Combi Boilers: The Ultimate Guide

What is a Combi Boiler?

Combi boilers combine two functions in one unit: hot water and central heating.

So, instead of having to require a hot water tank like you would with a system boiler, everything can happen within one compact unit.

Combi boilers only heat up water when you need them to, which is not only better for the environment but great for your energy bills.

How Does It Work?

A combi boiler never sleeps and is ready at a moment's notice. Whatever time of day, combi boilers are ready to provide hot water and heat. This is because, instead of storing heated water in a big tank, combi boilers heat it instantaneously.

So when you turn on the hot tap or go to run a shower, cold water will be sourced from the mains, run through the combi, and get heated up.

This is due to the heat exchanger, which is powered by gas, oil, or electricity. What was cold moments before is now hot water pouring out of the tap or shower.

What About Central Heating?

The process is very similar for central heating. The cold water quickly turns into hot water and is then sent through to the radiators. Once the water has lost its warmth, it then goes back to the boiler and gets going again.

Although combi boilers can do both, they can never do them both at the same time. This means they can send hot water to your tap or shower or to your radiators. They aren't able to handle both simultaneously.

That doesn’t mean you won’t have hot radiators while your shower’s running – it just means you won’t be able to turn the heating up until you’re finished in the shower.

How Are They Installed?

Fortunately, the installation of combi boilers is a pretty straightforward process.

You don’t need a cold water tank or a hot water tank fitted, plus a combi boiler comes with a lot less pipework than any other type of boiler.

We strongly advise not to fit a combi boiler yourself, unless you’re a qualified plumber. If it’s going to be powered by gas, it's better to get yourself an engineer who is Gas Safe registered to do the job for you.

Not only is this a much safer, more effective option but performing the installation yourself will invalidate your boiler's warranty.

What Are Condensing Combi Boilers?

It is actually a legal requirement that every new boiler in the UK has to be condensing, so it might help to know what that means.

All boilers have a flue (a small pipe on an external wall that all waste gases, such as nitrogen dioxide, go through) before dispersing into the atmosphere. But these gases actually still have heat that can be used.

Whereas a condensing boiler has a flue gas heat recovery system. Meaning the gases are snatched up before they have a chance to escape the flue and are used again to heat more water.

This is not only better for the environment but reduces your energy bill, too.

Gas Boiler Ban

It's worth noting that gas fired boilers are being phased out over the next few years. See our page on everything you need to know about the gas fired boiler ban.

Pros and Cons of Combi Boilers

Here we’ve listed the advantages and disadvantages of a combi boiler.

Pros of a Combi Boiler

No storage tank means that the water comes directly at mains pressure, so you can enjoy a power shower without having to buy an extra pump.

The compact nature of combi boilers means there’s more space in your house for you to utilise.

Without the need for hot water cylinders and cold water tanks, your loft will be a lot more spacious.

Combi boilers should be fairly quick to install, seeing as they have fewer parts and less pipework than other types of boiler. Its shorter installation time means it will be cheaper to install.

Whereas energy is wasted on keeping the tank of water permanently hot with other boilers.

A combi boiler will heat water only when it is required, using minimal energy, and helping you to save money on your bills.

Due to having very few pipes, combi boilers have little chance of pipes freezing.

Cons of a Combi Boiler

Combi boilers can’t deal with high water demands. This is because it draws water directly from the mains and can’t support multiple outlets at the same time.

So if you have two hot showers running, there’ll be a drop in pressure. Adding more or turning on the taps and the pressure will get less and less.

A house with lots of people and high water demand is not a good place for a combi.

However, there are solutions. You can install an accumulator tank which costs between £500-£1,000. This will store some extra high-pressure water and give your combi a helping hand on those more demanding days for hot water.

Another option is a mains booster that costs around £300-£500 and designed to receive incoming mains water and give it an added boost.

Combi boilers combine central heating with hot water, but they don't work in unison.

Whenever you turn on a shower or a hot tap, the central heating will pause and the combi boiler will focus on your hot water needs. Once the hot water stops, the central heating will kick back in again. Although not ideal, the pause will rarely last long enough to cause any real problems.

If a traditional boiler were to give up, you can normally use an immersion heater to provide a quick fix. Unfortunately with a combi boiler, that isn’t an option.

Seeing as solar panels usually need a hot water tank, and combi boilers don’t have one, unfortunately there isn’t much compatibility between a combi boiler and solar thermal technology.

How Much Does a New Combi Boiler Cost?

Typically new combi boilers cost around £700, although if you include the cost of installation it will push the total up to around £2,500. Take a look at the graph below for a general idea of combi boiler costs.

If you’re looking for a combi boiler that’s reliable and at an affordable price, consider the Baxi 200 Combi. It ticks all the boxes without breaking the bank. If you'd like to know about how much a boiler service costs read our guide.

All Baxi boilers are endorsed by Energy Saving Trust. The Baxi 200 Combi is efficient, easy to install, and can be operated with a smartphone or tablet.

Here is the data for a 24kW model.

The Best Combi Boilers in 2023

Based on key factors such as efficiency, power output, warranty, and price. We’ve listed the seven best combi boilers of 2023.

All information is from manufacturers’ official data sheets.

All models listed here (excluding the Potterton Assure) have 10 year warranties available, dependent on particular terms and conditions.

Combi Boiler Grants

If you’re unsure whether you can afford a combi boiler, the UK Government’s Energy Company Obligation (ECO) might be able to help you.

You could be eligible to have your old boiler replaced for free or at least subsidised, if you meet certain requirements.

You need to be living in social housing, and own a non-condensing boiler that’s a minimum of five years old.

For more information, you can take a look at the official government page to find out how to apply, although the scheme now only applies to gas boilers.

Summary

That’s it. That’s our guide on combi boilers and hopefully now you should have a better idea on whether a combi boiler is the right boiler for you.