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Updated: 17 Dec 2021

What are the different types of boiler?

Our experts tell you everything you need to know about combi, heat-only and system boilers.
Sam Morris

No matter what type of fuel your boiler runs on, there are different types of boiler available. 

A combination (combi) boiler is the most common type of boiler the UK. But it's not the best option for everyone. 

Heat-only and system boilers are better suited for a variety of situations, and the wrong type could lead to significantly higher bills or long delays for hot water.

Read on to learn more about the various types of boiler and to find out the pros and cons of each, plus how a condensing boiler could save you hundreds on your heating bills. 

Half of homeowners, on average, consider changing brand when getting a new boiler installed (Which? boiler survey 2021). Read our best gas and oil boiler brands guide to discover the ones to go for.

What type of boiler do you need?

Every home is different, and so the right type for your household may be different to your neighbour's. Read on to discover more pros and cons for the various types of boiler. 

Combi boilers

Combination boilers connect directly to the mains cold water supply, providing hot water on demand. They are a great choice for many homes, especially for smaller houses and flats that only have one bathroom, as they have a lower hot water and heating demand than larger properties.

Pros of combi boilers

  • Take up less space - a combi boiler doesn't need a water tank, so you'll have more space.
  • Aesthetics - all its components are within the boiler casing, making it more compact and neater to look at.
  • Easier to service - due to their popularity, most engineers are trained to work on combi boilers, making it easier to find a qualified one in your local area.
  • Instant hot water - you don't have to wait for a water tank to heat up; a combi boiler will immediately supply unlimited hot water.

Cons of combi boilers

  • Only one shower at a time - the rate at which water is supplied reduces when the boiler is trying to provide hot water to two or more places at the same time. This typically means it isn't possible to run more than one bath or shower at a time, or a shower while someone else does the washing up using hot running water.
  • Limited by mains pressure - they don't work as well in homes with poor water pressure.
  • Moving parts - the boiler unit contains moving parts, so there is more potential for things to go wrong than with a conventional boiler.

Decided a combi boiler is for you? Find out what the best combi boilers brands are, or go straight to our expert reviews of combi boilers

Heat-only boilers

Also known as 'conventional' or 'regular' boilers, heat-only boilers provide heat directly to your radiators and connect to a water tank to provide hot water. You will also normally have an on/off switch - so you can heat water in the storage tank when you know you're going to need it, and leave it switched off when not in use. 

An 'open-vented' heat-only system - the most common heat-only boiler system - requires a cold water feed and expansion tanks in the loft.

Heat-only boilers typically have components housed externally from the boiler, such as a circulation pump.

Pros of heat-only boilers

  • Good for larger households - if you have multiple bathrooms with people frequently needing to use hot water at the same time, a heat-only boiler will meet this demand.

Cons of heat-only boilers

  • Doesn't give hot water on demand - you have to wait for the water in the cylinder to heat up.
  • Temporarily run out of hot water - if lots of people are using the hot water, it's possible to run out and you'll have to wait for the hot water tank to refill.
  • Takes up space - you'll need room for cold water feed and expansion tanks to be installed.
  • Less efficient than combi boilers - as you will lose heat from the stored hot water in the hot water cylinder.
  • Installation costs - can be more expensive to install than a combi boiler if you don't already have the cold water feed and expansion tanks.

Have you decided that a heat-only boiler is the type for you? Take a look at our best heat-only boiler reviews.

System boilers

Sometimes known as 'closed vent' or 'sealed system' boilers, system boilers require a hot water cylinder. However, unlike conventional boilers, they don't require a cold water tank, and components that are external in a conventional boiler, such as pumps and valves, are built into the body of a system boiler.

A system boiler is the perfect choice if you think you'll need a heat-only boiler but don't want lots of external components.

Pros of system boilers

  • Good for larger homes - if you have multiple bathrooms with people frequently needing to use hot water at the same time, a system boiler will meet this demand.
  • Takes up less space then a regular boiler -  you don't need the space for a large cold water tank in the loft.

Cons of system boilers

  • Take up more space than a combi boiler - you still need space for a hot water cylinder.
  • Doesn't give hot water on demand - you have to wait for the water in the cylinder to heat up.
  • Temporarily run out of hot water - if lots of people are using the hot water, it's possible to run out and you'll have to wait for the hot water tank to refill.
  • Less efficient than combi boilers - as you will lose heat from the stored hot water in the hot water cylinder.

Have you decided a system boiler is for you? Head straight to our best system boiler reviews

Storage combi boilers

A less common system,  storage combi boilers work like a combination boiler, but have an internal hot water cylinder built into the boiler.

They are a good solution if you have a higher hot water demand but don't have the space for the separate hot water cylinder that a system or heat-only boiler requires. 

Pros of storage combi boilers

  • Benefits of a combi and system boiler in one - hot water on-demand, but the hot water cylinder means hot water can go to several outlets at the same time without the pressure dropping (like it would with a regular combi boiler).
  • Takes up less space than a system boiler - the hot water cylinder is built into the boiler body, rather than being separate like a system boiler.
  • Cheaper to install than a system or heat-only boiler - as everything is built in, the unit is typically cheaper and is less expensive to install.
  • Solar thermal panel compatible - as it stores hot water, you can keep that water warm with solar thermal, which isn't possible with just a typical combi boiler.

Cons of storage combi boilers

  • Take up more space than a combi boiler - due to the hot water cylinder built into the boiler's body. 
  • System boilers can meet larger hot water demands - storage combi hot water tanks are typically sized between 40 and 200 litres, but some hot water tanks for system boilers can be 500 litres or more.  
  • Less choice - being a less common type of boiler, there are fewer storage combi systems to choose from.
  • Engineer knowledge - being a less common type of boiler, some engineers may be less familiar with a storage combi boiler, making it trickier to repair or service.

 Decided that having a storage combi boiler is right for you? Read our best storage combi boiler reviews.

What is a condensing boiler?

Parents and child holding tablet on sofa under a blanket

Condensing boilers are more efficient and 'greener' than older non-condensing boilers. They make the fuel you burn go further, meaning you'll produce fewer carbon emissions while reducing your heating bills. 

Replacing an old, less efficient, non-condensing boiler with a new, high-efficiency, condensing boiler could save you between £45 and £300 a year on your heating bill (Energy Saving Trust, 2021).

All types of boiler can be condensing boilers. Since 2007, except in exceptional circumstances, building regulations have stated that all new boilers installed in a domestic home should be high-efficiency condensing boilers.

So the chances are you already have one if you've had a new boiler installed since 2007.

How does a condensing boiler work? 

With a non-condensing heat-only boiler, some heat is wasted in the form of hot gases released from the flue. 

A condensing boiler captures some of the heat from these gases and uses it to heat water returning from your central heating system. It therefore requires less heat from the burner and is more efficient.

All new modern boilers are condensing boilers. So if you're thinking of replacing an old boiler with a new one, you will be buying a new condensing boiler and gaining all of the efficiency savings that come with it.

Pros of condensing boilers

  • More efficient - condensing boilers are typically 25% more efficient than a non-condensing model. This results in lower heating bills and fewer carbon emissions.
  • Safer - there's much less risk of anything getting sucked into a condensing boiler as it takes in air from outside. Whereas non-condensing boilers typically take in air from inside the room, running the risk of sucking small objects (like small stones and debris) into your boiler.

Cons of condensing boilers

  • Poor installations limit efficiency - a poor installation, or not using a boiler correctly, means a boiler may not actually condense efficiently. Check our top tips to reduce heating bills to learn how to check your boiler is condensing

Getting a Gas Safe qualified heating engineer to install a new boiler will ensure it operates efficiently. Find a Which? Trusted Trader or use our Trusted Traders search tool below to find a local trader in your area

How do biomass boilers work?

  1. Pallets, wood chips or logs are stored, and are automatically or manually fed into the biomass boiler
  2. Biomass boilers burn this to generate heat.
  3. Heat is sent to radiators and/or underfloor heating.
  4. Heat is also stored in a hot water cylinder for showers, bather and taps.

Stoves can be connected to a regular boiler, such as a gas boiler, or a biomass boiler. Biomass boilers, like stoves, burn wood to generate heat. They can be used along with a stove, or on their own, and are particularly good for homes not connected to mains gas.

You can also get a 'wet' stove, where an integral biomass boiler is built into the firebox. This should make the most of the heat. In some cases, you can get these retrofitted to the stove using a 'clip'.

There are also pellet boiler stoves, another form of stove and boiler in one, and flue boilers, which fit onto the flue connection between the stove and the chimney.

We wouldn't recommend adding a 'back boiler' to an old open fire as these are a lot less efficient and more polluting than modern stoves.

You can use a thermal tank to link up a range of other heating systems too, such as solar panels and a gas boiler with a wood-burning stove. 

A thermal tank (also known as a buffer tank or accumulator) will help regulate the fluctuating use of the different energy sources - eg sun for solar panels in the summer and logs for a wood-burning stove in the winter.

If you’re thinking about replacing your boiler, use our research to buy one you can rely on. See our boiler reviews. You can also use our wood-burning stove reviews to find out which companies have boiler stove options.

Low carbon heating systems

We will all be making the transition to low carbon heating over the next few decades. A gas boiler may be the best option for you currently, but these produce carbon emissions and will eventually be phased out. If you want to consider a lower carbon heating option now, read our expert guides on:

  • Solar water heating Solar thermal panels that use heat from the sun to heat water to use in your home.
  • Ground source heat pumps A network of water pipes buried underground, extracting natural heat from the ground that is then transferred to your home heating system. 
  • Air source heat pumps Works in a similar manner to a ground source heat pump, but instead extracts heat from the air, which is then boosted to a higher temperature using a compressor.
  • Electric central heating Reduces carbon emissions from your home (though only zero carbon if the electricity is produced by renewables). Examples include immersion heaters and storage heaters