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Boiler Types Explained: Combi, System and Conventional

Adam Cherry
By: Adam Cherry
Updated: 7th October 2021

When it comes to finding a new boiler, one of your first tasks is to choose the best type of boiler to suit your home and lifestyle.

This article will guide you through the various types of boiler and central heating systems that are available.

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Condensing vs Non-condensing boilers

Blink and Blaze
When researching types of boilers, you are likely to come across the terms condensing and non-condensing boilers. These terms describe the type of technology within the boiler, i.e. how the boiler uses fuel to produce heat for your home.

You will not have to make a choice between these types as, by law, all modern boilers now use condensing technology as standard. This is because they are more energy efficient than non-condensing boilers. While non-condensing boilers would only be able to extract between 50-80% of heat from the fuel, modern condensing boilers can typically extract 90-92% of the heat.

Condensing boilers are so much more effective because they are sealed systems meaning they extract heat from flue gases which would have otherwise been lost by a non-condensing boiler. Condensing boilers were not overly popular when they first hit the market, with early teething problems and a collection of myths about their reliability putting many homeowners off. Since then condensing boilers have come a long way and many of those old issues have been resolved.

Visit What is a Condensing Boiler? for more information.

Internal vs External boilers

Another possible choice you might need to make is whether to have your boiler installed inside or outside your home. The majority of gas domestic boilers are installed inside the home, e.g. in a kitchen cupboard, but some people choose to install the boiler in a garage, outhouse or outside in the garden. These boilers are usually oil fired boilers for properties which are not connected to the gas grid as the oil is stored outside in a large tank.

Boiler fuel types

The type of fuel your boiler will use – gas, oil, LPG, electricity or biomass – might be restricted based on where you live, but some homeowners may find they have a choice.
  • Most UK homes are connected to the gas grid which means they can have a natural gas boiler which is supplied with fuel automatically.
  • Properties which are not connected to the gas grid, e.g. those in rural areas, will need to use an alternative fuel for the boiler such as oil, electricity or biomass (wood pellets).
  • More and more homeowners (even those who have access to gas) are making the choice to install a boiler or heating system which uses renewable fuel such as a biomass boiler or a heat pump in order to lower their carbon emissions.

Natural gas

Gas is the most common heating fuel type in the UK and the majority of homeowners who are connected to the gas grid opt for a gas boiler.

Advantages of a gas boiler

Cheaper to run than electric boilers

Gas is around 3-4 times cheaper than electricity per kWh making it a much more economical option when it comes to heating your home. Whilst a gas boiler is not 100% efficient (and an electric radiator is), you can still get more heat for your money from gas.

Gas is the cleanest fossil fuel

Whilst gas is a fossil fuel and therefore not environmentally friendly, it is the cleanest fossil fuel of those available. In fact, gas creates less than half of the CO2 emissions of oil, and a third of those produced by coal.

Disadvantages of a gas boiler

You must be connected to the gas grid

In order to power a boiler with natural gas you must be connected to the gas network. It can be extremely costly to have your property connected to the grid if it is not already, so if this is the case you may want to opt for a different fuel type.

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Many homes that use an oil boiler choose to do so because they have no connection to the gas line, however there are a lot of considerations you should make before going ahead with this type of system.

Advantages of an oil fired boiler

Oil combi boiler

No need for a gas supply

If your property is not connected to a gas line, using an oil fired boiler can be one of the most cost effective alternatives. It is however more expensive than using gas to heat your home, so if you have access to gas it may be worth considering that option also.

Disadvantages of an oil fired boiler

You can run out of oil

The fuel required for an oil boiler is not available on demand as it is with gas or electricity, it instead must be ordered and then stored in a tank. You will need to monitor the amount of fuel you have left and order it as needed.

Oil may have to be paid for in bulk

Ordering oil for your system can also present financial issues as some suppliers will require payment on delivery (as opposed to spread monthly).

Storage space for an oil tank

Oil tanks can in fact be sunk into the ground to save space, however if you opt to store it at ground level you will need to make a few considerations. This includes ensuring there is a level, non-combustible solid base for it to sit on, which can take up a great deal of space.


LPG or liquefied petroleum gas is a combination of gaseous hydrocarbons, produced from natural gas and oil extraction (66%) and from oil refining (34%). It is stored and transported in small canisters, large tanks and lorries.

Advantages of an LPG boiler

Cheaper than an oil boiler

An LPG boiler is significantly cheaper to buy than an oil boiler. For example it's possible to find an LPG boiler for as little as £350, while an oil boiler will usually cost £800 or more.

Cleaner fuel emissions

LPG is still a fossil fuel so it's not a 'clean' source of energy, but it is 'cleaner' than oil, i.e. it produces around 15-20% less carbon.

More compact size

LPG boilers tend to be smaller in size than oil fired boilers and in many cases they have a lower noise output.

Wider choice

There is a much wider choice of LPG boilers than oil boilers with many of the market leaders like Worcester Bosch, Vaillant, Ideal and Baxi offering highly popular models.

Disadvantages of LPG Boilers

Higher fuel cost

The unit price per kWh of LPG is higher than both natural gas and oil.


You'll need to keep a close eye on your fuel tank as leaving it too late to top up could leave you without fuel if it cannot be delivered in time.

Buying a tank

An LPG system requires a storage tank which you will either need to buy or rent from your supplier and store in your garden, so you'll need to factor this in as an additional cost.


An electric central heating boiler is a great option for anyone looking for a compact boiler for a smaller household. They are also a good alternative for homes that cannot install a gas boiler since they are not connected to a gas line.

A typical electric boiler will work by heating the water that runs through it with a heating element and this hot water is then pumped to where it is needed. Thanks to the way they heat water, electric boilers are considered one of the least wasteful options as there is extremely minimal heat loss like there is with a gas boiler.

Advantages of an electric boiler

High efficiency

An electric boiler can run at around 99% efficiency as it doesn't lose heat in the same way as a gas boiler would. Whilst this does mean that the boiler itself is more efficient, it doesn't necessarily make an electric boiler eco-friendly.

Simple, compact and often cheaper to install

Electric boilers use fairly simple technology when compared to a gas boiler. For this reason they are usually compact and in many cases cheaper to install than other boiler types, especially as there is no flue to be fitted. Since they don't use gas, there isn't the same risk present from carbon monoxide and you don't need to use a Gas Safe Registered installer.

No need for a gas supply

For anyone without access to a gas line an electric boiler can be the ideal solution for heating a property effectively. Connecting your home to the gas line can be extremely expensive, so many homeowners opt for an electric boiler instead.

Disadvantages of an electric boiler

High cost of electricity

Despite being so efficient, electric boilers can be expensive to run due to the high cost of electricity. With gas costing less than half of the price of electricity, you should consider this factor before opting for an electric boiler.

Not always ideal for large properties

An electric boiler can only heat small amounts of water at a time, and usually they cannot store it for later use. This means a larger property which uses a lot of hot water, or has multiple bathrooms being used at once may not find an electric boiler adequate.

Reliance on electricity

Since this style of boiler runs on electricity, it might not be the best option for a property that is affected by power cuts or outages on a regular basis.

Visit our guide to electric boilers for more information.


A biomass boiler provides an alternative option to a traditional gas or electric boiler. It works in a similar way, usually utilising the same kind of pipes and radiators, however the main difference is the fuel.

Biomass usually refers to wood pellets or wood chips which are burnt to heat up a tank of hot water, which is then used to heat your home and provide hot water.

Advantages of biomass boilers

Can be more environmentally friendly than other fuels

Certain fuels used by biomass boilers are actually considered carbon neutral, making them much more environmentally friendly than traditional boilers. This includes wood, as it only releases around the same amount of carbon when burnt as it absorbed when it was growing. If you use locally sourced biomass this will reduce the carbon needed to transport it, making it greener.

No reliance on gas or electricity

Having independence from gas and electricity means you won't be affected by any price rises from suppliers.

You may qualify for the Renewable Heat Incentive

Some biomass boilers are eligible to receive the government's Renewable Heat Incentive (RHI), a series of quarterly payments received over several years.

Disadvantages of biomass boilers

Installation costs can be high

A biomass boiler can be very costly (depending on the type you opt for) when compared to something like a condensing boiler. There is also a range of other costs you may want to factor in for the life of your biomass boiler, including the delivery costs of fuel and any electricity used to power the system.

You need to remember to buy the fuel

A biomass boiler can run on a range of fuel types, but you will need to ensure they are suitable for use (for example you cannot just burn household waste). This means it's important to make sure you have a reserve of fuel to use at all times.

Storage space

Not only will you need to purchase the fuel regularly, you will also need a reasonably large amount of storage space for it. It can take a lot of wood, or other solid fuel, to heat your property well so make sure to consider the need for additional storage space.

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Which type of boiler is best?

Our guide covers the three most common types of boiler and central heating options; system boilers, conventional boilers and combi boilers. The type of boiler you need will depend on the size and age of your property as well as your lifestyle.

Conventional boilers

Also known as a regular boiler, a conventional boiler system is made up of a number of parts including a boiler, heating controls, a hot water cylinder, a cold water storage cistern plus a feed and expansion cistern.

A conventional boiler system is fed by two tanks which are located in the loft. One of these, the cold water storage tank, draws cold water from the mains supply. The other is the feed and expansion tank, which feeds the boiler system and manages any water that expands in the system after being heated.

The boiler heats water which is then stored in a hot water cylinder until it is needed for radiators or domestic use.

A conventional boiler is often found in older homes, but a lot of modern homeowners choose to replace them with a system or combi boiler as they deliver higher water pressure. However, some older properties may not have pipework which can cope with higher water pressure and so need to keep using a conventional boiler system.

Advantages of a conventional boiler

Regular Gas Boiler

You can use multiple taps at once

Since a high volume of hot water can be stored in the cylinder, you can use multiple sources such as taps and showers, without feeling a drop in water pressure or temperature.

Works well when replacing an older system

When it comes to replacing the heating system in an older property, a conventional boiler can be a good option as it requires minimal changes to the pipework, if any.

It can work with a backup immersion heater

An electrical immersion heater can be installed into the hot water cylinder, so if your boiler breaks down you have an alternative way to heat water.

Compatible with solar thermal

A conventional boiler system is compatible with solar thermal panels, which harness the sun's energy to create warm water which is then fed into your central heating system.

Disadvantages of a conventional boiler

Takes up a large amount of space

A conventional boiler requires both a loft and an airing cupboard to house the tanks and cylinder which means it is often not ideal for smaller homes.

Hot water tank heat loss

A conventional boiler system uses a hot water storage cylinder. Whilst hot water is stored in there it will lose heat over time, in order to prevent this you should insulate your tank well.

You can't have instant hot water

Once you've used all of the hot water in your storage cylinder, you will need to wait for the boiler to heat another tank full before you can use more. This means you should consider the amount you are likely to use when choosing a water cylinder.

Complicated to install from scratch

Due to all of its separate parts and pipework, a conventional boiler system is one of the more time consuming and costly heating systems to install.

Read more in What is a Regular (Conventional) boiler?

System boilers

A system boiler works in a similar way to a conventional boiler, except for the fact that it does not require a feed tank or expansion vessel to be kept in the attic as this technology is built into the boiler unit itself. It takes its cold water supply directly from the mains, heats it up and sends it to a hot water cylinder to be stored until it is needed.

Advantages of a system boiler

System Gas Boiler

Fairly straightforward to install

Many of the components needed for a heating system are already built into a system boiler. This makes it more straightforward to install than a conventional boiler.

You can use multiple taps at once

The storage tank means that you can get hot water from multiple sources like taps and showers at the same time without losing water pressure or seeing a temperature drop.

Doesn't need a cold water feed tank

A system boiler doesn't require a cold water feed tank, which would usually be located in a loft. This makes it ideal for smaller homes or those without attic space.

Delivers stronger water pressure

A system boiler takes cold water directly from the mains which means it usually delivers a stronger water pressure than a conventional boiler (assuming water pressure is adequate in your area).

Compatible with solar thermal

System boilers can work with a solar thermal system, which uses the sun's energy to heat water for your home. This can reduce your household's carbon emissions and energy bills.

Disadvantages of a System Boiler

Not as compact as a combi boiler

Unlike a combi boiler, a system boiler requires a hot water storage tank, making it far less compact.

Hot water tank heat loss

Any hot water created will be kept in a hot water storage tank until it's needed. During this time heat will be lost, so it's important to try and prevent as much of this heat loss as possible by insulating it.

You are limited by the size of your hot water tank

With a system boiler you can only use as much hot water as your storage tank can hold. If you require more you must then wait for your boiler to heat it again, and this is usually on a set timer.

Visit What is a System Boiler? for more information.

Combi boilers

A combi boiler, also known as a combination boiler, is highly efficient and compact, making them ideal for smaller homes. The name combi refers to the fact that these boilers are able to act as both a water heater and also a central heating unit.

Thanks to their space saving features, combi boilers are one of the most popular choices in the UK - accounting for over half of domestic boiler installations each year.

Combi boilers work by heating water directly from the mains, so you don't need a hot water storage cylinder or cold water tank. This is ideal for smaller properties without airing cupboard or roof space. This heating method also makes them very energy efficient and affordable to run, as water is not heated and stored (and wasted if it's not used). While combi boilers have some great advantages, there are also considerations you should make before going ahead with an installation.

Advantages of a Combi Boiler

Combi Gas Boiler


Combi boilers are ideal for small homes as they require no bulky water storage tanks, they also require less pipework.

Energy and cost efficient

Since you only heat the water you need, when you need it, a combi boiler wastes very little energy (and therefore money).

Good water pressure

Providing you have an adequate level of mains pressure, you should experience a good level of pressure from your shower and taps with a combi boiler.

Quick, easy and generally cheap to install

Since a combi boiler requires no tank, it is one of the more straightforward boiler types to install. This also means it tends to be a cheaper option both for installation and repairs.

Hot water on demand

You don't need to wait for a water tank to heat up as a combi boiler heats water on demand. This also means you can have an unlimited amount of hot water.

Disadvantages of a combi boiler

Requires a good level of mains pressure

It's important to make sure that your mains supply can deliver an adequate flow rate and pressure before choosing a combi boiler. If not, it may not be the best type of boiler for your household.

Not compatible with all types of shower

You cannot use a power shower with a combi boiler, as the pressure is dictated by that of the mains supply.

Can't run more than one shower or bath at a time

With a combi boiler you cannot run more than one shower or bath at a time (or run a hot tap at the same time). This makes it a less ideal option for larger households with multiple bathrooms.

No backup water heater

As there is no hot water cylinder with an immersion heater, if your combi boiler breaks down you will not have a backup supply of hot water.

Visit What is a Combi Boiler? for more information.

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Which boiler type is best for your home?

The most commonly installed type of boiler in the UK is the combi boiler (small to medium-sized homes), followed by system boilers in larger homes with multiple bathrooms.

Older properties may need to install a conventional boiler to avoid having to replace pipework and radiators, and homes in areas of low water pressure may find that a conventional boiler delivers better water pressure via its gravity fed system.

If in any doubt about the type of boiler you should install, contact a professional heating engineer in your area who can provide you with advice and a quote for the work.

Should you switch boiler type?

Swapping an old, inefficient boiler (of any type) for a newer model has many benefits including improved efficiency, cheaper heating bills and reduced emissions. If you are trying to keep installation costs to a minimum, a like-for-like boiler replacement would be the cheapest option in terms of upfront costs.

However, there are several possible reasons why a homeowner might want to change the type of boiler in their home. In most cases, homeowners choose to replace a conventional boiler with either a system or a combi boiler.

Boiler change Potential benefits Things to consider
Conventional to system You can get rid of the feed and expansion tanks in the attic. Stronger water pressure (if mains pressure in your area is adequate). Older pipework and radiators may not be able to cope with stronger water pressure. Mains water pressure may be weak in your area. If so, an additional pump may need to be installed.
Conventional to combi You can get rid of the feed and expansion tanks in the attic. You can get rid of the hot water cylinder in your airing cupboard. Stronger water pressure (if mains pressure in your area is adequate). Older pipework and radiators may not be able to cope with stronger water pressure. Mains water pressure may be weak in your area. If so, an additional pump may need to be installed. A combi boiler may not be able to produce enough hot water for homes with lots of radiators and/or multiple bathrooms without losing pressure and/or temperature.
System to combi You can get rid of the hot water cylinder in your airing cupboard. Hot water would be available on demand so no more waiting for the cylinder to refill. Reduced heating bills due to not having to heat a hot water cylinder. A combi boiler may not be able to produce enough hot water for homes with lots of radiators and/or multiple bathrooms without losing pressure and/or temperature.
Combi to system Higher volume of hot water stored in the cylinder which can supply multiple bathrooms and radiators without losing pressure and/or temperature. When the hot water in the cylinder has been used up, you will need to wait for it to refill. Heating bills can increase as more energy is required to heat water in the cylinder.

What size boiler do you need?

Boiler Size
Now you know that you know the fuel and type of boiler you should be using, the next decision relates to the size of the boiler.

A central heating system needs to be powerful enough for your home's needs. The size or output of a boiler is measured in kW. If the boiler is too big for your home, it is likely to waste fuel and make your heating bills unnecessarily high. If a boiler is too small, it will struggle to heat your home.

To work out what size of boiler you need, you should consult a professional heating engineer. They will consider factors like the number of radiators, the number of inhabitants and the number of bathrooms in your property.

The table below gives you a rough guide as to the size range you should be looking for in your boiler search depending on the size of your home and hot water demand. As combi boilers need to be able to produce heating and hot water on demand, they are generally more powerful than system or regular boilers.

Number of bathrooms Number of radiators Combi boiler recommended size System or conventional boiler recommended size
1 10 24-27 kW 10-18 kW
2 15 27-32 kW 18-26 kW
3+ 20 34-42 kW 27-40 kW

How much does each boiler type cost?

When weighing up the potential cost of a new boiler, you will also need to consider the installation itself. As the price of boilers can vary, so can the rates charged by heating engineers for the installation. Factors that will have an impact on how much the installation will cost include:

  • Type of boiler being installed
  • Complexity of the installation (a like-for-like replacement will cost less than a complete change of heating system)
  • Whether the boiler is moving to a new location
  • The rates charged by the installer (installers in London tend to be more expensive than those in other areas of the UK)
  • Below is the potential cost of a gas boiler replacement in a home with 3 bedrooms. All of the figures in the table are averages and could go up or down depending on a number of factors. Firstly, there's the type of boiler being installed. Gas boiler prices can range from as little as £400 to right up in the region of £3,000 depending on the manufacturer and model.

    Boiler Type 1-2 bedroom home 3-4 bedroom home 4+ bedroom home
    Combi £1,000 – £1,500 £1,500 – £2,000 £2,000 – £2,500+
    System £1,500 – £2,000 £2,000 – £2,500 £2,500 – £3,000+
    Conventional £1,500 – £2,000 £2,000 – £2,500 £2,500 – £3,000+

    As there are so many variables that can have an impact on boiler replacement costs, all of the costs in the table are estimates. To get a more precise idea of boiler replacement costs, we highly recommend getting quotes from multiple Gas Safe registered engineers.

    Finding the best boiler installer

    A new boiler of any type is a big investment, so you'll want the installation to be carried out to the highest standard by a competent installer for the best possible price. A gas-fired system boiler must be installed by a Gas Safe registered engineer (an OFTEC technician is needed for the installation of oil boilers).

    Rather than accepting the first quote you receive, we highly recommend comparing at least 3 quotes from different companies.

    At Boiler Guide we have a network of over 5,000 Gas Safe registered boiler installers. Send us an online enquiry today to receive up to 3 quotes from installers near you. That way, you can compare different quotes without any obligation whatsoever to use one, and it's completely free of charge.

    Adam Cherry

    About the author

    Adam Cherry

    Adam is our resident home heating expert. His experience and advice has helped millions of customers improve the efficiency of their homes and save money.

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