condensing combi boiler

A condensing combi boiler is one of the highest efficiency boilers you can buy in the UK at the moment.

This is the most complete guide on the internet when it comes to a condensing combi boiler.

If you’re still confused between the two, let us make it easier for you.

First things first:

All boilers manufactured in the last 10 years are condensing boilers.

So, whether you buy a boiler from us or from any other manufacturer/heating company, you can rest assured that you will be purchasing a condensing combi boiler.

To learn more, pick from any of the topics below:

What is a Combi Boiler?

what is a combi boiler

Combi is short for Combination

Unlike Open Vent Boilers (also called regular or conventional boilers), combi boilers do not require cold water tanks in the attic space or hot water cylinders elsewhere in the house.  Sealed System Boilers don’t require cold water tanks in the attic, but still require a hot water cylinder for DHW (Domestic Hot Water) storage.

A Condensing Combi Boiler combines high-efficiency water heating and central heating within one compact unit.  It heats your drinking water on-demand, so you don’t need to store water in a tank.

Furthermore, these combi boilers deliver hot water through your taps or showers at mains pressure.  So, if your mains water pressure is good enough, you can enjoy hot showers without the need of any extra pumps.

If you are new to the world of combi boilers, we’ve published easy-to-understand guides on our blog which may help:

What is a Combi Boiler?

How a Combi Boiler Works

Which boiler is best when it comes to combi boilers?  This post may help: Best Combi Boilers in the UK

How does a Combi Boiler Work?

Whether your boiler is running on Mains Gas, Oil or LPG, the fuel enters a sealed combustion chamber inside the boiler and an electric ignition sets it alight.  The primary heat exchanger within the boiler then heats incoming water up to 60 degrees, which then flows through all the radiators in your home, which is how central heating works.

how a combi boiler works, combi boiler diagram

A condensing combi boiler combines home heating and on-demand hot water within the same compact unit.  In direct contrast, Open Vent (also called Regular, Standard or Conventional) boilers and System Boilers heat water that’s stored in a tank.

As you can see in the cross-section diagram of a condensing combi boiler above:

They have two heat exchangers inside them;

  1. A primary heat exchanger. (For central heating)
  2. Secondary plate heat exchanger. (For hot water)

The primary heat exchanger heats and recycles the water that goes through the pipes into the radiators.

The secondary plate heat exchanger uses that returning dirty hot water from radiators, and heats up sealed pipes with drinking water that goes into your taps.

Interested in a combi boiler?

Other Types of Boilers

Conventional/Regular/Standard/Open Vent Boilers

which boiler is best 2019, conventional boilers

These are also called standard boilers.  The technically accurate term is “Open Vent Boilers”.  Conventional or standard boilers require a cold water tank in the loft, and a hot water cylinder elsewhere in the house.

If you have a large house that has more than 2 bathrooms, and hot water may be called simultaneously, a conventional boiler may suit you best.

Sealed System Boilers

which boiler is best, system boilers

The heating and hot water components are built into the boiler.  It needs a pressurized unvented cylinder for storing hot water.  Unlike a conventional boiler, you don’t need a cold water tank in your loft or attic.

Interested in a conventional or a system boiler?

How does Condensing Technology Work?

All boilers manufactured in the last 5-10 years are condensing boilers.

A condensing combi boiler (or any condensing boiler) recovers more heat by recycling exhaust gases (mostly hot steam with some acidic content), which older non-condensing boilers used to waste.  Hot steam contains energy, and in a modern condensing combi boiler, the hot flue gases go through a heat exchanger first before they exit through the flue outside.

The diagram below explains how a condensing boiler works:

condensing combi boiler

In fact, as per the latest regulations around this, titled Boiler Plus Legislation, every new boiler installation in England or Wales needs to be a condensing boiler.

Whether it’s combi, open vent or sealed system.

So, whether you buy a Worcester Bosch or Viessmann, or Vaillant or Baxi or another brand of your choice, you can be rest assured in the knowledge that you are buying a condensing boiler.

Unless someone convinces you to buy a 15 year old boiler, I think we’re safe here.

When Should You Switch to a Combi Boiler?

When you want to Save Space

Whether you’re changing from a Regular/Open Vent/Conventional setup, or a Sealed System setup, a condensing combi boiler doesn’t need any attic tanks or hot water cylinders to store or heat water.  That frees up extra space in your home.

When you want to Save Energy

Because a condensing combi boiler heats water on-demand i.e. only when you need it, there is potential money to be saved in terms of saved energy.  With a hot water cylinder, whether you use that water or not, it eventually cools down and needs to be reheated to be ready for use, at least twice every day.  While that is convenient and a nice-to-have, it’s extra energy getting wasted.

When you want to Save Money

According to the Energy Saving Trust, replacing a boiler that’s older than 12-15 years 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.  A brand new condensing combi boiler is above 90% efficiency, can get to 94% with programmable and learning thermostats along with weather compensation.  Premium combi boilers like the Vitodens 200-W are between 98% to 100% efficient!

That extra £30 to £50 saved over the short term can result in huge savings over the long term.

When you want to be Environmentally Responsible

condensing combi boiler, be environmentally responsible

Changing to a condensing combi boiler, especially a modern, A-rated condensing combi boiler can help save not only your hard-earned money because of higher efficiency, but also help reduce your carbon footprint and leave a cleaner, greener planet behind for your future generations.

While it’s not destroying our planet on an extreme level like animal agriculture, it’s still a step in the right direction.

Which Boiler is Best for Your Property?

It is important that you choose the best boiler for the size of your property before installation.  Whether that’s a combi boiler or not is not important.

We’ve done an extensive, factual and in-depth article on this topic on our blog.  You can read it in full here: Which Boiler is Best

  • Flat/1-2 Bedroom House (1 Bathroom)
  • Combi Boiler
  • Storage Combi Boiler
  • Conventional Boiler
  • Sealed System Boiler
  • Semi Detached House (3 Bedrooms & 2 Bathrooms)
  • High Output Combi Boiler
  • Storage Combi Boiler
  • Conventional Boiler
  • Sealed System Boiler
  • Mid or End Terraced House (3 Bedrooms & 2 Bathrooms)
  • High Output Combi Boiler
  • Storage Combi Boiler
  • Conventional Boiler
  • Sealed System Boiler
  • Large Detached House/Bungalow (4 Bedrooms & Multiple Bathrooms)
  • High Output Storage Combi Boiler
  • Conventional Boiler
  • Sealed System Boiler
  • Sealed System Boiler
Need a new boiler?

What is a Storage Combi Boiler?

condensing combi boiler

Storage Combi Boilers are a new innovation in the world of combi boilers for larger properties.  They’re larger combi boilers with a water storage tank contained within the unit itself.

An example of a storage combi boiler is the Viessmann 111-W Storage Combi which houses a 46 litre stainless steel thermal store within the boiler itself.  This is a great option, because even if you call for hot water, you still get it at mains pressure in two bathrooms.

It’s prudent to communicate everything about your property to the heating engineer from the boiler installation company prior to installation.

Condensing Combi Boiler Efficiency

combi boiler efficiency

55% of what you spend in a year on energy bills is spent on heating.

According to the Energy Saving Trust:  “Heating accounts for about 55% of what you spend in a year on energy bills, so an efficient boiler makes a big difference.”   The biggest reason for modern combi boiler efficiency in 2018 is down to condensing boiler technology.

How is Efficiency for a Condensing Combi Boiler Calculated?

“Seasonal Efficiency of Domestic Boilers in the UK” or SEDBUK is used to compare and classify boilers on energy efficiency.  Combi boilers are measured on how good they are at converting fuel to heat.

There are two different types of SEDBUK ratings on labels:

SEDBUK 2005 – Rates boiler efficiency from A to G.  A-rated boilers are above 90% efficient.

SEDBUK 2009 – This is a newer system which shows combi boiler efficiency as a percentage instead of letters.  All new boilers should be rated using SEDBUK 2009, but you may still see older models which were tested under the old SEBUK 2005 system. A combi boiler which was A-rated 90% under SEDBUK 2005 will be 88% under the new SEDBUK 2009.  The difference is mainly down to differences in efficiencies between using a boiler to heat a home in the winter months, and only using it for heating water in the summer.  Boiler manufacturers display SEDBUK 2005 rating on their website and manuals because it gives them a higher figure for combi boiler efficiency.

The table below shows SEDBUK figures for Annual Energy Costs to heat your home for different types of Boilers:

Type of BoilerEfficiencySemi-DetachedDetachedFlat
Modern Condensing Boiler89%-92%£740£1050£450
Non-Condensing Boiler75%-78%£850£1210£580
Old Back Boiler (Light Weight)60%-65%£1020£1450£670
Old Back Boiler (Heavy Weight)50%-55%£1210£1720£790
Interested in a combi boiler?

Condensing Combi Boiler Reliability

Heat Exchanger

As we explained above, the primary heat exchanger of a combi boiler is responsible for central heating.  To heat all the radiators in the house quickly, the boiler blasts the primary heat exchanger with flames at around 700 to 900 degrees.  It’s like a furnace inside, which means continuous, extreme thermal stresses on the internal components of a boiler, especially the heat exchanger, on a daily basis, and almost 24/7 during winters.

Because of that fact, it is crucial that the primary heat exchanger is made of a reliable, durable and highest quality metal, or else it will corrode and lose its efficiency over time.

Best Metal Type for a Heat Exchanger

The best possible metal for a heat exchanger is a stainless steel and chromium alloy.  Stainless steel is extremely resistant to corrosion in a wide range of pH levels.  Whereas for aluminum, if the special manufacturer-recommended heat transfer fluids and inhibitors are not used to produce and maintain a narrow pH range, it corrodes at a rapid rate and gets damaged.

And if your boiler’s aluminium heat exchanger is damaged, you will void your warranty, because the manufacturer will cite improper use of fluids.

A replacement heat exchanger may cost anywhere between £400 and £500.

In direct comparison, stainless steel heat exchangers do not require special fluids, are compatible with plain clean water and propylene glycol (commonly available).

The graph below compares a stainless steel heat exchanger (in blue) with an aluminium heat exchanger (in red):

combi boiler reliability, aluminium vs stainless steel heat exchanger

As you can see above, due to better thermal conductivity, aluminium starts off at a higher output compared to stainless steel.  But between the first 1000 to 1500 operating hours, aluminium starts to decline rapidly.

In some extreme acidic or base environments, aluminum may corrode rapidly with catastrophic results.

Furthermore, the aluminium surface of your heat exchanger may be porous.  So, on top of high corrosion, clients have complained about aluminium heat exchangers getting blocked due to sludge, debris and durt.  No such problem with stainless steel.

Aluminium and stainless steel may start at similar efficiency and output, but over years of daily use stainless steel wins hands down.  There is no competition, and no real reason to buy a boiler with an aluminium heat exchanger, no matter how amazing the brand reputation is.

The only advantage aluminium has over stainless steel is that it’s lighter in weight and cheaper to produce.

Can you Detect Heat Exchanger Corrosion?

combi boiler reliability, aluminium heat exchanger corrosion

If your aluminium heat exchanger has corroded heavily and lost efficiency, you have no way of knowing, unless the boiler breaks down or you open the casing inside the boiler.  Which is NOT recommended at all, unless you are a Gas Safe registered heating engineer.  A corroded heat exchanger will give no warnings, and make no noise.  Heating and hot water will still work like normal.

But due to the corrosion, the boiler’s energy efficiency will go down, and your energy bills will start going up ever so slightly.  And that slight increase in lost efficiency = lost money compounds over time.

How much is a New Combi Boiler?

If you were to buy a combi boiler online through a plumbing merchant’s website, that’s the boiler-only price.  Occasionally there will be deals with free flues, or packages with flues and other extras.  There are plumbing merchants where you can buy complete “ready to install” boiler packages as well.  Then all you need to do is find a reliable, Gas Safe registered and accredited plumber to install it for you.  (Shameless plug incoming) Or find a boiler installation company like Boilerhut who will do everything for you.

Anyway, let’s look at Plumbnation for example, it’s an online plumbing merchant website.  If you searched Google for how much is a new combi boiler, chances are you will land on their website.  Some of the highest selling boilers are listed below with their price.  Please note, these are boiler-only prices.  They do not include extras or installation.

  • Viessmann Vitodens 050-W Combi Boiler (29kW) – £842.78 inc VAT
  • Worcester Greenstar 29CDi Combi Boiler (29kW) – £1,110.88 inc VAT
  • Viessmann Vitodens 100-W Combi Boiler + Flue (30kW) – £1,167.66 inc VAT
  • Vaillant ecoTEC Pro 30 Combi Boiler (30kW) – £1,083.29 inc VAT
  • Ideal Logic+ C30 Combi Boiler (30kW) – £889.90 inc VAT

Pros and Cons


  • You may get a much cheaper price compared to a quote from a massive energy company like British Gas.
  • Online boiler + local plumber may be slightly cheaper than a boiler installation company’s online boiler quote.


  • If the local plumber is not company trained/accredited, you will not get extended 10+ year warranties.
  • Without talking to a heating expert, you may end up choosing the wrong boiler for your property.
  • A local plumber may install it for cheap, but may not be available in case of a boiler breakdown.
  • A local plumber or small heating firm may also take weeks depending on availability.
  • No finance options available.
Interested in a combi boiler?

New Combi Boiler Installation Cost

If you see a website advertising a new combi boiler installation cost, for example, “£1695 inc. VAT for a new combi boiler on your wall!” that is possibly for the easiest new combi boiler installation cost, for example, swapping your old combi boiler with a new one.  In many cases, low prices may also be down to a cheaper, lower efficiency boiler with substandard parts.

Whichever boiler system you convert to combi from, below are the steps taken as standard for each installation:

  1. Drain existing heating & hot water system.
  2. Remove existing boiler, and any hot water cylinder and attic tanks.
  3. Controls will be upgraded to ones suitable for a combi boiler.
  4. Existing smart controls like Nest or Hive can be configured to work with the new combi boiler.
  5. All scrap to be carted away from the site and recycled.
  6. The central heating system will be converted to combi.
  7. The gas run will be upgraded to 22mm pipes for modern, condensing combi boilers.

Once all of that is done, the following will be installed:

  • Brand New, Modern Condensing Combination Boiler
  • Horizontal or Vertical Flue
  • Microbubble deaerator (if required)
  • Magnetic Filter
  • Limescale Filter
  • Boiler Plus Compliant Programmable Thermostat
  • OR Internet Enabled Learning Thermostat
  • North Facing Weather Compensation
  • Chemical Flush & Add an Approved Inhibitor
  • Associated Pipe Work to Complete Installation
  • Registration of Extended 7-10 Years Warranties
  • Commission & Test to MFI & Complete Benchmark
  • Building Regulation Notification with Certification including Landlord Certificates
  • Hand Over & Induction on Boiler, Controls & Best Usage

Below are the approximate costs for installation and conversion from different boiler setups to combi:

Cost to Convert Open Vent to New Condensing Combi Boiler

new combi boiler installation cost

With higher capacity combi boilers and storage combi boilers available now, this has become very common.  A decade or so ago we didn’t have powerful combi boilers, so the only option was to either get rid of attic tanks and convert to a system boiler or replace the old open vent boiler with a new one.  But now we have larger capacity combi boilers (from 35kw to 38kW) and storage combi boilers for larger properties with 2 bathrooms and higher demand for simultaneous hot water demands.

Average Installation Cost for Conversion from Open Vent to a Condensing Combi Boiler:

£1,800 to £3,800* inc. VAT

*Depending on the options you choose.

Average Installation Cost for Conversion from Sealed System Boiler to New Condensing Combi Boiler

new combi boiler installation cost

Pictured above is a Viessmann 100-W Sealed System Boiler and an Unvented Cylinder

Sealed System boilers don’t need to store cold water in expansion tanks in the attic/loft.  They only need the hot water cylinder for domestic hot water storage.

Average System to New Combi Boiler Installation Cost:

£1,800 to £3,800* inc. VAT

*Depending on the options you choose.

Cost to Swap Old Combi with New Condensing Combi Boiler

old boiler to new condensing combi boiler swap

Possibly the easiest and quickest form of combi boiler installation.  Because your central heating is already working around a combi setup, all the installer needs to do is take out the old combi boiler, and install the new one.  In some cases, the gas run will have to be upgraded to 22mm for a modern condensing combi boiler.

Average Old Combi to New Combi Boiler Installation Cost:

£1,600 to £3,500* inc. VAT

*Depending on the options you choose.

Cost to Convert Back Boiler to New Condensing Combi Boiler

new combi boiler installation cost

This may be the most complicated combi boiler installation because there is a lot of work involved.  The old back boiler needs to be decommissioned and either left in place or removed if you want it.  Removal may require a lot of make-good work.  This installation in most cases also connects to a vertical flue instead of the commonly used horizontal flue, which adds to the cost.

Average Back Boiler to New Combi Boiler Installation Cost:

£2,200 to £4,500 inc. VAT

*Depending on the options you choose.

Cost to Install a Full Central Heating System with a Condensing Combi Boiler

new combi boiler installation cost

If you’re installing a brand new central heating system around a combi boiler, the cost can be significantly higher compared to a simple combi boiler replacement.  A new central heating system installation includes radiators and TRVs (Thermostatic Radiator Valves) and Lock-Shield Valves.  Because it’s a brand new installation, installers don’t need to drain the existing system or remove anything.

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Average Central Heating New Combi Boiler Installation Cost:

£4,000 to £6,000 inc. VAT

*Depending on the options you choose. Includes radiators, pipe runs, gas upgrades etc.

Need a new boiler?

Should You Try and Install a Combi Boiler Yourself?


Please DO NOT try and install a boiler yourself.

New gas connections will 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.

Why does the UK law require you to do so?

  • 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.

Best Combi Boilers 2021 UK – Gas/LPG

Below is a list of not only the best condensing combi boilers in the UK, but most of these are also the most energy efficient combi boilers on.

1. Viessmann Vitodens 200-W

best combi boilers 2019, viessmann vitodens 200-w

This is the most powerful and highest efficiency condensing combi boiler money can buy in the UK right now.  There’s nothing else quite like it in the domestic boiler range in the UK from any of its rivals.  It’s also number 1 in condensing combi boiler list in Germany.

  • 98% Energy Efficiency. Almost all of the fuel used is converted into heat
  • 5 inch colour touchscreen. Easy to operate Vitotronic control unit.
  • Control via app and internet (with optional Vitoconnect 100 WLAN adaptor)
  • High Quality Inox-Radial Stainless Steel Heat Exchanger
  • Quietest boiler in the UK at only 34 decibels
  • Compact dimensions, only 450 mm wide
  • Optional remote fault notifications for Viessmann Trained Installers

2. Viessmann Vitodens 100-W

best combi boilers 2019, viessmann vitodens 100-w

This boiler bridges the gap between affordable combi boilers and the absolute premium.

For a full review of the 100-W, please click here.

  • High Quality Stainless Steel Inox-Radial Heat Exchanger
  • Backlit LCD Touch Screen for Easy Operation
  • Class A rated for Energy Efficiency
  • Available in outputs from 4.7 to 35 kW
  • Up to 10 year extended warranty available
  • LPG Conversion Kit Available

Definitely one of the best boilers available in the UK right now.  It sits right between the entry-level Vitodens 050-W and the high end, premium Vitodens 200-W.

3. ATAG iC Economiser Plus

best combi boilers 2019, ATAG iC Economiser Plus

This is a surprise entry on the list of best combi boilers 2019.  Very few people know about ATAG in the UK.  That’s because this is an unbiased list focusing mostly on technologically brilliant, energy efficient combi boilers.

  • 10 Year Warranty
  • Unique iCon stainless steel heat exchanger
  • Instant hot water with pre-heat facility
  • Built in Passive Flue Gas Heat Recovery Device

4. Vaillant ecoTEC Plus Combi

best combi boilers 2019, vaillant ecotec plus combi

Another big German brand operating in the UK for many years, manufacturing high quality, high efficiency, robust boilers.

The Vaillant ecoTEC Plus Condensing Combi Boiler features:

  • Stainless Steel Heat Exchanger
  • 5-year guarantee as standard, with 7 to 10 years available when you book a Vaillant Advance installer
  • Dimensions in mm (825 model): H 720 x W 440 x D 338
  • Works with LPG where required

5. Worcester Bosch Greenstar CDi Classic

best combi boilers 2019, worcester bosch greenstar cdi classic

Market leader in the UK for many years, but lagging behind in terms of technology, especially quality of internal components as the company has chosen to stick with aluminium.

Some features of their flagship Greenstar CDi range:

  • Simple boiler control knobs with clear temperature display
  • Larger two bathroom properties can be supplied by higher output options
  • Wide choice of controls enables greater comfort and economy
  • Instantaneous hot water, always available

6. Viessmann Vitodens 050-W

best combi boilers 2019, viessmann vitodens 050w

Winner of the Which? Best Buy Award for the second year running in 2018, this condensing combi boiler is comparable to the Worcester Bosch and Vaillant models, but cheaper.

  • Ultra Compact – Just 707mm in height
  • DHW output ratings of 5.9 to 35 kW
  • High quality stainless steel Inox-Radial heat exchanger
  • High efficiency – up to 97% (Hs) [gross cv]

7. Worcester Bosch Greenstar i

best combi boilers 2019, number 7 worcester bosch greenstar i

Worcester Bosch’s more affordable boiler.  This is in the price range of the Viessmann Vitodens 050-w which is one place higher on this list of best combi boilers 2019.  In some cases, it’s more expensive than the 050-w, which is crazy considering this is a lower quality boiler with an aluminium heat exchanger.

  • Wide choice of controls enables greater comfort and economy
  • Low energy pump reduces electrical usage
  • Hot water instantly heated from cold mains

Buy a Combi Boiler Online

Who Should You Buy a Combi Boiler from?

buy a condensing combi boiler online

It’s probably a good idea to look for online reviews.  But be careful as a lot of online “review collector” companies have popped up recently.  To our knowledge, two of the best review websites are Trustpilot and FreeIndex.

Click to read our Trustpilot reviews

Every review is verified by Trustpilot, and they won’t allow a company to remove negative reviews.  A negative review can be reported by a company, and Trustpilot will investigate with the customer before taking any action.

Need a new boiler?