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Witten by Bernard Morgan (Engineer).
First Published: September 4, 2018. Updated: November 28, 2023

Changing to a Combi Boiler

Changing to a combi boiler can be a walk into the unknown, especially if you’ve never had a combi boiler in your property before and you’re switching from an old back boiler, standard boiler, or a system boiler to a combi boiler.

Some clients may want to avoid changing to a combi boiler and stick with the known quantity, i.e. their comfort zone. This is especially true if they have a larger property and require hot water simultaneously from multiple outlets.

As well as choosing the right type of boiler, in order to replace the conventional boiler with combi cost must be considered as well. However, the cost for a combi boiler can also change depending on your previous boiler type. If you need to replace a hot water tank with a combi boiler, for example, your installers will have to do something about the tank in the attic as well, incurring additional cost.

If you’re new to the idea of ‘convert to combi boiler’, you’re asking ‘What are combi boilers?’ or you’re interested in what is involved in changing to a combi boiler, you can read more of our posts by clicking one of the links below!

What does this guide cover?

Is it worth changing to a combi boiler?
Why change to a combi boiler?
Benefits of changing to a combi boiler
What is involved in changing to a combi
How hard is it to change to a combi boiler?
How much does it cost to change to a combi boiler?

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 >

Is it Worth Changing to a Combi Boiler?

The short answer to that question is it really depends on your needs.

Whether you’re changing from conventional boiler to combi, it’s important to consider a combi boiler replacement as a new way for you to save money.

Changing to a combi boiler gets rid of attic tanks or hot water cylinders or extra pipework. Combi boilers generate heating and hot water on-demand, so you no longer need that hideous old boiler tank sitting in your attic.

This frees up a lot of space in your house. Because you don’t need to keep reheating the water in a DHW cylinder, changing to a combi boiler can also be more energy efficient in many cases.

So, if your hot water needs suit the combi boiler, there is really no reason against changing to a combi boiler. Even if you have a larger property, and you need your boiler to be able to handle high flow rates and hot water being called over two taps, there are storage combi boilers available now.

To read a detailed post on storage combi boilers, please click the following link: Storage Combi Boiler

Why Change to a Combi Boiler?

One of the most important reasons for changing to a combi boiler is energy efficiency. According to the Energy Saving Trust, replacing a 12-15 year older boiler with a new A-rated condensing combi boiler can save you up to 40% on your energy bills. Internal components of a boiler go through ridiculous thermal stresses on a daily basis. Over time this results in corrosion. Corroded parts can bring the efficiency of a non-condensing boiler down to 70% to 50% over years of use.

In monetary terms, that means you may be losing £30 to £50 of every £100 you’re spending on your energy bills. Changing to a combi boiler, especially a modern, A-rated condensing combi boiler can help save you that money because they are over 90% efficient. There are some premium combi boilers like the Viessmann Vitodens 200-W which are 98% efficient and above!

Another reason to consider a combi boiler replacement is you don’t need to store hot water, because combi boilers produce hot water on-demand. Whereas a regular or a system boiler stores hot water in a cylinder. And whether you use that water or not, it eventually cools down and the boiler has to fire up again to heat it. That is extra energy getting wasted.

Benefits of Changing to a Combi Boiler

Higher Energy Efficiency for Modern Condensing Combi Boilers.
High-quality internal components such as the primary heat exchanger.
Great Space Saving Solution. No need to have a cylinder or attic tanks.
More room in the airing cupboard and attic.
Combi boilers may be more energy efficient in some cases compared to conventional boilers.
Condensing technology makes modern combi boilers over 90% efficient.
Flagship models from top UK boiler brands are 92% to 94% efficient.
Premium, high-end boilers like Viessmann 200-W are above 98% efficient.
Perfect central heating solution for flats and small houses.
Regular and System boilers produce hot water twice a day, 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, and uses the diverter valve to easily switch between hot water and central heating.
Storage combi boilers are available for larger properties with 2 showers.
Compact combi boilers also available which can fit inside a kitchen cupboard.

Okay, you’ve got the quote, what next? It’s good practice to always communicate clearly your heating and hot water needs to a surveyor or heating expert you will be talking to on the phone.

You will answer property size related questions going through the quoting tool anyway, so it should pick the most suitable boiler for you.

If not, our in-house heating experts can always make that decision easy for you. Once you’ve agreed to the combi boiler installation, and agreed to a date, your next steps are simply to wait.

What is Involved in Changing to a Combi Boiler?

Once you’ve made your decision, it pays to be aware of what is involved in changing to a combi boiler, so what are the next steps? 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:

Drain Existing Heating & Hot Water System.
Remove Existing/Old Boiler.
Remove Existing Thermostat and Controls [If Needed].
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 Smart Wireless/Internet Enabled Programmable/Learning Thermostats. Not sure which thermostat is best? Check out our Nest vs Hive comparison article.
Configure Smart Thermostats to work with the new boiler.
Register Extended Warranties*.
Commission and Test to MFI and Complete Benchmark.
Supply Building Regulation Notification with Certification.
Hand Over & Induction on Boiler, Controls and Best Usage.

How Hard is it Changing to a Combi Boiler?

Changing to a combi boiler can be very difficult and time-consuming, depending on the boiler system you’re changing from. For any gas boiler conversion, new gas connections will also have to be made, which means only Gas Safe Registered engineers should 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 remove 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 & £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 needs to be checked.
Boiler siting needs to be safe and comply with Gas Safe & Building Regulations.
Manufacturer specified appropriate materials and fittings have to be used.
If you or your plumber are not Gas Safe Registered and make a new gas connection to a boiler = heavy fines or prison.
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 test.
Flow rates need to be checked and adjusted.
Gas Safe Engineers need to gas rate the boiler and check gas pressures and flue integrity.

How much does it cost to change to a combi boiler?

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 replace the conventional boiler with combi cost? Below, you can see average costs for the most common combi boiler conversions:

PLEASE NOTE: These are average, approximate costs including installation and extras listed in the installation steps section earlier in the post. The reason we’ve given an approximate cost range is that these costs can vary greatly depending on the type of boiler (whether low-end, mid-range or high-end flagship premium boilers).

Existing Combi to New Combi (Straight Swap. No Conversion)

New Combi Installation	Average Cost	Time Taken to Install
Combi to Combi Swap Same Location	£1700 to £2200 inc. VAT	1 Day
Combi to Combi Swap New Location	£2200 to £2500 inc. VAT	2 Days

Open Vent/Regular/Conventional Boiler to New Combi Boiler (Conversion Needed)

Regular to Combi	Average Cost	Time Taken to Install
Remove DHW Storage Cylinder and Attic Water Tanks and Replace Open Vent Boiler with a Combi.	£2200 to £2500	2 Days

Back Boiler to Combi Boiler (Conversion Needed)

Regular to Combi	Average Cost	Time Taken to Install
Decommission Back Boiler, Leave in Place or Cart Away. Remove any Water Cylinders or Attic Tanks. Install new Combi Boiler.	£2400 to £3000 inc. VAT	2 to 2.5 Days

System Boiler to Combi Boiler (Conversion May Be Needed)

Regular to Combi	Average Cost	Time Taken to Install
Remove DHW Storage Cylinder. Convert to Combi. Install a New Combi Boiler	£2000 to £2450 inc. VAT	2 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.