changing to a combi boiler

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.  Especially if they have a larger property and require hot water simultaneously from multiple outlets.

But we’re getting close to 2020, and these days there are solutions for every problem as we’ll discover later in this post.

Before we delve deeper, if you’re completely brand new to the world of combi boilers, we’ve done posts on our blog in the past explaining what are combi boilers, and how they work.  Please click the following links to read those posts in detail:

What is a Combi Boiler

How a Combi Boiler Works

To get an online boiler quote for changing to a combi boiler, simply enter your postcode below and hit the green button:


Is it Worth Changing to a Combi Boiler?

The short answer to that question is it really depends on your needs.  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.  This frees up a lot of space in your house.  And 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 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

changing to a combi boiler

  • Higher Energy Efficiency for Modern Condensing Combi Boilers.
  • Great Space Saving Solution. No need to have a cylinder or attic tanks.
  • More room in 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.
  • Storage Combi Boilers available for larger properties with 2 showers.
  • Compact combi boilers also available which can fit inside a kitchen cupboard.

What is Involved in Changing to a Combi Boiler

Once you’ve made your decision of changing to a combi boiler, what are the next steps?  It helps to get an online boiler quote on the website of a respectable boiler installation company.  Online boiler quotes are free, no obligation, and take a few minutes, giving you at least a rough estimate of what your boiler installation might cost.  Rough estimates can then be confirmed the same day by having a chat with a heating expert.  You can get a quote using our intelligent quoting system by hitting the green button below:


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.

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.
  • 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.

This video below shows a brief overview of what’s involved with changing to a combi boiler:

How Hard is it Changing to a Combi Boiler?

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.  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 heavy fine or even prison time.  So, 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.
  • 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.

Changing to a Combi Boiler Cost

Installation costs can vary depending on what type of boiler you’re changing to a combi boiler from.  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 because 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 InstallationAverage CostTime Taken to Install
Combi to Combi Swap Same Location£1700 to £2200 inc. VAT1 Day
Combi to Combi Swap New Location£2200 to £2500 inc. VAT2 Days

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

Regular to CombiAverage CostTime Taken to Install
Remove DHW Storage Cylinder and Attic Tanks and Replace Open Vent Boiler with a Combi.£2200 to £25002 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 Cylinders or Attic Tanks. Install new Combi Boiler.£2400 to £3000 inc. VAT2 to 2.5 Days

System Boiler to Combi Boiler (Conversion May Be Needed)

System Boiler to CombiAverage CostTime Taken to Install
Remove DHW Storage Cylinder. Convert to Combi. Install a New Combi Boiler£2000 to £2450 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.


PLEASE NOTE:  The price you’ll get through the quoting system for changing to a combi boiler is a rough initial estimate.  Always check with our team to confirm prices and see if we have any offers available.  You can call us on 029 2009 9898 or email us at sales@boilerhut.co.uk.

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