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Witten by Bernard Morgan (Engineer).
First Published: December 15, 2023. Updated: February 8, 2024

Changing to a Combi Boiler

Are you thinking about changing your current boiler for a new combi boiler? Has it broken down, or is it burning through gas because it’s become inefficient?

When replacing an old boiler, there are many options, but we’ll stick to combi boilers in this guide. Whether you know a lot about boilers or hardly anything, we’re here to help. A conversion from a back, system or regular boiler to a combi boiler is pretty simple.

Replacing your boiler

There are many reasons to replace a boiler:

  • it’s broken down
  • it keeps needing to be reset
  • the heat exchanger needs to be replaced
  • it’s inefficient and costing you money

There is never a good time to have to convert your system to a combi. Especially in winter. But it’s a simple process that heating engineers perform all the time. Here at Boilerhut, we convert heating and hot water systems to combis boilers every day.

In this guide, we’re going to get right into the details. Why move to combi, is it worth it, what are the benefits and negatives, how long would it take and how much will it cost. Because we’re going to get detailed we’ve added a contents with links to specific sections. This will help you find the information you are after more.

This video shows the process of installing a combi boiler in place of an open vented system.

Why change to a combi boiler?

Combi boilers aren’t new to the market, the first was released in the mid-1960s. Combi boilers are fantastic options for a lot of properties across the UK. But they won’t suit every home. They’re not great for large homes, for example. This is because hot water is heated on demand, and there will be a limit to the water flow rate. Storage combi boilers may be an option if a standard combi wouldn’t suit your needs.

One of the most important reasons for changing to a combi boiler is energy efficiency. The Energy Saving Trust explains you could save 40% on your bills by upgrading an old boiler. Over time, thermal stress and corrosion cause losses in boiler efficiency. This could mean having a boiler running at 50% efficiency.

That means you may be losing £50 of every £100 you’re spending on your energy bills.

Changing to a combi boiler can help save you money. All new boilers have to be over 90% efficient. If you are swapping a combi for a combi boiler, or changing from system or regular there could be other savings too. For example, combi boilers don’t need an external hot water tank.

Some premium combi boilers like the Viessmann Vitodens 200-W are 98% efficient. Learn more about Boilerhut boiler replacement >

Benefits of changing to a combi boiler

  • High energy efficiency for condensing combi boilers; 90 – 98%+.
    • Flagship models from top UK boiler brands are 92% to 96% efficient.
    • Premium, high-end boilers like Viessmann 200-W are 98%+ efficient.
  • Great Space Saving Solution.
    • No need to have a cylinder or attic tanks.
    • More room in airing cupboards and attics.
    • Perfect central heating solution for flats and small houses.
    • Compact combi boilers are also available, which can fit inside a kitchen cupboard.
  • May be more energy efficient than conventional boilers in some circumstances.
  • Regular and System boilers produce hot water whether it’s used or not.
    • If the hot water isn’t used, it cools down, wasting energy.
    • Combi boilers produce hot water on-demand, i.e. only when it’s needed.
  • Storage combi boilers are available for larger properties with 2 showers.

Is it worth changing to a combi boiler?

The short answer to that question is it depends on your and your property’s needs.

Whether you’re changing to a combi, or replacing one, it’s essential to consider the costs over time, not upfront.

Changing to a combi boiler gets rid of attic tanks, hot water cylinders and extra pipework. They also run the heating and hot water on demand, so you don’t need a tank in your attic. Also, old tanks may not be well insulated, creating a large, inefficient radiator.

So, if your hot water needs aren’t that demanding a combi boiler may be perfect. If you think you will need to run many showers and hot water taps a system boiler may be more appropriate. But there is a middle ground; a storage combi boiler.

You can find out more about storage combi boiler in our handy guide.

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What is involved in changing to a combi boiler?

Once you’ve decided, it pays to be aware of what is involved in changing to a combi boiler, so what’s the process? Below, we detail what is involved in changing to a combi boiler.

Changing to a Combi Boiler – Installation Steps

Once the Gas Safe installers are at your property, they will perform the following steps – not all steps are required in all circumstances:

  • Drain existing heating & hot water system.
  • Remove existing/old boiler.
  • Remove existing thermostat and controls.
  • If you’re switching from open vent/regular to combi, remove cylinders and attic tanks.
  • Convert heating system to combi.
  • Upgrade gas run to 22mm.
  • Conversion to combi can sometimes add an extra day.
  • Install the new combi boiler.
  • Perform a chemical/power flush on the entire heating system.
  • Install deaerators and/or magnetic filters.
  • Install thermostat. Not sure which thermostat is best? Check out our heating controls.
  • Configure smart thermostats to work with the new boiler.
  • Register extended warranties.
  • Commission and test the boiler.
  • Supply building regulation notification with certification.
  • Hand over & induction on boiler, controls and best use.

How hard is it to change to a combi boiler?

Changing to a combi boiler is something our engineers do very regularly. It can be difficult, but it’s also pretty routine. It can be time-consuming, depending on the boiler system you’re changing from.

New gas connections will also have to be made for any gas boiler conversion, which means only Gas Safe registered engineers can legally do it. If you try to do it yourself or get a plumber without the proper certification to do it without the required tools, you can face a heavy fine or even prison time.

For any gas boiler conversion into combi, there are always steps like removing a hot water tank that you’ll need to be aware of. Because of this, it’s always best practice to have Gas Safe Registered engineers who are experienced in Combi Conversion to do the job for you.

There are a couple of more reasons why:

  • Gas Safe registered engineers spend 5 years studying and hit their true level of experience in 5-10 years.
  • Flue gas analysers, which cost between £600 and £1000, are required by law.
  • Flues gas analysers need to be calibrated annually and hold a certificate.
  • The suitability of the combi boiler for the property must be checked.
  • Boiler siting needs to be safe and comply with Gas Safe and building regulations.
  • Manufacturer specified appropriate materials and fittings have to be used.
  • Equipment like manometers test the gas supply and make sure the pressures are safe.
  • Gas integrity needs to be tested prior to connecting the boiler. And again, once connected.
  • Let-by test & tightness tests need to be carried out.
  • Water flow rates need to be checked and adjusted.
  • Gas Safe Engineers must rate the boiler and check gas pressures and flue integrity.
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How much does it cost to change to a combi boiler? / Combi boiler prices

Installation costs can vary depending on what type of boiler you’re changing to a combi boiler from.  So what needs to be done to work out the replacement of the conventional boiler with combi cost? Below, you can see the average costs for the most common combi boiler conversions:

PLEASE NOTE:  These are approximate costs. Include installation and extras listed in the installation section earlier in the post.  We’ve given an approximate cost range because these costs can vary greatly depending on many factors, including the type of boiler (whether low-end, mid-range or high-end flagship boilers).

Open vent/regular/conventional boiler to new combi boiler (conversion needed)

Regular to CombiAverage CostTime Taken to Install
Remove Hot Water Storage Cylinder and Attic Water Tanks and Replace Open Vent Boiler with a Combi.£2,500 to £3,3002 Days

Back boiler to combi boiler (conversion needed)

Back Boiler to CombiAverage CostTime Taken to Install
Decommission Back Boiler, Leave in Place or Cart Away. Remove any Water Cylinders or Attic Tanks. Install new Combi Boiler.£2,600 to £3,900 inc. VAT2 to 2.5 Days

System boiler to combi boiler (conversion may be needed)

System to CombiAverage CostTime Taken to Install
Remove Hot Water Storage Cylinder. Convert to Combi. Install a New Combi Boiler£2,600 to £3,400 inc. VAT2 Days

How much to change from a water tank to a combi boiler? (conversion may be needed)

Regular/System to CombiAverage CostTime Taken to Install
Remove Hot Water Storage Cylinder. Convert to Combi. Install a New Combi Boiler.£2,600 to £3,400 inc. VAT2 Days

To get a more accurate price estimate for your property size and other options, our intelligent quoting tool can help you. Our online boiler quote is free, you are under no obligation to buy, and the whole process takes a few minutes to complete. Simply enter your postcode below and hit the green button.

What’s next?

The easiest thing is getting a quick quote using our online tool. It will ask you between 10 and 16 questions about your property and current setup. Next it’ll ask about what you’d like: combi or system etc. It will then suggest up to three options for you.

Getting a couple of quotes for any big job like this is good practice. We know we’re competitive, especially with the premium Viessmann Vitodens range.

Once you have an online quote, we’ll call you to discuss the finer details. This will include:

  • The flue
  • The gas pipe and meter
  • Where you want it
  • If you want a smart thermostat
  • If what you’ve selected is the best option for you…an online system can only do so much.
  • And some other details.

Our in-house heating experts can help you with every part of the process. Once you’re happy with the details, we’ll send you the full & final quote. There is no need to visit until you decide you want Boilerhut to install this boiler.

If you do, we arrange payment options and an installation date. We get busy in the background arranging all the parts required for the work.

Then, you just need to wait for the day of the installation.

More Information About Combi Boilers

< Back to the main combi boiler installation and details page

REVIEW: Viessmann Vitodens 111-W Storage Combi >
How do combi boilers work? >
Combi vs Conventional boilers >

Further Reading…

Types of boilers explained >
Condensing or Combi – A full guide >
Pros of upgrading to a high-efficiency boiler >
Understanding boiler efficiency >

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Save up to £590 PER YEAR on your heating bills.
Quote in 30 seconds. No obligation. Next Day installation.
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Frequently asked questions

Existing combi to new combi (straight swap – no conversion)

NEW COMBI INSTALLATION AVERAGE COST TIME TAKEN TO INSTALL
Combi to Combi Swap Same Location £2,100 to £2,600 inc. VAT 1 Day
Combi to Combi Swap New Location £2,300 to £3,100 inc. VAT 2 Days

If you already have a combi boiler, you don’t need to convert any part of your system. Read more about combi boilers and replacing them >

Yes. Old boilers are usually very inefficient meaning they waste gas which isn’t heating your water or home. Replacing the boiler is a great way to reduce heating costs and carbon emissions.

No. The UK government advised they wanted to stop gas boilers being installed. But this is for new-build properties only. They want to try and increase the number of air source heat pumps in homes across the country. But, regarding older homes air source heat pumps are very unlikely to be suitable. The government won’t be able to ban gas burners for many, many years.

Typically, a boiler can last about 10 to 15 years with proper maintenance, and while annual servicing can extend its life, an ageing boiler often requires more effort to heat your home effectively. If your boiler is due for replacement, it might lead to increased costs due to diminished energy efficiency, and there’s also a small risk that it may no longer be safe.

This will depend on how old the current boiler (of any type) is.

  • Old non-condensing boilers will require plastic condensate pipes to a drain.
  • Gas pipes need to be 22mm in diameter.
  • In most cases when installing a new boiler a new flue will also need to be installed.
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