Boiler Types Explained: Combi, System, Conventional & More

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Last updated: October 31, 2023

If you are looking for a new boiler, then you need to pick the best type for your home. But with a number of designs on the market, how do you know which boiler is best for you? We take a look at the main types of boilers available and weigh up the pros and cons for each, to help you make the right choice.

Types of boilers

When it comes to choosing a boiler, there are three main designs to select from: the combination or Combi boiler, the closed vent or System boiler and the Conventional boiler, also known as the heat-only or regular boiler. All three of these boiler types are also known as condensing boilers, which means they are energy efficient and are a legal requirement for all UK homes.

While the Combi boiler is the most common boiler type today, it’s not necessarily the best boiler for all homes and central heating system designs. So, to help you understand the differences between the main boiler types, we will explain each one in turn, starting the with the universal condensing boiler.

Condensing Boilers

All UK homes today are legally required to have a condensing boiler, so whichever type of boiler you opt for your home heating and hot water storage, it’s important to ensure it complies with the regulations. Unlike the older ‘non-condensing’ models, a condensing boiler has a larger heat exchanger which captures and recycles heat that would otherwise escape, making it more fuel efficient, saving on your heating bills and lowering your carbon footprint. Any boiler that has a flue gas recovery system is a condensing boiler.


  • Condensing boilers are more than 90% energy efficient, and are between 15-30% more efficient than non-condensing boilers
  • You have lower energy bills as the boiler recycles heat that would otherwise be lost
  • They are safer as they are a sealed system and draw air in from outside the home
  • They are more compact than non-condensing boiler systems


  • The condensation process produces an acidic waste that requires a fluid draining system, which must be fitted to a waste pipe
  • Due to the steam produced, the location and placement of the boiler has to be carefully considered, reducing flexibility when it comes to installation
  • The presence of exhaust gases inside the boiler means it also needs an internal fan, making its mechanisms more complicated than a non-condensing boiler

Combi Boilers

Generating both heat and hot water on demand, the Combi boiler is the most common type of boiler for your home and central heating. The Combi boiler is a single unit, meaning there are no separate hot water tanks, unlike more traditional boilers, as the water is delivered from the mains, making it one of the most compact boilers you can buy. Installing a Combi boiler is also one of the quickest to do. This type of boiler is energy efficient, and as Combination boilers heat water directly from the mains via a built-in heat exchanger, it provides the convenience of an instant supply of hot water, whenever you need it.

A gas combination boiler system contains almost all components within the Combi boiler itself, which makes the boiler system more compact and there’s no need for a cold water storage tank.


  • A Combi boiler is a self-contained system, meaning it is one of the most compact boilers you can buy
  • There is no need for a separate hot water tank, so it will take up less space as a tank in the loft or airing cupboard
  • You get hot water and central heating on demand
  • Lower maintenance as the heating and hot water functions are integrated


  • May not be the best choice for homes with low water pressure or very large houses
  • The moving parts inside the boiler does mean there is more potential for things to go wrong
  • The hot water can only be used for one task at a time, eg. you can run a bath and a shower at the same time
  • You need to ensure the Combi boiler is compatible with your shower system

System Boilers

Also known as closed vent boilers, System boilers have all the built-in components of a Combi boiler but without the option for hot water production as it doesn’t have the internal heat exchanger. This makes a System boiler a good choice if you want heat only boilers that are compact and has minimal external components. System boilers also take up less space and don’t require a water tank in the loft, as they have their own hot water cylinder and so are easier to install. System boilers are a good option for larger homes with multiple bathrooms or if there is no space for a large water tank in the loft.


  • Uses a hot water cylinder so you don’t need a large cold-water tank in the loft
  • System boilers are a good solution if you need hot water at more than one tap at a time
  • Works well with homes that have low water pressure
  • Integrated components mean the boiler is a compact size
  • System boilers are compatible with solar heating systems


  • You will still need a hot water cylinder which can be expensive to install
  • Not as efficient as Combi boilers as you lose heat from the stored hot water
  • Will require insulation with a System boiler to minimise heat loss
  • You need to wait for the water to heat up again, once the hot water runs out
  • System boilers still need more space than Combi boilers

Conventional Boilers

Unlike a Combi boiler or a System boiler which are both closed vented, a Conventional boiler is open vented and only provides heating, not hot water. Also known as a heat-only or regular boiler, the system needs a separate water storage tank or hot water cylinder, where water is heated, stored and ready to use. The components of a Conventional boiler are housed externally, including the circulation pump. Due to the size and need for a separate hot water tank for hot water storage, a Conventional boiler is not a good choice is space is a premium, but they work well if you have a large household. They also suit multiple bathrooms or if you already have the traditional heating system installed and need a replacement boiler.


  • Conventional or heat only boilers work well with old radiators that can leak if used with a more high-water mains pressure system
  • Conventional boilers are a good choice for large families where you need to use hot water and multiple taps at the same time
  • As they are heat only, they are typically the cheapest boilers to buy
  • Good for homes that have low-water pressure and they are compatible with solar thermal systems


  • You don’t get hot water on demand and it needs to heat up once the storage tank runs out
  • They are less efficient than Combi boilers as you do lose some heat from the hot water storage cylinder
  • They are not small and so need sufficient space to set up and install
  • A lot of the components are external and more vulnerable to damage

Boiler Fuel Types

As well as considering which type of boiler best suits your home, the type of boiler fuel you want to use will also influence your final decision. From what fuel is available to your home to whether you want to use renewable energy, your options will may also be influenced by efficiency and lower fuel bills. Here are the main boiler fuel types widely available today:

Gas boilers

Then most common heating fuel in the UK, most homeowners whose property is connected to the gas grid, will choose a gas boiler system. Gas boilers are fuelled by natural gas which is delivered by the gas mains and is predominately used on closed vent models such as Combi boilers. With a gas boiler, gas is injected into the burner, with the pilot used to ignite the gas. The heat produced is then transferred via the heat exchanger into the central heating system. Fumes from the combustion chamber are then extracted via fans and vented safely through the gas flue.

A gas boiler is cheaper to run than electric boilers, meaning it is economical but not as efficient. And while it is a fossil fuel, gas is one of the cleanest you can use. You do have to be on the grid however, and the cost to get your property connected if it is not already, can be high.

Oil boilers

If your home is not connected to the gas mains, an oil boiler is a popular option as they operate in a similar way to a gas boiler system. When firing up your oil boiler system, the oil is pumped into the boiler’s combustion chamber, and ignited, where the gases are then vented out through the boiler flue. You will, however, be required to have a separate outdoor tank to store your oil supply and need to ensure you have a reliable supplier to ensure you don’t run out. This means you also need to monitor the levels in your oil tank and order a new supply in plenty of time. And the oil tank will need a decent amount of outside space.

LPG boilers

Liquefied petroleum gas or LPG combines gaseous hydrocarbons from gas and oil extraction as well as oil refining to create a fossil fuel that’s cleaner than oil, as it produces up to 20% less carbon. LPG boilers are also cheaper to buy than oil boilers, there are a wide variety of models available and they are more compact in size. With an LPG boiler you still do need to get your fuel delivered to you, just like oil, and the unit price of LPG is typically higher than both natural gas and oil. There’s also a need for a tank to store the LPG outside, which will add to the installation cost. A key advantage to LPG is that some regular boilers can be converted to LPG if you purchase the appropriate conversion kit, so make you chat with the right boiler engineers if you are looking to make the switch.

Electric boilers

For compact boilers and smaller households, an electric boiler is worth considering. Electric boilers are also another option if you are not connected to a gas supply. Electric boilers work by using an internal heating element to heat up the water running through, which is then pumped to where it is needed, radiator or hot water taps. The benefit of electric boilers is that they are one of the lowest waste options, as minimal heat is lost, compared to gas boiler systems. Electric boilers are mostly easy and convenient to install and need less pipes and components compared to other boilers so are compact and affordable. However, you do need to factor in the potentially higher cost of electricity and the fact that electric boilers are not necessarily the best option for larger homes or households, requiring the use of multiple bathrooms. They are also considered greener than the main fossil fuels as there are no gas emissions.

Biomass boilers

Our final fuel type is the biomass boiler, which can be more environmentally friendly than the more traditional boiler fuels. Biomass boilers in principle, work in the same way as other boilers but use biomass fuel, which is either wood pellets or chips that are burnt to provide the heat source for your boilers system.  Opting for a biomass boiler can help you to reduce your home’s carbon footprint, in that the fuel is largely considered carbon neutral and you can shop locally for your biomass supply. It also means you are not reliant on gas or electricity suppliers, meaning you can keep your energy bills down. And you can even potentially receive payment for using a biomass boiler, if you qualify for the Renewable Heat Incentive. However, biomass boiler installation can be costly and you need storage space for your fuel.

What Type of Boiler Do I Need?

So, now you know more about the different types of boilers and boiler fuel, and are considering a new boiler or boiler replacement for your home, how do you decide which boiler type you need?

Each home has different heating needs as well as variants in property and household size, the number of bathrooms and water pressure, depending on where you live. For most homes, Combi boilers are a good solution, as they are energy efficient, provide heat and instant hot water and are easy to install. Although, as we have seen, Combis are not necessarily the best choice if you have low water pressure or live in a large household where you need to service multiple bathrooms. The takeout from this boiler guide is to consider the size and available space in your home, your fuel preference, the number of your household, level of water pressure and budget to whittle the boiler choice down to the right one for you. And if you are still unsure, then discuss your new boiler needs with a heating engineer who will be able to give you his expert advice.


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