Best Condensing Boilers

Updated March 2021

If you’re due a boiler replacement, we are here to assist you. 

Since 1st April 2007, it has been a legal requirement that all gas and oil boilers installed in England and Wales must be condensing boilers. Whilst in Scotland, this has been the law since 1st April 2005 and 1st April 2007 for gas boilers.

In short, every boiler manufactured in 2021 is a condensing boiler, regardless of the configuration being open vent, sealed system, or a combi.

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In 2021, the options for condensing boilers are vast. Due to this, it can be difficult to know which is the best condensing boiler for you. This is made increasingly more difficult due to the Boiler Plus regulations brought in from April 2018. The regulations mean that all boilers installed in England, Wales and Scotland must have a minimum performance standard of 92% ErP and must also include one of the following technologies: 

  • Flue gas heat recovery systems
  • Weather Compensation
  • Load Compensation
  • Smart controls featuring automation and optimisation functions

Whilst these measures were brought in to reduce carbon emissions and in turn, bring down consumer energy bills, they have also made all condensing boilers seem very similar at first glance. However, this is where we come in. Our guide to the best condensing boilers available in 2021 will breakdown each boiler option and highlight to you our most recommended boiler options. 

To compile this guide, we have considered the following factors:

  • Price
  • Warranty
  • Design
  • Material quality
  • Customer feedback
  • Customer service
  • Awards and accolades

What is a Condensing Boiler?

How to condensing boilers work?

All boilers, whether non-condensing or condensing, have combustion emissions, like heated gases, which pass through heat exchangers and transfer the generated energy to radiators or under-floor heating, which then spread the heat around your house. After this is done, the remaining combustion gases are released into the air outside through the flue, usually up through the roof or through the wall.

In older, non-condensing boilers, some portion of this heat is lost during this process, because along with the waste gases a large amount of steam that forms during the burning process (due to the water contained in natural gas in its initial state) is being pushed out.  The released steam carries an untapped amount of evaporation energy that older, non-condensing boilers are unable to make use of.

Regardless of the boiler, you buy in 2021, it will recycle the water vapour through a primary heat exchanger and recover latent heat utilising vaporisation.  This condensed vapour leaves the system in liquid form, via a condensate pipe into a drain.

For more information on condensate pipes, read the following guide – ‘What is a condensate pipe?

Condensing Boiler Technology

Condensing boilers have a fan installed next to the heat exchanger working at variable speed.  For these to work at optimum efficiency, a condensing boiler houses a closed combustion chamber connected to a coaxial flue, so the combustion gases can pass through it.  Controlling the fan’s speed helps maintain an optimum combustion ratio of air and gas.  To minimise any heat loss from the flue gases, it’s important for the boiler’s heat exchanger to allow water vapour condensation.

Internal Boiler Fan Example

A condensing boiler works at its best when the heat exchanger surface is equal to or below what’s called the dew point temperature.  Normally, the dew point temperature for natural gas is around 55 degrees Celsius.  And so, for the boiler to work in condensing mode, the temperature of the coolant in the return loop (when the coolant returns back to the boiler) should not exceed 57 degrees Celsius.

Saving you Money on your Energy Bills

If properly installed, a condensing boiler will not only heat your home and provide hot water consistently, it can also reduce your gas consumption by 20% to 25%.  According to the Energy Saving Trust, you can save anywhere between £300 to £500 per year on your energy bills by switching to a condensing boiler along with other energy-saving measures like draught-proofing your doors and windows, insulating your walls and using a boiler with smart controls or weather compensation.

Condensing Boiler Cost

Working out a condensing boiler cost is something you don’t have to do alone. By speaking to one of our in-house heating experts, you’ll receive reliable and trustworthy advice that can ensure you get a fantastic boiler for a reasonable price. To do so, you can either call us directly on 029 2009 9898 or get a FREE, no-obligation quote by filling in your postcode below and then hitting the green button…easy!

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How are energy savings Calculated?

When you buy a condensing boiler, you can rest assured that it will be at least 90% energy efficient out of the gates, and can go up to 98% efficiency by utilising programmable thermostats or internet-enabled smart thermostats like Tado or Nest. One such boiler which can achieve these levels of efficiency is the Viessmann Vitodens 200-W. You can find out more about this in our Viessmann Vitodens 200-W review

Compared to this, older non-condensing boilers started at about 80-85% energy efficiency, and over years of daily use dropped down to 70% or even 50% efficiency.  This is because the internal components of a boiler go through massive amounts of thermal stresses on a daily basis for years, resulting in corrosion which reduces the efficiency.  If your existing boiler is older than 12-15 years, chances are it’s a non-condensing boiler, and you may be paying more on energy bills than you need to.

In monetary terms that means for every £100 that you spend on your energy bills, with an older non-condensing boiler operating at 70% or 50% efficiency, you may lose £30 to £50.  And with a modern condensing boiler operating at 94% efficiency, you may lose only £6 for every £100 spent on energy bills. That’s a lot of money wasted that could otherwise be saved.

Condensing Boilers are Kinder to the Planet

As less heat is lost through the flue gases, you will be increasing your energy efficiency and wasting less energy. On top of saving money on energy bills, you will also be putting out fewer emissions into the atmosphere and being more environmentally responsible.  A condensing boiler can go a long way in reducing your carbon footprint.  

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Quality of Internal Components

When the condensing process happens and the vapour turns into a liquid, it releases an amount of thermal energy.  Whilst older, non-condensing boilers were designed to avoid the condensation process, modern condensing boilers are designed to utilise this process to their benefit. They do this by utilising condensation in the heat exchanger to absorb the heat and transfer it to radiators around your house. 

Stainless Steel vs Aluminium for the Primary Heat Exchanger

The secret to exploiting condensation for energy efficiency has been known for a long time, but it hasn’t been implemented effectively in home or industrial heating appliances until the most recent technological innovations.  To maintain high efficiency for longer, the primary heat exchanger needs to be robust.  Boiler manufacturers in the UK use either aluminium or stainless steel for their heat exchangers.

Below is a graph that shows the longevity comparison between aluminium and stainless steel heat exchangers.  While aluminium starts off strong, it drops off massively in later years, whereas stainless steel maintains its efficiency:

best condensing boiler

It is no secret that stainless steel is much more durable, reliable and less prone to corrosion compared to aluminium.  Viessmann are one of the only boiler manufacturers in the UK who use a patented Inox-Radial Stainless Steel Heat Exchanger for their best condensing boiler range.  And because they manufacture it in-house, they are able to compete at the same price point as market leaders in the UK, despite having a technologically superior boiler.

They are also one of the 3 “Best Buy Boiler” award winners in the UK.  The other two being Worcester Bosch and Vaillant, both of whom use an aluminium heat exchanger.

For more information on your boiler’s heat exchanger and how it can help you save on your energy bills, please read our informative ‘What is a Heat Exchanger?’ blog post

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Our recommendations for the best condensing boilers 2021

1. Viessmann Vitodens 200-W

Fuel TypeAvailable in:Efficiency Hot water flow rateWarrantyPrice (Installed)
Gas26kW, 30kW, 35kW98%10.3 l/min (26kW), 12.3 l/min (30kW), 14.4 l/min (35kW)Up to 10-year comprehensive warranty when installed by a Viessmann trained installerFrom £3300 (inc. VAT)

Available in Open Vent, System and Combi Versions

The opinion of our heating engineers is that this one of, if not the best boiler that money can buy in the UK currently. It’s one of the most powerful domestic boilers available and it is comparable to many commercial boilers. 

Having been awarded Which? Best Buy status every year since 2018, the Vitodens 200-W also comes highly recommended by the 219 registered Which? Trusted Traders heating engineers and 9,610 domestic customers surveyed. 

As mentioned above, the Vitodens 200-W offers an efficiency level of up to 98%. This is due to the boiler’s high quality internal components, innovative engineering solutions, and weather compensation technology which takes into account the outside temperature and adjusts the temperature inside.

The Vitodens 200-w also has WiFi capabilities, which enable it to be controlled remotely and enable it to be used with smart thermostats for increased efficiency and energy savings.

Our Viessmann Vitodens 200-W Review will be able to tell you more about this boiler.

Vitodens 200-W Key Features

  • Up to 98% of the fuel used is converted into heat
  • Quiet operation thanks to low fan speed
  • Compact dimensions, only 450 mm wide
  • Easy to operate Vitotronic control unit with 5 inch colour touchscreen
  • Control via app and internet (with optional Vitoconnect 100 WLAN adaptor)
  • Energy efficiency class: A
  • Optionally available as combi (type BK2B) or system boiler (type B2HB)
  • System boiler: Cascade with up to six boilers up to 594 kW possible
  • Environmentally responsible MatriX cylinder burner
  • Optional remote fault notifications for Viessmann Trained Installers

2. ATAG iC Economiser Plus 35

best condensing boiler, ATAG

Fuel TypeAvailable in:Efficiency Hot water flow rateWarrantyPrice (Installed)
Gas27kW, 35kW, 39kW94%12.6 l/min (27kW), 16.1 l/min (35kW), 17 l/min (39kW)Up to 14-year comprehensive warrantyFrom £2600 (inc. VAT)

Available in Open Vent, System and Combi Versions

Whilst a lesser-known brand in the UK, ATAG has been designing and manufacturing boilers for almost 70 years. In The Netherlands where the company was established, ATAG has been the recipient of numerous awards and for this reason, are held in high regard. Whilst in the UK, ATAG is loved by customers and are the number one rated boiler manufacturer on Trustpilot.

The ATAG iC Economiser Plus is an excellent example of quality from ATAG. This is firstly because the boiler is available with a very impressive standard warranty of 10 years and an even more impressive warranty which can be extended to up to 14 years. We like this because it means you know straight away that you are buying a boiler which is going to last almost a decade and a half. Not only this, the heat exchanger on the boiler is guaranteed for the lifetime of the boiler.

Next, we like the efficiency of the iC Economiser Plus. ATAG boilers are some of the most energy-efficient boilers in Europe and the iC Economiser Plus operates a great 94% efficiency rating. 

The efficiency of the iC Economiser Plus is largely down to the quality of components used within the boiler. One of the most important components within this boiler is the iCon heat exchanger, which is made from high-grade stainless steel and has been developed to maintain 98% of its original efficiency levels throughout its lifetime. In addition to the quality of this component, other components within the boiler are made from high-quality stainless steel and brass, as opposed to the plastic components utilised within lesser boilers.

The efficiency of this boiler is also due to ATAG’s design engineers developing the boiler to utilise untapped residual heat left in the waste flue gases when the boiler is being used for hot water.

Like the Vitodens 200-W above, this ATAG boiler also has built-in weather compensation technology, allowing the boiler to adjust its temperature based on the temperature outside. It also has Wi-Fi capabilities and can be controlled remotely using a smartphone or tablet.

This boiler comes highly recommended by us.

ATAG iC Economiser Plus 35 Key Features

  • Up to 14 year warranty available
  • Stainless steel heat exchanger
  • Stainless steel and brass components
  • Instant hot water with pre-heat facility
  • Built in Passive Flue Gas Heat Recovery Device
  • Built-in weather compensation technology
  • Achieves ErP ‘A’ rating for heating and hot water
  • Wi-Fi compatible

3. Viessmann Vitodens 100-W

best condensing boiler, viessmann vitodens 100-w

Fuel TypeAvailable in:Efficiency Hot water flow rateWarrantyPrice (Installed)
Gas26kW, 30kW, 35kW94%10 l/min (26kW), 12 l/min (30kW), 14.5 l/min (35kW)Up to 10-year comprehensive warranty when installed by a Viessmann trained installerFrom £2500 (inc. VAT)

Available in Open Vent, System and Combi Versions

The Viessmann Vitodens 100-W is Viessmann’s best selling gas condensing boiler and this is for good reason. Offering efficiency, compact dimensions and quiet operation, the Vitodens 100-W sits at an affordable price and fits in nicely between the entry-level Vitodens 050-W and the high-end Vitodens 200-W.

Like all of the other boilers in the Viessmann range, the Vitodens 100-W is equipped with Viessmann’s patented stainless steel Inox-Radial heat exchanger, ensuring excellent efficiency which won’t be affected by corrosion like boilers with aluminium heat exchangers.

As with the Viessmann Vitodens 200-W, the Vitodens 100-W has also been awarded Which? Best Buy status every year since 2018 and is a favourite amongst the Which? Trusted Traders heating engineers and 9,610 domestic customers surveyed.

Our Viessmann Vitodens 100-W Review will be able to tell you more about this boiler.

Viessmann Vitodens 100-W Key Features:

  • Quiet operation thanks to low fan speed
  • High-quality stainless steel Inox-Radial heat exchanger
  • Environmentally responsible MatriX cylinder burner
  • Class A-rated for energy efficiency
  • Modulation range up to 1:6
  • Low CO2 emissions
  • User-friendly backlit LCD touch screen for easy operation
  • LPG conversion kit available
  • Up to 10 year extended warranty available if installed by Boilerhut


4. Vaillant ecoTEC exclusive with Green iQ combi

best condensing boiler, vaillant ecotec plus with green iq combi

Fuel TypeAvailable in:Efficiency Hot water flow rateWarrantyPrice (Installed)
Gas35kW, 43kW89%14.5 l/min (35kW), 17.8 l/min (43kW)Up to 10-year warranty availableFrom £3000 (inc. VAT)

One of the reasons why Vaillant and Worcester Bosch don’t usually score highly in reviews is due to the use of aluminium heat exchangers. However, the Vaillant ecoTEC exclusive with Green iQ combi is an exception to this rule, as it is a Vaillant boiler that features a stainless steel heat exchanger. For this reason, it is a worthy addition to this list. 

Due to the stainless steel heat exchanger, the ecoTEC exclusive with Green iQ combi will retain its efficiency levels longer than other boilers in the Vaillant range. It also has a fantastic modulation of 1:10, which gives the boiler a higher level of efficiency and will result in lower fuel bills by adapting to provide only the heating output you require.

This boiler is also hand-assembled by one operative in Derbyshire which has its merits. Whilst once you’re finished with it, Vaillant claims that 85% of its parts are recyclable.

Providing future proof technology, the boiler is also Wi-Fi compatible and can be programmed and controlled from a smartphone or tablet using Vaillant’s vSMART™ heating control.

Vaillant ecoTEC exclusive with Green iQ combi Key Features:

  • Quiet operation
  • Modulation range up to 1:10
  • Excellent flow rate of 17.8 litres per min on the 43kW output boiler, delivering fast instant hot water and Vaillant’s highest ever flow rate
  • ErP A rated for both hot water and heating efficiency
  • Works with LPG 

5. Viessmann Vitodens 050-W

best condensing boiler, vitodens 050-w

Fuel TypeAvailable in:Efficiency Hot water flow rateWarrantyPrice (Installed)
Gas29kW, 35kW93%12.3 l/min (29kW), 14.5 l/min (35kW)Up to 10-year comprehensive warranty when installed by a Viessmann trained installerFrom £2100 (inc. VAT)

Viessmann’s entry level model in the condensing boiler arena, it has surprisingly good performance at an incredibly affordable price.  What’s more, it has the same patented Viessmann stainless steel heat exchanger as every other boiler in the Viessmann range, and is also compatible with weather compensation.  And although it starts off at around 90% efficiency, with programmable thermostats and weather compensation it can go up to 93% efficiency, which is mighty impressive for its price.

Along with the rest of the Vitodens range, it has also been awarded Which? Best Buy status every year since 2018, so with this boiler you will know you are choosing a boiler which is loved by engineers and current owners alike.

For a detailed look at why this could be a fantastic choice of boiler for you, take a look at our Viessmann Vitodens 050-W review.

Viessmann Vitodens 050-W Key Features

  • Ultra-compact – just 707mm in height
  • High-quality stainless steel Inox-Radial heat exchanger
  • Integral instantaneous water heater
  • Low CO2 emissions for reduced environmental impact
  • Digital user interface with LCD display for easy operation
  • Up to 10 year extended warranty available
  • Class A-rated for energy efficiency

For a detailed look at why the Viessmann Vitodens 050-W could be a fantastic choice of boiler for you, take a look at our detailed review or watch our video review below: