condensing or combi boiler

Condensing or Combi Boiler

We’ve had a lot of queries from clients asking about condensing or combi boiler, or what is the difference between a combi boiler and a condensing boiler.  To make it clear from the start, those are incorrect questions because all modern combi boilers are condensing boilers.  If your boiler has suddenly broken down after 20 years of daily use, and you’ve never had to shop around for a new boiler, we don’t blame you for being stumped at the moment and wondering whether to buy condensing or combi boiler.  Which is why we’ve written this in-depth, factual article and tried to answer all questions, to help you make an informed decision.

When people search condensing or combi boiler online, they probably mean “conventional” or combi boiler.  Conventional boiler or regular boiler are just different names for an Open Vent boiler.  Open Vent boilers have hot water cylinders and attic tanks.  Which is what most of the average semi-detached family homes in the UK will have.  Sealed System boilers are newer, and just have a cylinder for DHW (domestic hot water) storage.  Along with modern combi boilers, all modern Open Vent and Sealed System boilers are also condensing boilers.  Just to make it really simple, ALL boilers manufactured in 2018 are condensing boilers.

Before we get into the nitty-gritties, if you’d like a quote for a condensing or combi boiler and talk to a heating expert for any free heating advice, please enter your postcode below and hit the green button:


What is a Combi Boiler?

condensing or combi boiler

We’ve done an extensive post on this subject which you can read by clicking on this link: What is a Combi Boiler? It looks at pros and cons of combi boilers and conventional boilers.  We’ve also done a post on How a Combi Boiler Works.

To summarize, “Combi” is short for combination boiler.  Because it combines heating and hot water, you no longer need a separate DHW storage cylinder or attic tanks like you do for an Open Vent Boiler or a Sealed System Boiler.  A condensing or combi boiler has a primary heat exchanger for heating, and a secondary plate heat exchanger for hot water.  A basic combi boiler will suffice if your property has 1-2 bedrooms and 1 bathroom and there is no simultaneous hot water requirement from multiple taps.  Even if your property is larger than that and you need on-demand hot water from multiple taps simultaneously, there are larger capacity combi boilers and storage combi boilers available too.

What is a Condensing Boiler?

According to Wikipedia, “Condensing boilers are water heaters fueled by gas or oil.  They achieve high efficiency (typically greater than 90% on the higher heating value) by condensing water vapour in the exhaust gases and so recovering its latent heat of vaporisation, which would otherwise have been wasted. This condensed vapour leaves the system in liquid form, via a drain.”  

In simpler terms condensing boilers recover more heat by recycling exhaust gases (mostly hot steam with some acidic content), which older non-condensing used to waste.  It’s no rocket science that hot steam contains energy, as the inventor of the first ever steam engine found out.  So, in modern condensing boilers, the hot flue gases go through a heat exchanger first before they exit through the flue outside.

Condensing or Combi Boiler – Is that the Right Question?

All combi boilers in 2018 are condensing boilers.  However all condensing boilers in 2018 may not be combi boilers.  If you’re even more confused than before when you started to read this post about condensing or combi boiler, do not panic.  To better understand the distinction, we suggest reading our What is a Combi Boiler post again.  In summary, there are different types of boilers; Open Vent, Sealed System and Combi boilers for different sized properties.  Again, to emphasize: All boilers manufactured in 2018 or in the last 8-10 years are condensing boilers.  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.

Benefits of Switching to a Condensing Combi Boiler


Even if your older non-condensing boiler (older than 12 years) hasn’t broken down, it’s still a good idea to switch to a modern condensing boiler.  Reason being older boilers lose their efficiency due to corrosion on internal components because of thermal and mechanical stresses on a daily basis over years and years of constant use.  Unless you had old back boilers with cast iron heat exchangers, a lot of manufacturers over the last decade or so switched to aluminium, which is not as corrosion resistant or durable as other metals like stainless steel.  In any case switching to a modern condensing or combi boiler offers the following benefits:

  • Higher Energy Efficiency of above 94%
  • Save Money on Energy Bills (40% or £500 Per Year)
  • Wireless Digital Programmable Thermostats
  • Internet Enabled Smart Thermostats
  • Boiler Plus Compliance
  • Use with Weather Compensation
  • Space Saving
  • Reduced Carbon Footprint
  • Compact Size

Best Condensing or Combi Boiler Brands UK 2018

Before we give you a list of best condensing combi boiler brands in the UK, it’s probably prudent to find out which condensing or combi boiler is best suited to your property.  We’ve done a detailed, in-depth article on the same on our blog in the past, which has also become one of our most popular articles.  Please click on the following link to read it in full: Which Boiler is Best in 2018.  If you’d also like to read an extensive guide on best combi boilers for 2018, please click on the following link: Best Combi Boilers 2018

In summary, the best condensing or combi boiler brands in the UK are:

1. Viessmann

condensing or combi boiler

UK Best Buy award for the second year running in 2018, world leaders in the commercial boiler space for many decades, their transition into the domestic boiler market has been fairly recent.  But that is only positive, good news for the consumers.  Commercial boilers are much bigger, powerful and technologically advanced, because they’re supposed to efficiently heat large buildings, factories and stadiums.  And because Viessmann have been manufacturing these mammoth boilers for years, their know-how translates well into a domestic condensing or combi boiler.  Plus they have the facilities and the technical expertise to manufacture all of their internal components in-house, many of which are patented features unique to Viessmann boilers, like the Inox-Radial Stainless Steel Heat Exchanger (pictured below):

condensing or combi boiler, viessmann heat exchanger

So, why should you care about that?

The primary heat exchanger of a boiler is similar to the engine of a car.  So, the more efficient and durable the engine is, better the car performs.  Viessmann have patented this design, so this is unique to their range of boilers.  Stainless steel is a much more durable and corrosion resistant metal compared to aluminium.  And pretty much all of Viessmann’s rivals in the UK source their heat exchangers from a third-party manufacturer based in France.  Below is a diagram showing the durability comparison of stainless steel vs aluminium, and what it means for your central heating condensing or combi boiler:

condensing or combi boiler

As you can see, the aluminium heat exchanger drops off in performance dramatically over time.  So, over years of daily use, a Viessmann boiler will not only maintain its efficiency longer, it will also be more durable and resistant to corrosion and break down less often.  In monetary terms, this means your investment in a Viessmann boiler will give much better returns.  Whereas another brand with an aluminium will be ready to be replaced in half that time.

And because Viessmann manufacture most of their components in-house, they can compete on price with their UK rivals.  It’s really a no-brainer to invest in Viessmann boilers.

Coincidentally, German car engines are some of the best in the world, in fact we’d even say the absolute best.  Mercedes’ F1 team’s German engineering prowess from Stuttgart combined with British aerodynamics/mechanical expertise + driving talent has been beating the might of the Italians (Ferrari) in the Formula 1 World Championship for years now.  And even when it comes to boilers, as much as it hurts us to say this, none of the British boiler manufacturers (if there are true British boiler manufacturers left) hold a candle to Viessmann in terms of technology and efficiency.  Some more Viessmann condensing or combi boiler features:

  • 98% Energy Efficiency and Above (Viessmann Vitodens 200-W)
  • Class A rated for energy efficiency
  • Patented High Quality Stainless Steel Inox-Radial Heat Exchanger (Manufactured in-house)
  • Robust, Reliable and Durable
  • 10 Years Extended Warranties on Boiler + Heat Exchanger
  • Combined PRV (Pressure Relief Valve) and Condensate Combined
  • Easier to Site and Install
  • In-Built Shock Arrester
  • Compact design and low weight
  • Quiet operation thanks to low fan speed
  • Available in outputs from 4.7 to 35 kW
  • Low CO2 emissions for reduced environmental impact
  • User friendly Backlit LCD Touch screen for easy operation
  • Optional room thermostats available

Mythbusters – Answering Doubts and Concerns around Viessmann

There are some myths around Viessmann condensing or combi boiler based on outdated information.  Let’s clear some of those:

1) Are Viessmann spare parts readily available throughout the UK?

Answer: Yes. Their UK headquarters are based in Telford, Shropshire, and all Viessmann spare parts are stocked at Plumbase and several other plumbing merchant locations around the UK.

2) With Brexit looming, is it safe to buy a Viessmann boiler? What will happen to them?

Answer: Nothing. They will continue to function the same way as they are now, like several other European companies, because they have invested heavily in the UK.  That is assuming Brexit even goes through.


2. Worcester Bosch

Acquired by the German company Bosch in 1992, Worcester Boilers have been the market leaders in the UK forever now.  And for good reason.  They are known for manufacturing high quality, efficient, reliable and durable boilers which have been the boiler of choice for UK consumers for decades now.

condensing or combi boiler, worcester bosch

You simply cannot go wrong with buying a Worcester condensing or combi boiler.  The only reason they’re second in this list is because of Viessmann’s stainless steel heat exchanger.  They have a vast boiler range suitable for almost every property size, except they don’t have wall mounted storage combi boilers for larger properties.  They do have the floor-standing Greenstar Highflow series though.  One of the reasons Worcester Bosch are so successful is because they’ve been present in the UK market forever, in fact the name “Worcester Boiler” is synonymous with central heating for generations.  We do feel however the technology these boilers are running is a bit dated compared to the likes of Viessmann.  Probably because they’re the market leaders and it’s the “If it’s not broken, don’t fix it” mentality, but in any case there is no way the aluminium heat exchangers inside these can compete with the ones from Viessmann.  In any case, you still get your money’s worth when you buy a Worcester Bosch boiler.

  • One of the highest hot water flow rates
  • Simple boiler control knobs with clear temperature display
  • Low energy pump reduces electrical usage
  • Frost protection is included, reducing frost risk if installed in a garage or a loft
  • Inbuilt keyless filling link making it easy to refill your system
  • Large Condensesure style siphon built into the boiler – eliminates frost risk


We’ve mentioned the top two premium boiler brands in the UK.  The brands listed below are also good, but not in the league of the top two in our humble opinion:

3. Vaillant

4. Ideal

5. Baxi

Vaillant is comparable to Viessmann and Worcester Bosch in terms of technology and boiler range, with Ideal and Baxi fast catching up.  There are also the affordable condensing or combi boiler brands like Ferroli, Vokera etc. which are cheaper, but not up to par when it comes to quality.

You’ve landed on this page because you’re obviously an educated, intelligent, internet-savvy buyer doing research before making one of the biggest buying decisions for your household.  The boiler you choose will keep you and your loved ones comfortable and warm for the next 10-15 years at least.  We hope we’ve given you all the relevant facts to make your decision easier.  So, happy trails and choose wisely!

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