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Condensing Boilers vs Combi Boilers: What are they?

Condensing boilers are ultra-efficient. Combi boilers are ultra-efficient. When you mix the two together, bill-saving magic happens.

What’s the difference between a condensing boiler vs a combi boiler? 

If you are on the hunt for a new boiler for your home and have come across these two terms, you may be wondering what the difference is? And which you should choose for your home? 

SPOILER ALERT! They are ultimately the same thing. In fact, all modern boilers are condensing boilers, whether that be a combi, system or a regular boiler. 

You no longer need to be concerned about avoiding a non-condensing boiler, since legislation banned their sale in the UK back in 2005. 

Want to know which condensing boiler is best? Check this out.

What is a Condensing Boiler? 

What is a condensing boiler?

The term "condensing" relates to the technology the boiler uses and in the past all boilers were non-condensing. In this old design of boiler all of the hot waste flue gases were released outside of the home, which itself wasted a substantial amount of heat and therefore money. 

Condensing Boilers Save CO2It also released a significant amount of carbon emissions, a greenhouse gas that’s partly responsible for climate change. 

In response to increasing energy costs and environmental concerns, Boiler Plus legislation was established in 2005. This meant that from then on all new boilers had to be condensing, which vastly improved the efficiency of boilers. 

How do Condensing Boilers Work? 

How do condensing boilers work?

Condensing boilers are designed to recycle the heat from the waste flue gases to preheat the water entering the boiler. 

Condensing boilers are able to do this as they are installed with a Flue Gas Heat Recovery System and utilise two heat exchangers. This prevents energy waste and means you are getting more heat for the same amount of money. 

During the recovery process, the water vapour in the gasses is transformed from a vapour to a liquid, resulting in the process of condensation, hence the name. 

The temperature of the flue gas can fall as much as 80℃ and the heat which would have been wasted in old non-condensing models can be put to use. During this process as much as two litres of wastewater is generated each hour, which exits the boiler via the condensate pipe. 


Due to this heat recovery process, condensing boilers are much more efficient than old non-condensing boilers, resulting in reduced carbon emissions, energy waste and lower heating costs. 

What is a Combi Boiler?

What is a combi boiler?

A combi boiler or combination boiler is the most common type of boiler installed into UK homes. It is able to provide both hot water on demand and heating water from a single unit, hence the name “combi” boiler. 

Imagine if Usain Bolt was your postman, but he also did your bins... yeah, that.

Unlike system boilers and regular (heat-only) boilers, combi boilers do not require a separate hot water storage cylinder, a separate overhead tank in the attic and all components are internal. This means they do not require as much space and so are ideal for smaller properties, with some even being compact enough to fit into the average sized kitchen cupboard. 

Combi boilers heat water directly from the mains water supply, providing instant hot water on demand to taps and showers. Since they only heat the water you need, this also means that less energy is wasted, since there is no need to keep any stored and heated in its own tank. 

How is a Combi Boiler, Condensing?

Combi Boilers are Condensing 

All modern domestic boilers including combi boilers, system and regular boilers now use condensing technology. 

All domestic boilers produce hot waste exhaust gases, which exit the boiler via the flue, however, all new boilers are fitted with a Flue Gas Heat Recovery System, making them condensing. 

A Rated BoilersAll condensing boilers are at least 90% energy efficient, making them around 40% to 50% more efficient than old non-condensing models. They can reuse the heat in flue gases, are better for the environment and will help to reduce your heating bills.

Looking for a new boiler? Get the best price on a condensing combi boiler.

Condensing Boiler Regulations

BoilerPlus Condensing Boilers  

On the 6th of April 2018, the Department for Business, Energy and Industrial Strategy (BEIS) implemented Boiler Plus Legislation. 

The aim of this new legislation was to drastically increase the energy efficiency of residential boilers, helping to reduce carbon emissions, while decreasing heating costs. As a result, all new boilers installed into UK homes must meet these new standards.


These new standards do not just cover combi boilers either, in summary, the regulations state that: 

  • Gas-fired boilers, both new installations and replacement, must be condensing
  • Oil-fired boilers, both new installations and replacements, must be condensing
  • All condensing boilers that are installed need to have an efficiency rating no lower than B

Advantages of a Condensing Combi Boiler

Advantages of Condensing Boilers

Here are the main advantages of condensing boilers: 

Superior energy efficiency

Energy Efficient Condensing Boilers

New condensing boilers benefit from a Flue Gas Heat Recovery System that makes them much more efficient than old non-condensing models. This means they produce more heat from the energy used and waste less energy during the process. 

Non-condensing boilers could be less than 60% efficient and were rated as G under the ERP (Energy Related Products Directive) boiler rating system.

Reduce energy waste and your heating bill

Reduce Bills Condensing Boilers

According to the Energy Saving Trust by replacing your old G-rated non-condensing boiler with a modern A-rated condensing boiler you could save as much as £300 a year.

Paired together with a smart thermostat and thermostatic radiator valves you could save even more. For example, by installing a Nest, you could save up to £75 and 330kg of carbon a year, thanks to its adaptable learning schedule.

Reduce your carbon footprint

Reduce Bills Condensing Boilers

Modern A-rated condensing boilers have considerably lower carbon emissions than old non-condensing boilers. In fact, by upgrading your old boiler for a modern condensing boiler you could save up to 1,220 kg of CO2 per year.

Disadvantages of a Condensing Combi Boiler

Downsides of a Combi Boiler

Although combi boilers are the UK’s most popular type of boiler, the reality is that they are not suitable for some homes. 

Water flow rates are reduced when two or more outlets are being used at the same time.

So, if you have got a larger property with a few bathrooms (which may be used at the same time), a combi might not be the right choice for you. 

Ultimately, a combi boiler can only heat the water which gets fed to it from the mains. So, if your mains water pressure is low, your combi boiler won't work very well.

In summary, a combi boiler may not be suitable if: 

  • You have high hot water demands e.g. more than one bathroom 
  • Your mains water supply pressure is weak 
  • Your pipework is not compatible 

If a combi boiler isn’t suitable for your home, there are other types of boilers available that will be, including system and regular boilers. These are also condensing and highly-efficient A-rated boilers. 

How Much Does a Condensing Combi Boiler Cost? 

How Much Condensing Boilers Cost

The cost of a new condensing combi boiler is dependent on a variety of different factors, these include the following:

  • The type of boiler you have installed e.g. combi, regular or system.
  • The boiler model you choose e.g. mid or premium range.
  • If you want to relocate your boiler e.g. from the garage to the attic.
  • If you want to convert your boiler e.g. from combi to regular.
  • Any add-ons you purchase e.g. filters, smart controls etc.
  • Check the latest prices on our combi boilers, here

The most common type of boiler installations and easiest is a relatively uncomplicated combi swap i.e. replacing your old combi with a new combi. For example, this is much simpler than replacing a combi with a system boiler.


What Size Condensing Combi Boiler do you Need? 

Size Condensing Boiler

Here is a quick overview of the condensing boiler size you need:

Best small condensing combi boilers:

Best large condensing combi boilers:

Best offensively large, powerful combi boilers:

  • #1: Worcester CDi Highflow 44kW / 55kW
  • #2: Viessmann 111-W 35kW

Want to know the kW you need for your home? Check out our "what size boiler do I need" guide. Alternatively, you can use our quick boiler finder tool, which will recommend the boiler that is most suitable for your home. 

If you live in an average-sized home, see our guide on choosing the best boiler for a 3 bedroom house

Looking for the Best Deal on a New Boiler? 

Heatable New Boiler

If so, get your new condensing combi boiler installed with Heatable, here’s why: 

  • Gas Safe installation within 24 hours
  • Thousands of happy customers – an average score of 4.9 on Trustpilot (that’s better than the market leader).
  • Price match guarantee – if you find a cheaper quote, we will match (must be like for like)
  • Multiple payment options - interest free available
  • 10 year guarantee
  • Fixed price guarantee – no hidden costs
  • Save your quote until later

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Heatable is registered in England and Wales No. 08804726 (VAT No. GB205805920 ). Trading address is 1 Station Road, Great Sankey, Warrington, Cheshire, WA5 1RH. Heatable acts as a credit broker (not as a lender) and offers credit products through a panel of lenders. Heatable is authorised and regulated by the Financial Conduct Authority (reference no. 805259).

[1] Price based on a like for like combi swap in a 2 bed, 1 bathroom terraced home with a standard flue

[*] Installation dates are subject to installer availability

[ยน] only available on combi boiler installations

Financial Support: If you are in receipt of certain benefits, you may be entitled to claim a free boiler grant.