A guide to combi boilers: how much they cost and how they work

11 February 2021 | Aimee Tweedale

If your old boiler has packed up and you’re looking for a replacement, you may be feeling overwhelmed by all the different types of boiler out there.

The combination boiler – better known as a ‘combi’ for short – is the most popular type of gas boiler in the UK1. Consider it the boiler equivalent of cups of tea, or fish and chips!

A couple dancing by their radiator in the kitchen

So, to make your decision a bit easier, we’ve put together this complete guide to combi boilers. Keep reading to find out what a combi boiler is, what you’ll pay to have a new one installed, and how much it might save you in the long run2

For more on conventional or system boilers, read our guide to different boiler types.

And for a comprehensive look at condensing boilers, read this one. 

What is a combi boiler?

So, what even is a combi boiler? A combi boiler is a system that provides both heating and hot water directly from one boiler – hence the name, ‘combination boiler’. 

System boilers and conventional boilers – the old staples – usually store hot water in a cylinder or tank, but combi boilers are much smaller and more efficient. They’re linked to your mains water supply, and so can heat up the water as soon you turn on your tap. It’s no wonder they’re the most popular choice of boiler in the UK: they’re small but powerful.

When we talk about a ‘combi boiler’, we’re generally talking about one using natural gas, as most homes are connected to the gas grid. For the roughly 4 million homes that aren’t, many use oil boilers or electric heating systems. Find out more about those in our complete guide to oil central heating, and learn more about the pros and cons of gas, oil, and electric heating systems.

How does a combi boiler work?

While system and conventional boilers will hold on to a supply of hot water, combi boilers work by instantly heating up water from the mains supply when you need it. 

This makes them the most efficient kind of boiler, because they’re not constantly using energy to heat a tank full of water. Instead, they just use energy when you need it – which is good news for your carbon footprint, and your energy bills!

How do combi boilers heat water? 

Most combi boilers have two devices called “heat exchangers” inside them. One is linked to your central heating, and keeps your radiators hot. The other is linked to your mains water supply. When you turn on a hot tap, a sensor is activated inside the boiler which tells it to burn fuel, heating up the heat exchanger. Voila: hot water!

The boiler might also keep a very small amount of water heated in reserve, to make sure you can always get that instant warmth when you need it.

Worcester Greenstar combi boiler

What are the benefits of getting a combi boiler?

There’s a reason combi boilers are the most popular central heating system in Britain – in fact, there’s a few! Read on to find out the biggest benefits of combi boilers, and why they might not be for everyone.

Advantages of combi boilers

  • They’re highly efficient. Combis use a lot less energy than older models, because they’re not constantly heating a hot water tank. In fact, replacing your old boiler with a combi could save you as much as £300 per year in bills3. To get the most out of your central heating, read our guide to making your home more energy-efficient, and how to set your boiler timer.
  • Combi boilers cost less. Get more details on exactly how much a combi boiler usually costs below.
  • They give you hot water on demand. Just turn on the tap, and you’re ready to go!
  • They’re space-savers. Unlike other types of boiler, combis don’t need extra space in your airing cupboard or attic to store hot water. They’re compact and sleek. 
  • Combis are cleaner, too. Cutting out the hot water tank means there’s less potential for sludge and other nasties to build up in your heating system.

Disadvantages of combi boilers

  • They can’t keep up with the hot water demand in big houses. If you have multiple bathrooms, and family members who are likely to shower at the same time, then a combi boiler probably won’t be powerful enough for you. 
  • They need strong water pressure. Combis rely on mains water pressure to move hot water around the house. If your water pressure isn’t that great, you might want to consider a different type of boiler.

Want to keep warm in a more eco-friendly way? Check out our guide to low carbon options for heating your home

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What’s the cost of a new combi boiler?

According to the Energy Saving Trust, the total cost of getting a boiler replacement is usually in the region of £2,3004. But the good news is that replacing your old boiler with an up-to-date system could save you hundreds per year in energy bills. 

Combi boiler prices usually start at around £600-£900 for a budget model. If you’re looking at more premium models, you could pay up to £2,000.

But that’s just the cost of the boiler itself. You also need to think about the price tag that comes with getting it installed.

Combi boiler installation costs

The overall cost of getting a new combi boiler, with installation included, can go up to around £3,5005. It all depends on factors like:

  • The brand and model of boiler you choose
  • What type of boiler you’re replacing
  • Whether you want to move your boiler
  • Any extras you would like (such as smart features)
  • What floor the boiler will be installed on
  • Your location

Let’s take a couple of example scenarios:

Say you’ve got a pretty modern flat, where you’ve already got a gas combi boiler which needs replacing. It might cost you £880 for the boiler, £600 to install it, and another £150 to add a magnetic filter (to keep your boiler squeaky clean). That brings your total to £1,630 – which is not bad going!

At the other end of the spectrum, let’s say you live in a bigger house, and you want to get a combi boiler installed, but in a different place to where your current boiler is. You choose a £1,000 boiler, and you pay £1,100 to get it installed in a new location. You might even splash out on some extras, like extra pipework for £300. That brings your total to £2,400.

These two examples are just estimates – but they give you an idea of how boiler installation prices can vary so much. When you’re thinking about replacing your boiler, always make sure you shop around, to get the best quote for you.

Read more about new boiler costs and how to pay

Children play games next to the radiator

How much money could replacing an old boiler save me?

In the graph below, you can see an overview of the savings you could make by switching to a more efficient boiler. These estimate figures are courtesy of the good people at the Energy Saving Trust

How much you could save by replacing an old boiler

As you can see,  the less efficient the old boiler is (G is the lowest rating) and the larger the house is (detached homes need the most heating), the more savings you could make.

For example, changing from an E-rated boiler to a new A-rated combi in a mid-floor flat only saves £55 a year on gas. Replacing an F-rated boiler in a detached house, however, could save £210. 

When considering how much money you’ll save, it’s also important to consider the brand of boiler you’re choosing. Make sure to stick with a reliable, reputable brand – because you won’t save money if you have to keep shelling out for repairs! Head over to Which? to read reviews of the major boiler brands – including the 4 biggest names, Worcester Bosch, Vaillant, Baxi, and Ideal – and find a combi boiler that’s right for you.

When to get a new boiler: 5 signs your old boiler is due a replacement

  • Your energy bills are slowly but surely rising. If your boiler is past its best days, it will be less efficient, which means it uses more energy to get the same results. This means you’ll be burning through a lot of fuel, which doesn’t come cheap – so you could likely save yourself a lot of money in the long run by replacing it.
  • Your heating is constantly cutting out. If your boiler is breaking down every week, the repair costs are likely to soar (along with your frustration). This is a sure sign that it’s time for a new boiler. Read here to find out more about common boiler problems and how to fix them.
  • There’s a leak. Puddles of liquid appearing around your boiler? It’s likely to have a faulty part. Leaks can cause damage to your home and pipework, so it’s important to get it fixed or replaced, pronto.
  • Your boiler or radiators are making strange noises. Central heating systems can make all kinds of noises; often, if you hear tapping or ticking, it’s a sign that your radiators need to be bled. But if you hear banging, gurgling, or any other strange sounds from your boiler, you should call for the help of an engineer.
  • Something stinks. If there’s any kind of unusual smell coming from your boiler, it could be a sign that it’s leaking gas. Faint smells could also be a sign of a carbon monoxide leak. (Carbon monoxide itself doesn’t smell, but it could cause a smell by stopping your boiler from burning properly.) Either way, this is really dangerous – make sure it’s switched off, and call the emergency gas line on 0800 111999. Other signs of carbon monoxide leaks include soot, condensation, and your blue flame turning yellow. Find out more in our guide to carbon monoxide and how to keep safe.

Time for a new boiler?

You can get an A-rated, award-winning boiler from CORGI HomeHeat (part of the OVO family). Want to know more? Visit our boilers page to check out the offers from CORGI HomeHeat. Don’t leave it to guesswork – get a new boiler installed safely by local engineers. 

Would you like to save hundreds on your energy bills, and cut back on carbon emissions, too? 

Switch to OVO today, and not only will you get competitive rates, but we’ll give you 100% renewable electricity6, and plant a tree for every year you’re with us.

Sources and references

2  Our website offers information about energy-efficiency solutions, but not personal advice. It is intended to provide information and guidance only. You assume responsibility for any decisions made or actions taken based on the information provided.



6 The renewable electricity we sell is backed by renewable certificates (Renewable Energy Guarantee of Origin certificates (REGOs)). See here for details on REGO certificates and how these work.  

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