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What is a combi boiler?

When the time comes to upgrade your heating system, it can be daunting to know where to start. Even deciding what type of boiler is best for your home can be a challenge when you’re unfamiliar with the jargon. Combination or ‘combi’ boilers are currently the most popular boiler in the UK, but what are they, and why are they so popular? In this handy guide, we’ll tell you everything you need to know about combi boilers, to help you to decide whether a combi’s a good option for your home.

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What is a combi boiler?

When the time comes to upgrade your heating system, it can be daunting to know where to start. Even deciding what type of boiler is best for your home can be a challenge when you’re unfamiliar with the jargon.

Combination or ‘combi’ boilers are currently the most popular boiler in the UK, but what are they, and why are they so popular?

In this guide, we’ll tell you everything you need to know about combi boilers, to help you to decide whether a combi’s a good option for your home.

For information on particular combi models, check out our blog: The best combi boilers 2022.

For tailored advice on your options, contact one of our friendly team at The Heating People.

What is a combi boiler?

A diagram of a combi system in a house.Combination boilers, or ‘combi’ for short, are compact all-rounders that provide both the heating and hot water for your home.

Unlike conventional or heat-only boilers, combis don’t need a separate cold water storage tank or an external hot water cylinder in your attic – everything it needs is inside the unit.

This is because they take their cold-water supply directly from the mains, and they don’t store hot water, because they heat water instantaneously, on demand.

Since combi’s only heat water as and when they need it, they can save you money on your energy bills because you’re not paying to heat and store water that you might not use.

Another plus? No waiting for a tank to heat up when you really want a hot bath. A combi is ready when you are.

Are combi boilers reliable?

An installation of a Vaillant ecoFIT by The Heating PeopleIn the past, combi boilers were not what they are today.

So, if you’ve heard mixed reviews about combi’s, you’ve probably heard about an old type of combi, which relied on a mechanical type of operation, which affected the delivery of hot water.

Over the last 15 years, combi boilers have improved significantly, and not only must they be at least 92% efficient, serious progress has been made with their effectiveness of performance.

Nowadays, a faster electrical process has improved the transfer of heat for hot water.

Rest assured, that today, combi boilers are compact, easy to use, reliable, efficient, and very effective at delivering heating and hot water straight from one unit.

How does a combi boiler work?

Diagram of a combi boilerCombi boilers are different from other boiler systems because all their operating components are contained within one compact unit.

It’s important to point out that whilst combi boilers provide both heating and hot water, they can only perform one of these tasks at a time, and they always prioritise hot water. 

Combi boilers work using two heat exchangers.

The primary heat exchanger heats and recycles the water that goes through the pipes into your radiators to heat your house. This water travels through the radiators and recycles back into the boiler to be reheated again. This is controlled via a programmer or timer, and a room thermostat.

The secondary heat exchanger heats up the hot water that goes into your taps. When you turn on a hot tap, a combi boiler pauses the central heating, and opens a diverter valve, which diverts water away from the radiator circuit and into a plate heat exchanger, which heats clean water from the mains, and delivers it to your hot water taps or shower. 

When this switching happens, you will hear some boilers switching on and off as you open the taps.

How efficient are combi boilers?

Diagram showing Boiler Plus.The Boiler Plus regulations have changed the law on the efficiency of boilers, to improve how you use energy in your home.

These rules give you greater choice and control over your energy usage, which results in savings on your energy bills.

From April 2018, all boilers must now have a minimum ErP (Energy related Products) rating of 92% and all gas and oil boilers must have appropriate time and temperature controls fitted.

In addition, when installed, combi boilers must have one of four additional energy saving devices fitted: flue gas heat recovery; weather compensation; load compensation; or smart controls with automation and optimisation functions.

All of this means that your combi is very efficient and could help you to make significant savings on your energy bills.

The Heating People will be able to guide you on which additional Boiler Plus measure should be used alongside your new combi boiler.

Two of our favourite combi boilers are the ATAG iC Economiser Plus and the Vaillant ecoTEC Exclusive with Green iQ, because these boilers have in-built flue gas heat recovery. This makes them automatically compliant with boiler plus, with only minimal controls added.

We also love weather compensation, where situations allow. And we find that optimum start and smart controls are a popular choice for many of our customers as they fit many boilers and are easy to use.

But don’t worry – we’ll explain all your options, as part of your free quotation.

Could a new combi boiler save me money?

The short answer to this, is yes!

But how much you stand to save will depend on what you’re switching from.

All new combi boilers are A-rated for energy efficiency, meaning that they’ll be at least 92% efficient.  Whereas, if you’ve got an old G-rated boiler, this could be less than 70% efficient.

To put this into context, with a G-rated boiler, for every £1 you spend on your heating bill you are wasting 30p on lost energy!

Combi boilers are generally thought to be more efficient than conventional or system boilers because they heat water on demand, rather than wasting energy heating and storing hot water that you may never use.

How does a combi boiler compare to the other types of boilers?

Three houses with different heating systems.Combi boilers are different to heat-only and system boilers because they provide all your heating and hot water from the same compact unit.

In contrast, heat-only boilers need an additional cold-water tank and hot-water cylinder; and system boilers need a hot water storage cylinder to heat and store hot water.

Some of the key differences between the different boiler types are summarised here:

Combi boilers take their water directly from the mains

Combi boilers and system boilers receive their cold water directly from the mains. With a heat-only boiler, depending on the system in place, water is supplied from the mains or from tanks.

Combi boilers are compact

Combi boilers need the smallest amount of storage space because all their component parts are held within one compact unit. With a conventional boiler, space is required for cylinders and other equipment. With a system boiler, space is required for a hot water cylinder.

Combi boilers don’t have back-up immersion heaters

In the event of a combi boiler breaking down, there will be no heating or hot water. With conventional and system boilers, you can install an immersion heater. This means that in the event of a breakdown, you’ll have no heating, but will still be able to get hot water.

Combi boilers are efficient

Combi boilers are highly efficient. Heat-only or conventional boilers can be less efficient if you don’t use much hot water each day.

System boilers have a wider range of control options available, compared to conventional boilers and are easy to integrate with renewable heating systems such as solar. You can read about the pros and cons of system boilers here.

Combi boilers are best suited to small to medium sized homes

Combi boilers are better suited to small to medium sized homes.

Conventional and system boilers are better suited to larger homes or homes with multiple bathrooms. This is because they store hot water and can therefore supply large amounts of it to multiple outlets at the same time.

Combi boilers – Installation cost

An engineer installing a boiler.The cost involved in installing the different types of boilers will obviously depend on the condition of your current heating system, and the make and model of boiler that you choose. But as a rough guide:

Replacing an existing combi boiler with a new combi boiler will generally cost between £1800-3000. Upgrading from a conventional boiler to a combi boiler will cost between £2200-£3500.

Switching from a heat-only boiler with a new conventional boiler will cost between £1800-3000.

Changing from a conventional boiler to a system boiler will cost between £1800-3000. Replacing a system with a new system boiler will cost between £1800-3000.

How do I know if I already have a combi boiler?

If you’re wondering if your existing boiler is a combi, there are a few things that you can check:

Do you have a cylinder?

A cylinder is a copper ‘tank’ that holds your hot water on a traditional system – these are sometimes known as immersion tanks. Often cylinders are in an airing cupboard, but we have also seen them installed in lofts, basements and everywhere in between!

If you have a cylinder, it’s very unlikely that you have a combi boiler.

How many pipes are connected to your boiler?

Combi boilers have between five and seven pipes connected to them (sometimes these may run straight into the wall or up the back of the boiler making them hard to spot).

If your boiler has less than five pipes, it’s not a combi boiler.

What make and model is your boiler?

Check your boiler for a badge with a manufacturer and model name. For example: Vaillant ecoTEC Pros 28.  A quick Google search may tell you if your boiler is a combi or not. Be careful though, as some manufacturers have very similar names for their combi and standard boilers.

A Google search of your boiler’s badge may tell you whether you have a combi.

When your boiler is not running, turn on a hot tap.

If your boiler starts to operate when a hot tap is opened and stops operating shortly afterwards, it’s a combi. Repeat the test several times to be sure that it’s not a coincidence.

If your boiler starts to operate when a hot tap is opened and stops shortly afterwards, it’s a combi boiler.

Is a combi boiler right for me?

A happy family and dog.In general, combis are best suited to small to medium-sized homes.

A combi might be right for you if:

  • You live in an area with normal or high-water pressure;
  • You have modest hot water demands at any one time.
  • Your boiler is going to be within 20m of the furthest bath, shower or sink.



What are the main advantages of a combi boiler?

A woman looking ay a diagram of a combi boiler system.Here are some of the main advantages of choosing a combi boiler:

Combi boilers are space saving

Since combis don’t need external water tanks or cylinders, they are space saving. Many models are compact enough to fit in a small cupboard.

This can be a real advantage in flats and small houses where space is at a premium. A combi might free up your airing cupboard for additional storage space.

Combi boilers can improve your efficiency and reduce your fuel bills

Unlike conventional boilers, combis don’t produce and store hot water. They only heat the water needed by your household.

With a conventional boiler, hot water tends to be produced twice a day, even if it’s not needed. If it doesn’t get used, this hot water cools down, wasting energy.

Since combis only produce hot water on demand (when you turn a hot tap on) so this can mean reduced energy bills. 

Combi boilers provide hot water on demand

With no water tank to heat up, combi boilers can provide an unlimited supply of hot water on demand.

This is ideal in busy homes where showers may be used back-to-back each morning. It also means no more waiting for water to heat up for your bath, a combi is ready when you are.

Combi boilers are less likely to suffer from pressure problems

Combi’s take their water directly from the mains, so they’re less likely to suffer from air locks or low pressure.

What are the drawbacks to a combi system?

Whilst there are many benefits to having a combi boiler, they’re not right for everyone.

Combis are best suited to homes with relatively small hot water demands at any one time.

If you’re a household where everyone wants showers at the same time, then a combi won’t be suitable. In this case a regular or system boiler might be your better option. 

You could also ask us about storage combis!

What size combi boiler do I need?

Choosing the correct size of boiler is vital to ensure that your boiler runs efficiently and keeps your fuel bills as low as possible.

This is such an important topic, and it’s often done incorrectly, so we’ve written a whole blog about it! You can read it here.

But rest assured that The Heating People conduct a heat-loss calculation as part of your free survey. This will ensure that we match you with your perfect boiler.

How much do combi boilers cost?

Every home has different needs, which means that there is no single answer to this question. This will depend on your current set up, and the boiler manufacturer that you choose.

The Heating People can offer new combi boiler installation and combi boiler replacements with guarantees of up to 14 years.

As we are not tied to any one manufacturer, we can offer new boilers from all the major manufacturers including Worcester Bosch, ATAG and Vaillant.

As a guide, replacing an existing combi boiler will cost approximately £1800-2500; whereas upgrading from a regular system typically costs £2200-£3500.

How long does it take to install a combi boiler?

This really depends on a range of factors, including the type of boiler you have now, and if it needs to be moved to another room.

However, a typical installation usually takes between 1-3 days to complete.

We hope that you found this blog helpful. For tailored advice on whether a combi is right for you, contact us to book your free survey.

You might like our related blog: Pros and cons of combi boilers.

Useful Links:

Government factsheet on Boiler Plus

Vaillant combi boiler range

Worcester Bosch combi boiler range

ATAG combi boiler range