generally less expensive than a system boiler equivalent despite being able to supply larger quantities of hot water. The moderate purchase price makes it an appropriate choice for budget-conscious homeowners. Secondly, a storage combi offers a reserve of hot water to accommodate high hot water demands. Thirdly, storage combi boilers are able to heat water straight from the mains. This capacity makes it impossible for a storage combi to “run out” of hot water as can happen with system and conventional boilers. Fourthly, combi boilers with a storage tank are small-flat friendly, as they do not need water tanks and thus take less space than heat-only or system boilers.

Conversely, combis with a storage tank have three disadvantages that make them unsuitable for certain homes. Firstly, storage combis are larger than standard combis and don’t fit inside or between cupboards. You need to find a dedicated area inside the home where you’re able to install this type of boiler while maintaining minimum clearances on all sides. Some of the smallest homes may not have enough space to accommodate a storage combi. Secondly, storage combi boilers are unable to supply the high hot water volumes made possible with system and heat-only boilers. Most storage combis’ internal tanks have a 50-100 litre capacity, whilst system and conventional boilers work with external hot water cylinders with volumes that range between 250 and 1,000 litres. Finally, storage combis cost £500-£1,000 more than standard combination boilers, which is a significant price difference for most people.

What is the combi storage boiler cost?

The typical combi storage boiler cost is £1,700 – £4,200, excluding installation. The following three factors determine the price of a storage combi.

• The boiler manufacturer: High-end manufacturers with household name recognition, like Viessmann or Worcester Bosch, charge more for their storage combis. However, their boilers are well-known for high durability and energy efficiency, which offset the maintenance and running costs, respectively.
• The boiler features: Some storage combis have features such as smart speaker integration and intelligent filling. These functions streamline your control over the boiler but come at a premium.
• The boiler capacity: Higher output storage combis boilers are able to support more radiators, water fixtures, and occupants. However, higher heating and hot water capacities often (not always) translate into higher boiler purchase costs.

A great option for an affordable installation of a new storage combi boiler is Heatable. Heatable is an online boiler installer that matches you with the perfect boiler for your property in just 90 seconds. Heatable keep costs low by not operating a physical storefront and pass those savings on to you in the form of the best possible price on a storage combi boiler installation. Check out Heatable’s online boiler quotation tool here.

What is a system boiler?

A system boiler is a type of boiler that works in conjunction with a hot water cylinder to produce high volumes of heated water for taps and showers whilst powering the home’s radiators. System boilers are among the most common boiler types in the UK, following combis.

How does a system boiler work?

A system boiler works by directly heating water for the home’s radiators and indirectly heating a reserve of domestic hot water inside a separate, insulated cylinder. The boiler brings the central heating water to the desired flow temperature, and this water passes through a coil inside the domestic hot water cylinder, thus transferring the heat to the potable water stored there. The heating and domestic hot water systems stay entirely separate, so you never drink the water from your central heating. A system boiler does not use a feeder cold water tank to maintain water pressure, as all its water comes straight from the mains.

How can I tell if I have a system boiler?

You can tell if you have a system boiler if your central heating setup has the following four characteristics.

• A hot water cylinder: System boilers work in concert with a hot water cylinder to store domestic hot water, which is generally located in an airing cupboard. Having a hot water cylinder means you either have a system boiler or a heat-only boiler.
• No cold water tank: System boilers do not require cold water tanks, unlike heat-only boilers. There is no water tank in your loft if you have a system boiler.
• Three copper pipes: System boilers typically have three copper pipes coming from the boiler’s base. You have a combi boiler if you have seven such pipes and a heat-only boiler if you have fewer than four.
• No external pump: System boilers do not use an external pump, whilst heat-only boilers do.

Is a system boiler appropriate for my home?

A system boiler is appropriate for your home if you meet the following four criteria.

• Flexible budget: System boilers are generally more expensive than combi boilers. You may not wish to buy a system boiler if you’re operating on a very tight budget. System boilers are a bit less energy efficient than combi boilers, so a combi boiler may be more suitable if you need to cut down on your gas bill.
• Sufficient storage space: System boilers take up more space than combi boilers because they operate a hot water cylinder but less space than a heat-only boiler because they do not need a separate cold water tank. This size profile makes system boilers a fitting choice if you have enough floor area for a boiler and a hot water cylinder, but a poor choice if you live in a small apartment.
• High central heating needs: Larger properties come with 20 or more radiators and thus need a boiler with a sufficiently high heating capacity. System boilers are often an appropriate fit for larger properties with many radiators, but you must still select a system boiler of the right size to ensure it produces enough heat and works efficiently.
• High hot water needs: System boilers are a particularly suitable choice if your household uses a lot of domestic hot water. System boilers store domestic water in a cylinder ready for use, and can thus handle the load imposed by multiple occupants using taps or showers simultaneously. In contrast, combis are unable to cope with an elevated hot water demand because they’re only able to supply as much hot water as the mains pressure allows.

Are system boilers any good?

Yes, system boilers are good for three reasons. Firstly, system boilers produce a larger supply of hot water than combis because they store pre-heated domestic water in a cylinder. This extra water capacity means system boilers are better able to meet the higher hot water demands of multiple people running taps and showers at once. Secondly, system boilers take up less floor area than heat-only boilers. System boilers have a smaller profile than regular/heat-only boilers because they do not need cold water storage tanks to operate. This smaller footprint gives system boilers the edge if you live in a smaller home but your hot water usage is too high for a combi to handle. Thirdly, system boilers integrate with dual-coil cylinders, which are compatible with photovoltaic (solar) panels. Dual-coil cylinders have secondary coils that may be connected to a second heat source, such as a solar energy system.

However, system boilers have three key drawbacks. Firstly, system boilers take up more space than their combi counterparts because they require a hot water cylinder to operate, and the cylinder requires more room. This extra space requirement means that system boilers are not suitable for small apartments. Secondly, system boilers run out of water if your household exhausts the cylinder’s reserves, at which point you must wait for the cylinder to heat up the water again. Running out of hot water is not a problem if you have a combi boiler, since a combi boiler heats water as it flows from the mains (and can do so infinitely in theory). Thirdly, system boilers are less efficient than combi boilers because they consume fuel constantly to keep the tap water inside the cylinder hot, even if the central heating system is not functioning. In contrast, combis operate both the central heating and domestic hot water systems separately, supplying heat for each function as required.

What are the system boiler costs?

System boiler costs range between £800 and £1,200, excluding installation. Getting a system boiler entails purchasing and fitting a hot water cylinder if you haven’t got one already. The cylinder adds another £400 – £1,600 to the total costs, whilst installing the boiler and the cylinder costs around £1,000 on average. Overall, you’re looking at paying between £2,200 for a budget system boiler and cylinder, and £3,800 to fit the best system boiler model from a top-tier manufacturer.

What is the best system boiler?

Worcester Bosch Greenstar 4000 is the best system boiler in the UK in 2024 for three reasons. Firstly, the Greenstar 4000 offers advanced features, such as intelligent filling to make repressurisation as layperson-friendly as possible, and wireless integration for easy connections to smart thermostats and smartphone control apps. Secondly, the Greenstar 4000 boasts an energy efficiency of 94%, which means that the boiler converts 94p into heat for every pound you spend on your gas bill. Finally, the Greenstar 4000 system boiler comes at a moderate price of around £2,400 (fitted), which is low compared to equivalent boiler models from other top-shelf manufacturers.

Get a fixed-price quote on a Greenstar 4000 system boiler using Heatable’s online quotation tool.

What is a heat-only boiler?

A heat-only boiler (also known as a “regular” and “conventional” boiler) is a boiler type that works with hot and cold water storage tanks to supply heated water to the home’s taps, showers, and radiators. The heat-only boiler setup, comprising a hot water cylinder in the airing cupboard and the two cold water tanks in the loft, was the traditional setup used in most UK homes for the greater part of the 20th century. Conventional boilers are steadily ceding popularity to the more compact and energy-efficient combi and system boilers nowadays, although they’re still widely available on the market and are suitable for large properties with robust hot water demand.

How does a heat-only boiler work?

A heat-only boiler works in concert with a multi-component heating setup, which comprises the following seven essential elements.

• Boiler
• Domestic water storage tank
• Feed and expansion tank (F&E tank)
• Hot water cylinder
• Radiators and heating pipework
• Taps and showers
• Mains supply

Below is an overview of the process via which the seven aforementioned components produce heat and hot water in a conventional boiler setup.

1. Cold water enters the domestic cold water storage and F&E tank.
2. The cold water destined for the home’s taps and showers travels to the hot water cylinder and keeps this cylinder full.
3. The cold water from the F&E tank travels to the boiler, where the heat exchanger heats it up to the required flow temperature.
4. The water leaves the boiler’s heat exchanger and flows to the home’s heating pipework and radiators. On route to the heating pipework, this heated water passes through a coil inside the hot water cylinder, thus transferring heat to the potable water stored there. The central heating and potable tap water never come in contact with each other inside the cylinder.
5. The hot water cylinder supplies the indirectly heated domestic water whenever there’s a call for it at any of the home’s water outlets.
6. The central heating water continues to circulate throughout the home’s network of pipes, radiators, and towel rails.
7. The F&E tank allows the central heating water to rise back up if the boiler pressure gets too high.
8. The cycle continues as described in steps 1-7 unless the boiler gets shut off.

How do I know if I have a conventional boiler?

An easy way to know if you have a conventional boiler is to check the space at the highest point of your home, typically the loft. The presence of working water storage tanks in the loft indicates that you have a heat-only boiler. Further proof of your boiler being conventional is the presence of a hot water cylinder inside your airing cupboard.

However, having a hot water cylinder but no cold water tanks in the loft means you have a system boiler. The lack of any discernable water storage facilities in your home means you’ve got a combination boiler.

Is a heat-only boiler suitable for my home?

A heat-only boiler is suitable for your home if you have a large property with several occupants and have the space to accommodate a hot water cylinder, cold water tank, and a feed and expansion tank. A heat-only boiler is most suitable when your home already has the conventional setup with all the cold and hot water storage facilities and associated pipework. Getting a system boiler may be the wiser choice if you don’t already have the cold water and F&E tanks up in your loft. A system boiler gives you access to ample amounts of hot water, and you don’t end up wasting loft space on the two cumbersome water tanks that a heat-only boiler needs to operate.

That said, heat-only boilers are tried-and-tested, formidable heating machines that offer plenty of advantages over other types of boilers. Consider the pros and cons of a heat-only boiler carefully before deciding whether it’s the most suitable boiler type to heat your home.

What are the pros of heat-only boilers?

Below are the three pros of heat-only boilers.

• Ability to handle high demand: Conventional boilers are able to meet high hot water demand, making them well-suited to high-occupant properties.
• Integration with solar thermal systems: Heat-only boilers are compatible with solar thermal systems, which may be used to heat the domestic water in the cylinder.
• Compatibility with immersion heaters: The hot water cylinders that conventional boilers heat are compatible with electric immersion heaters. An immersion heater works as a backup hot water source if the gas supply is interrupted or the boiler breaks down.

What are the cons of heat-only boilers?

The three cons of heat-only boilers are listed below.

• Significant space requirement: Conventional boilers require hot and cold water tanks, which take up a significant amount of space in a home. Accommodating these tanks may be an issue in smaller properties.
• High installation costs: Installing a conventional boiler is often more expensive than other types of boilers since there are several components required to store cold and hot water.
• Lower energy efficiency: Conventional boilers are generally less energy-efficient than combi boilers.

What are the heat-only boiler costs?

A heat-only boiler costs £1,500 – £3,000 without installation costs and £2,000 – £5,000 fitted. These fitting costs assume a like-for-like installation where you replace an old heat-only boiler with a new one. Switching from a conventional boiler to another boiler type costs £500-£1,000 more than a like-for-like swap since such a conversion entails the removal of the water storage tanks and the associated pipework. Switching to a heat-only setup from another boiler type adds over £1,000 to the boiler installation costs, as you need to buy and fit the cold water tank, the F&E tank, the hot water cylinder, and the pipework that connects the system.

The installer you use has a significant impact on the overall boiler costs. Nationwide heating companies tend to charge more for boiler installations simply because of their reputation and high overheads. In contrast, online boiler installers like Heatable typically offer attractive deals on premium heat-only boilers thanks to their online business model and consequent low operating costs.

What is the best heat-only boiler?

Worcester Bosch Greenstar Ri is the best heat-only boiler in 2024 for three reasons. Firstly, the Greenstar Ri is a compact boiler. Compact dimensions mean that you’re able to fit the boiler inside a kitchen cupboard or between two cupboards, thus conserving precious space in your home. Secondly, the Greenstar Ri has an energy efficiency level of 92%, which means that it effectively converts 92p of every £1 spent on gas into heat. Thirdly, the Worcester Greenstar Ri is hydrogen-ready, which means it’s able to run on a 20%-80% hydrogen-natural gas mix, if and when such a mix gets supplied in the nation’s gas grid.

Use Heatable’s boiler quotation tool to get a fixed-price quote on a Worcester Greenstar Ri.

What is a back boiler?

A back boiler is a type of boiler that attaches behind a fireplace hearth and heats water using its proximity to the fireplace’s burner. Back boilers are no longer being installed in new properties in the UK due to their poor emissions record. You’re still permitted to use a back boiler if one is already fitted in your property, but you may find it difficult and expensive to source new parts for it.

How does a back boiler work?

A back boiler works by utilising the heat a fireplace produces to raise the temperature of central heating and domestic hot water. Back boilers have a cold water inlet that brings water from the mains and a heat exchanger that transfers heat from the fireplace into the back boiler. Hot water is then delivered to the output via an electric pump. Excess hot water is stored in a hot water cylinder with an overflow vent.

How do I know if I have a back boiler?

Consider your boiler’s location to find out whether you have a back boiler. Back boilers are generally installed behind the hearth of the fireplace, regardless of the fuel type the fireplace uses. The boiler you have is a “back boiler” if it’s installed behind a fireplace. You should consider replacing your back boiler because it’s inefficient, and you’re bound to save on energy costs by converting to a different boiler type.

Can I get a new back boiler?

No, you cannot get a new back boiler. Installing a new back boiler is illegal in the UK. This regulation is in place because of health and safety risks associated with back boilers as well as their poor energy-efficiency record.

Are back boilers illegal?

Back boilers are legal if they are already installed on your property, so you may continue to use an existing back boiler without facing legal consequences. However, it is illegal to install a new back boiler or to replace a broken back boiler with a new one. These government regulations are intended to counter back boilers’ poor efficiency and high emissions records.

Are back boilers dangerous?

Back boilers are dangerous when they’re not decommissioned and removed properly after the owner stops using them. The danger lies in the back boiler’s water jacket, which may retain water even after it’s no longer in use. The back boiler’s jacket heats up every time the fireplace is on, and any water left inside it turns to steam. If the steam pressure rises significantly, it may cause the jacket to explode. Several such back boiler explosions have been recorded in the UK, with at least one fatality stemming from an exploding back boiler.

What’s the best alternative to a back boiler?

A combi boiler is the best alternative to a back boiler for the following two reasons.

• Higher energy efficiency: Back boilers are notoriously inefficient, often wasting as much as 30p for every pound you spend on natural gas. In contrast, combi boilers are the most efficient boiler type on the market. The most fuel-efficient combis convert up to 98p into heat for every pound spent on natural gas.
• Improved safety: Combi boilers are safer than back boilers, since they’re easily accessible for maintenance and replacement, and you have greater control over their pressure.

How much does it cost to replace a back boiler?

It costs £3,500 – £4,500 to replace a back boiler. This figure is determined by the following three factors.

• Your replacement boiler: Different boiler types and models have different installation costs so the cost of replacing a back boiler depends on the replacement boiler of your choice.
• Whether you decommission: An alternative to replacing your back boiler is to decommission it, meaning you leave the back boiler in place but disable it. Decommissioning your back boiler may be as much as £1,000 less expensive than replacing it, but it must be done by a qualified professional to avert the risk of the back boiler building pressure and exploding while you operate the fireplace.
• Back boiler complications: How your back boiler is installed affects the cost of removing it. Some back boilers are positioned awkwardly behind the fireplace, and this awkward placement makes removal more labour-intensive and expensive.

Heatable offer a cost-effective option for back boiler replacements. It is possible to use Heatable’s online tool to find the most suitable boiler for your property, including the costs of replacing a back boiler.

What type of boiler do I have?

The list below provides an overview of each boiler type’s most distinguishing characteristics to help you establish which boiler type you have.

• Combi boiler: You have a combi if your heating and hot water systems are not connected to any cold or hot water storage tanks. You may have a storage combi boiler if the boiler housing measures over 900mm in height and over 500mm in depth.
• System boiler: You have a system boiler if your heating and domestic hot water systems are hooked up to a hot water cylinder, but there are no storage tanks for cold water in the loft.
• Heat-only boiler: You’ve got a heat-only boiler if your heating and hot water systems operate with a hot water cylinder (typically installed in the airing cupboard) and two cold water storage tanks (usually in the loft).
• Back boiler: You’ve got a back boiler if the boiler is located behind the fireplace hearth.

What type of boiler do I need?

The following four factors help you decide what boiler type you need.

• Hot water usage: You may need a system or a heat-only boiler if your property has several occupants who are likely to use the hot water at the same time. Combi boilers have no reserve of hot water and so struggle to keep up when there are multiple taps or showers running at the same time.
• Energy budget: Combi boilers are generally more energy efficient than system or conventional boilers because they do not need to keep a hot water cylinder heated. Combi boilers may be the most suitable boiler type for you if you want to keep your energy bills low.
• Floor area: Combi boilers take up the least amount of space because they do not have any external water storage tanks. Conventional boilers take up the most floor area because they use both hot and cold water tanks, which add significantly to the footprint of the heating and hot water systems.

What type of boiler is best?

Each boiler type (except back boilers, which are obsolete) has advantages and disadvantages, so determining which boiler is best is only possible when you consider the future user’s budget, heating, and hot water needs. The list below explains the scenarios under which each type of boiler is best.

• Combi boiler: A combi boiler is best for households with limited water consumption and smaller properties. Combis are highly energy-efficient boilers that take up little space and generally don’t cost too much. However, combis rely on mains water pressure, so they’re unable to supply enough domestic hot water to multiple taps or showers running at once.
• Heat-only boiler: A heat-only boiler is best for busy households with robust hot water demand and an existing conventional setup. Heat-only boilers have the capacity to provide large volumes of domestic hot water, and installing a new heat-only boiler in the place of an old one is the most economical route since it doesn’t involve tearing out the hot and cold water tanks. That said, heat-only boilers are not ideal for smaller homes with modest hot water demand, since the associated water storage tanks require lots of floor space but aren’t needed given low water consumption rates.
• System boiler: A system boiler is best for families that consume lots of hot water but have no existing cold water storage tanks in the home’s loft. System boilers are connected to a hot water cylinder, which may store anywhere between 250 and 1,000 litres of hot water, which is enough to satisfy the hot water demand of medium and large households. However, the hot water cylinder gets filled directly from the mains, so a system boiler does not require cold water storage tanks to operate, and you don’t need to waste your loft space on these components.

Which type of boiler is most efficient?

Combination boilers are the most efficient type of boiler. Combis do not have to constantly burn fuel to maintain the 60°C minimum temperature inside the hot water cylinder, unlike system or conventional boilers. Combi boilers only consume fuel when there’s a call for heat or hot water, thus helping their users cut on energy costs.

What is the most common boiler type?

Combis are the most common boiler type, with 80% of UK households that rely on gas heating using combination boilers. Combi boilers are continuing to grow in popularity as the nation’s homes and households become smaller. Combi boilers are popular because they have a small footprint, are highly energy-efficient, and are able to meet the hot water needs of most small households. These features make combi boilers the most popular choice for small and mid-sized apartments and small-family homes. Conventional boilers are the second most common boiler type, followed by system boilers in the third place.

Which boiler type lasts the longest?

Boilers manufactured with reliable internal components last the longest, whilst a boiler’s type has little to do with its longevity. Modern boilers of any type are capable of having a 10-15 year lifespan. However, boilers made with durable heat exchangers and quality Grundfos pumps have longer, fault-free lifespans. Going with the top-of-range models from the nation’s most reputable boiler manufacturers should get you a long-lasting boiler that doesn’t develop expensive faults for over a decade or more of service.

However, even the UK’s best boiler brands have “budget” boiler models, which tend to break down sooner and more frequently. Avoiding boilers with a short warranty should help you prevent the aforementioned scenario from unfolding. The warranty attached to a boiler is your best indicator of durability since boiler makers offer the longest standard warranties on models they have faith in, and vice versa. As a rule of thumb, opt for a boiler with at least a 5-year warranty to ensure you have a long-lasting model. However, paying a few hundred pounds more typically gets you the maker’s top-of-range model with the longest warranty term possible. The longest boiler warranties in the UK are in the 10-12 year range.

Annual boiler service is an essential tool for prolonging your boiler’s service life. Boiler service generally costs around £100, but the cost includes a comprehensive checkup that’s able to flag faults early, giving you a chance to rectify small boiler problems before they progress. Annual service is a common condition for maintaining a boiler warranty’s validity, regardless of the manufacturer.