What are the different types of boiler?

Need a new boiler? There are several different types available and we walk you through what they are, below.

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

For some, a heat-only or system boiler will suit you best. 

Installing 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. 

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, 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, which have lower hot-water and heating demands 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 to have a hot 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.

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. They 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 in different places 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.
• Less efficient than combi boilers – 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.

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 a good 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 – you will lose heat from the stored hot water in the hot-water cylinder.

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, as is the case with 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 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.

What is a condensing boiler?

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. 

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

So, if your boiler was installed more recently than 2007, you'll probably already have one.

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.

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 efficiently.

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 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, baths and taps.

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 generated by a stove. 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 on to 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 also use a thermal tank to link up a range of other heating systems, such as solar thermal panels. 

A thermal tank (also known as a buffer tank or accumulator) will help regulate the fluctuating use of the different energy sources, such as sunshine for solar panels in the summer.

If you’re thinking about replacing your boiler, use our research to buy one you can rely on. See our boiler brand reviews. 

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 the sun's energy 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).