what is a combi boiler

What is a Combi Boiler?

“What is a Combi Boiler?” is a question you probably haven’t had to ponder upon, especially if your 40 year old back boiler is still working.  If you’ve had to suffer a boiler breakdown however, and you never had a combi boiler installed in your home, this post will help you understand what is a combi boiler.


Before we explain what is a combi boiler however, we need to explain the different types of boilers first, starting with the most common type of boiler installed in an average 3 bedroom semi-detached family house in the UK:

Open Vent/Conventional/Regular/Standard Boiler

what is a combi boiler, open vent boiler

Although their technically accurate name is Open Vent, they may be called regular or standard boilers in the UK.  Why is it called Open Vent?  It’s because these boilers are installed on an open vent central heating system.  Open vent means the heating system is open to atmospheric pressure.  Components of an open vent system include feed and expansion tanks in the loft space, and a hot water cylinder usually in the airing cupboard or next to the boiler.  Open vent boilers are also compatible with solar thermal.

System Boiler

what is a combi boiler, system boiler

A system boiler is slightly different from a conventional open vent boiler, in that system boilers don’t need to store cold water in the expansion tanks in the loft.  System boilers take their water supply directly from the mains and heat your central heating system and produce hot water for a storage cylinder.  If you want online quotes for an open vent or a system, or if you want to know what is a combi boiler discount price, please enter your postcode below and hit the green button:


And now onto the real deal, the pièce de résistance:

What is a Combi Boiler?

what is a combi boiler

Combi is short for combination boiler.  Unlike open vent or system boilers, combi boilers do not require expansion tanks or hot water cylinders.  If you’re short on space, combi boilers are a great idea because you don’t need to store cold water tanks in the attic or hot water cylinders in the airing cupboard.  They are becoming an increasingly popular choice in the UK for small to medium sized homes.  In fact, the best combi boilers now outsell all other types of domestic boilers in the UK at the time of writing this article in August 2018.

Why are they called Combi Boilers?

Another burning question on top of what is a combi boiler, is why is it called a combi boiler?  As we mentioned above, combi is short for combination.  It’s because they combine high-efficiency water heating and central heating, within one compact unit.  Furthermore, 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 needing any extra pumps.

Save Time & Money on Combi Boiler Installation

What is a combi boiler cost effectiveness, especially if you buy it straight or get it installed?  If you are installing a new combi boiler at your property, or replacing your existing boiler, a combi boiler installation will save you money on installation time and costs.  Open vent and system boilers are generally more expensive because of the extra equipment.  And because you don’t have to have extra space for tanks and cylinders, it means lesser pipe work and a shorter installation time.

What is a Combi Boiler Suitable for Larger Properties?

If you have a larger property with 2 bath/showers, and are thinking a combi boiler may be too small or inadequate for your property, don’t worry.  There are larger combi boilers available now which house a thermal water storage tank within the unit itself.  These are called storage combis.  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 in two bathrooms, you still get it at mains pressure.

Which Combi Boiler is Best for Your Property?

What is a combi boiler size perfect for your property?  It is extremely important that you choose the best combi boiler for the size of your property before installation.  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

That post goes into detail about the types of boilers suitable for your property.  Even if you believe you have a good enough understanding after reading that post, it’s prudent to communicate everything about your property, any complications around boiler installation or flue options to the heating engineer or heating expert from the combi boiler installation company.  If you would like some free, no obligation online quote along with free help and advice, please begin by entering your postcode below and hitting the green button:


Best Combi Boiler Brands in the UK

What is a combi boiler brand you can trust with your eyes closed?  Fortunately there are plenty of options in the UK market.  We’ve done a factual boiler brand comparison post on our blog earlier, which gives a pretty accurate indication of which brand comes out on top in battle of the combi boiler brands: Best Combi Boilers 2018

Briefly, below is a list of the best combi boiler brands in the UK based on energy efficiency, reliability, engineering and quality + durability of internal components:

Worcester Bosch have been the market leaders in the UK market for a long time, partly because of the Worcester name, and partly because they do make good quality boilers.  Having said that, they are not the best boiler in the UK market today.

Why are Viessmann Combi Boilers better?

what is a combi boiler, viessmann combi boiler

Model Shown: Viessmann Vitodens 100-W

Viessmann are a German brand with their UK headquarters in Telford, Shropshire.  They manufacture their boilers in Germany, but all Viessmann spare parts are stocked at Plumbases across UK.  They are global leaders in the commercial heating space, supplying heating solutions to some of the biggest commercial centers and stadiums around the world.  Their foray into the domestic boiler market was fairly late compared to their rivals, but with their commercial heating experience, the transition was easy.  They have quickly gained a lot of momentum in the UK market.  If you are wondering what is a combi boiler best for your house, we truly believe Viessmann is the best combi boiler money can buy in the UK at the moment, because of the reasons below:

  • A-Rated 94% Energy Efficiency on 050-W and 100-W.
  • 98% and Above Efficiency on Vitodens 200-W.
  • Inox-Radial Stainless Steel Heat Exchanger
  • 10 Years Warranty on the Heat Exchanger
  • 7-10 Years Parts & Labour No-Quibble Warranty
  • PRV (Pressure Release Valve) and Condensate Combined into One
  • In-Built Shock Arresters
  • All internal components Patented and Manufactured in-house
  • Durable, Less Prone to Corrosion, Reliable

Because the internal components inside a boiler go through ridiculous thermal and mechanical stresses on a daily basis, especially winters, stainless steel is the metal you want inside your boiler.  And Viessmann has the best stainless steel heat exchanger inside every single boiler in their entire range:  Viessmann Boiler Range

To get exclusive Viessmann combi boiler discounts and extended warranties, get an online quote today:


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