Most efficient boilers

When looking for a new boiler, efficiency is a key area to look into. The more efficient a boiler is, the more you will save on your energy bills. 

Since 1st April 2005, all new gas boilers installed into UK properties are required by law to be condensing boilers. Whilst since 1st April 2007, the same law applies to oil boilers too. With all boilers available for sale and installation in the UK needing to be condensing, boiler efficiency has become ever more important, with boiler manufacturers competing with one another to utilise innovative technologies to increase the efficiency of their appliances. In this guide, we look in detail at the most efficient boilers available in the UK in 2021. Before we get started though, we will refresh you should you be wondering what boiler efficiency refers to.

What is boiler efficiency?

Boiler efficiency refers to the percentage of the total energy used by the boiler to provide heating. For example, a 92% efficient boiler will use 92% of the supplied energy for heating your home but the remaining 8% of energy will be lost. To put this into context, for every £1 you spend on heating your home, only 8p is wasted. It should be noted though that a 92% efficient boiler is very good and much more efficient than the older non-condensing boilers that are still being used every day in households across the UK. 

A typical non-condensing boiler could be running at 70% efficiency, meaning 30% of the energy you pay for is being lost and not being used to heat your home. 

Boiler efficiency ratings 

In the same way as other domestic appliances sold in the UK such as fridges, freezers and washing machines, boilers are rated using an energy efficiency rating scheme from A-G. However, all condensing boilers sold in the UK are required to be A-rated by law and must conform to Boiler Plus regulations. The energy efficiency rating scheme can be seen below:

Efficiency Rating Efficiency (%)
A 90% and above
B 86-90%
C 82-86%
D 78-82%
E 74-78%
F 70-74%
G below 70%

Boiler Plus regulations 

Since 2018, all new oil and gas boilers must be installed with a thermostat and a programme timer. Whilst all new combi boilers must include at least one of the following energy-saving features:

Flue gas heat recovery system

This is the system that condensing boilers utilise to recover heat from exhaust gases. We describe how this works below.

Load compensating thermostat

This is a thermostat that adjusts the temperature of the radiators based on the actual room. To ensure efficiency, this thermostat will increase the temperature of your radiators when the room is cold and decrease the temperature of the radiators when the room is reaching the required temperature.

Weather compensation

Weather compensation works by using an outside sensor that monitors the outside temperature and regulates your boiler temperature to ensure your home isn’t being unnecessarily heated.

Smart thermostat

Smart thermostats are internet-enabled systems that enable you to control your heating system remotely by using your smartphone or tablet. In addition to offering remote control, smart thermostats are also available with functionality such as open window detection, geofencing and voice control.

Why are condensing boilers more efficient than non-condensing boilers?

Regardless of the boiler that you choose, by choosing a condensing boiler to replace your existing non-condensing boiler, you will immediately be making savings on your energy bills. The reason for this is the way that condensing boilers work compared to the way that non-condensing boilers work.

Condensing boilers work to recover heat that a non-condensing boiler would have lost via the flue. This is called a flue gas heat recovery system and works by recovering heat by recycling exhaust gases (mostly hot steam with some acidic content) that older non-condensing used to waste. In condensing boilers, the hot flue gases go through a heat exchanger first before they exit through the flue outside.

We explain further how condensing boilers work in our Best Condensing Boilers guide.

Which boiler is the most efficient?

The top 5 most efficient boilers 2021

As discussed above, boilers today are smaller, lighter and quieter. They also feature advanced technology such as weather compensation and smart, programmable thermostats. On the surface, many boilers look and sound the same. Boiler manufacturers are also competing to produce the most energy-efficient appliance, so it can be difficult to identify which will be the most efficient for your needs. However, we have done the hard work for you here. With over 40 years of experience, our heating engineers have utilised their knowledge and experience to recommend the top 5 most efficient boilers available in 2021.

Worcester Bosch Greenstar 24i Gas System Boiler

most efficient boilers 2018 4 stars

most efficient boilers 2018, worcester bosch

Efficiency rating: 94% efficiency

ErP rating: A

Condensing: Yes

Central Heating Max. Output: 24.6 kW

As the UK’s leading domestic boiler manufacturer, Worcester Bosch invests heavily in enhancing the efficiency of its boilers. The Worcester Bosch Greenstar 24i is an excellent example of this and utilises Worcester Bosch’s advanced technologies to achieve 94% efficiency.

Suitable for small to medium-sized properties with up to 15 radiators, the Worcester Bosch Greenstar 24i is a system boiler with a very reasonable output of 24kW. 

Due to its excellent efficiency rating, the Worcester Bosch Greenstar 24i is endorsed by the Energy Saving Trust. 

With the Worcester Bosch Greenstar 24i, you get:

  • A large and comfortable boiler control plan that is incredibly easy to use
  • An array of user-friendly controls that help you keep your home at your preferred temperature
  • A highly efficient pump utilising low energy and reducing your electricity usage 
  • A boiler that works alongside a hot water cylinder to supply the hot water demands for properties with up to two bathrooms
  • Compatibility with Bosch EasyControl, a smart thermostat that uses a smartphone and tablet app to enable you to control your heating and hot water remotely, check your energy usage, and turn off the heating room by room
  • A boiler that is compatible with Worcester Bosch’s Greenskies solar panels for further energy and cost savings
  • Up to a 7-year guarantee
  • An A-rated boiler for heating and hot water

Find out more in our Worcester Bosch Greenstar i Gas Boiler Review.

Interested in the Worcester Bosch Greenstar i Gas System Boiler?

Baxi 224 Combi Gas Boiler

most efficient boilers 2018 4 stars

Baxi 224 Combi Gas Boiler

Efficiency rating: 93% efficiency

ErP rating: A

Condensing: Yes

Central Heating Max. Output: 21.8 kW

Baxi are known for their expertise in manufacturing high-quality boilers that perform well, are very efficient and don’t cost a lot, and the Baxi 224 Combi Gas Boiler is no exception. Illustrating its energy efficiency, the Baxi 224 is an Energy Saving Trust Endorsed product.

With a max central heating output of 21.8 kW, the Baxi 224 is suitable for smaller homes with up to 10 radiators and one bathroom. 

As it is very compact, the Baxi 224 is great for properties where space is at a premium.

With a 93% efficiency rating and a ErP A rating for heating and hot water. The Baxi 224 is highly efficient and is compatible with Baxi’s uSense smart thermostat which complies with Boiler Plus when connected using OpenTherm and works to reduce your energy consumption and save you money. USense allows you to control your heating remotely via your smartphone or tablet, learns about your lifestyle and how you utilise your heating and utilises load compensation as standard. 

With the Baxi 224 Combi Gas Boiler, you get:

  • A highly efficient boiler that will save you money on your energy bills
  • Compatibility with Baxi’s uSense smart control to learn how you heat your home and adjust the heating accordingly to reduce your energy bills and save you money
  • Up to a 3-year warranty
  • An A-rated boiler for heating and hot water
Interested in the Baxi 200 Combi Gas Boiler?

Vaillant EcoTec Pro 28 kW Combi Boiler

most efficient boilers 2018 4 stars

Efficiency rating: 94% efficiency

ErP rating: A

Condensing: Yes

Central Heating Max. Output: 24.8 kW

most efficient boilers 2018, Vaillant EcoTEC Pro

The Vaillant EcoTec Pro is a combi boiler suitable for small to medium-sized properties with up to 10 radiators and one bathroom. 

Vaillant utilises super-smart technology inside the EcoTec Pro enabling it to adapt to your heating demands and only provide you with the heat you require, lowering your fuel bills. 

Key to the efficiency of the Vaillant EcoTec Pro is its use of a stainless steel heat exchanger which offers reliability and corrosion resistance that will retain efficiency for longer than aluminium alternatives. 

Further efficiency is achieved by the EcoTec Pro’s integration with Vaillant’s vSMART app. vSMART enables you to control your heating system using an iOS and an Android app and provides self-learning functionality. This functionality starts to understand your home’s heating needs and makes your boiler work smarter to increase efficiency and minimise your energy bills. In addition to this, vSMART features weather compensation to monitor local weather conditions and adjust your boiler so that your home isn’t being heated unnecessarily.

With the Vaillant EcoTec Pro 28 kW Combi Boiler, you get:

  • Innovative technology to maximise energy efficiency
  • A stainless steel heat exchanger featuring anti-corrosion properties to retain efficiency for longer 
  • Compatibility with a range of wired and wireless controls including Vaillant’s vSMart app for increased efficiency and smart learning about your home’s heating requirements
  • The option for weather compensation to automatically regulate the temperature of your home based on the outside temperature
  • A boiler featuring a high-efficiency pump, making the boiler less expensive to run
  • A boiler that can be easily converted to use LPG

Find out more in our Vaillant ecoTEC Pro Combi Boiler Review

Interested in the Vaillant EcoTec Pro Combi Boiler?

Navien LCB700 Blue Flame 28 kW Oil Combi Boiler

most efficient boilers

Efficiency rating: 93% efficiency

ErP rating: A+ (when used in conjunction with Navien’s Smart Plus control pack)

Condensing: Yes

Central Heating Max. Output: 28 kW

Navien LCB700

Navien is a relatively new entrant to the UK market and has only been supplying boilers here since 2014. However, they are a global leader in condensing technology and this shows with the efficiency gains provided by the Navien LCB700 Blue Flame 28 kW Oil Combi Boiler.

Offering an efficiency rating of 93% and an ErP A rating for heating and hot water, the LCB700 utilises Navien’s innovative technology to achieve this efficiency. Key to this is Navien’s use of a blue flame burner and a stainless steel heat exchanger.

Navien’s SEDBUK A-rated blue flame burner uses revolutionary technology that replaces yellow flames in oil boilers and guarantees superior efficiency, ultra-low NOx emissions and low pollutant emissions.

The stainless steel heat exchanger used in the Navien LCB700 is designed for resistance against corrosion and erosion which will ensure the efficiency of the boiler is retained for longer.

With a built-in Intelligent Mode, the Navien LCB700’s efficiency is increased by utilising self-learning that monitors your DHW usage and prepares it according to your typical usage routine, continuing to monitor and adjust it as necessary.

Further smart functionality and efficiency gains can be achieved by utilising the LCB700 with Navien’s Smart Plus controllers which are designed to increase the LCB700’s efficiency and provide you with savings on your energy bill. A Smart Plus controller will regulate your heating based on changes in outside temperature, utilise geofencing to regulate your heating when you’re not home, enable the boiler to run in condensing mode when operating at maximum efficiency, and allow you to monitor your oil consumption quickly.

Due to its excellent efficiency, the Navien LCB700 Blue Flame Oil Combi Boiler is an Energy Saving Trust Endorsed product.

With the Navien LCB700 Blue Flame Oil Combi Boiler, you get:

  • Innovative blue flame technology offering the most efficient way to burn oil,  maximising the boiler’s efficiency
  • A stainless steel heat exchanger featuring anti-corrosion and erosion properties to retain efficiency for longer 
  • A high/low oil pump system allows continuous optimisation of heat/DHW output
  • Built-in intelligent mode offering self-learning to increase the boiler’s efficiency based on your typical usage
  • An OpenTherm ready boiler allowing for instant access to the latest control technology
  • Compatibility with Navien’s Smart Plus controllers
  • Navien’s industry-leading condensing technology
  • Up to a 10-year warranty
Interested in the Navien LCB700 Blue Flame Oil Combi Boiler?

Viessmann Vitodens 100-W 30 kW Combi Boiler

Efficiency rating: 94% efficiency

ErP rating: A

Condensing: Yes

Central Heating Max. Output: 25.4 kW

most efficient boilers 2018 5 stars

Viessmann’s best-selling gas condensing boiler, the sales of the Viessmann 100-W can be attributed to the combination of excellent reliability, efficiency and advanced technology. This comes down to Viessmann’s continued investment into product development and continuously improving the efficiency of its boiler range. 

New for 2021, the Viessmann 100-W has been updated with all new features to improve its efficiency.

Offering a 94% efficiency rating and an ErP rating of A for heating and hot water, the Viessmann 100-W combines high-quality components and advanced technology.

The key component utilised by the 100-W is the stainless steel Inox-Radial heat exchanger. Constructed from materials just one grade lower than surgical steel, this features properties that are self-cleaning and highly resistant to corrosion and erosion. These properties will allow the boiler to retain its efficiency for longer. 

The 100-W’s MatriX-Plus cylinder burner is environmentally responsible, reducing the boiler’s CO² and NOx emissions, and increasing the boiler’s efficiency. This is complemented by Lambda Pro automatic combustion control for maximum efficiency at all times.

This efficiency is further enhanced by the Viessmann 100-W’s smart functionality and integrated Wi-Fi, which is a new feature on the 2021 model. The boiler’s integrated Wi-Fi means that it is compatible with Viessmann’s ViCare app which allows it to be controlled remotely using a smartphone or a tablet without the need for third party controls. In addition to allowing you to control it remotely, the ViCare app will also enable you to increase the boiler’s efficiency by learning your daily routine and automatically reducing your heating, and in turn, your energy bills. Further to this, it allows you to control the heating in specific rooms in your property, ensuring you aren’t heating any rooms unnecessarily.

Further increasing the Viessmann 100-W’s efficiency is its use of weather compensation technology which will regulate and adjust your heating based on the current weather by utilising an outside temperature sensor.

With the Viessmann Vitodens 100-W 30 kW Combi Boiler, you get:

  • 94% efficiency 
  • An ErP A rating for heating and hot water
  • An advanced self-cleaning and corrosion and erosion resistant stainless steel Inox-Radial heat exchanger, designed to retain the boiler’s efficiency for far longer than aluminium alternatives 
  • An environmentally responsible MatriX-Plus cylinder burner which extends the boiler’s service life and reduces CO2 and NOx emissions
  • Lambda Pro automatic combustion control for maximum efficiency at all times
  • Compatibility with Viessmann’s ViCare app for smart learning of your typical heating and hot water usage 
  • Weather compensation technology to regulate your heating based on the exact temperature outside 
  • Up to a 12-year warranty 

Inox-Radial Stainless Steel Heat Exchanger

inox radial stainless steel heat exchanger

  • One of the most important features in a Viessmann boiler is its patented heat exchanger
  • Many boiler manufacturers such as Worcester-Bosch, Baxi and Ideal use an aluminium heat exchanger manufactured by a third party based in France
  • Stainless steel is a more efficient and corrosion-resistant metal compared to aluminium

Interested in the Viessmann Vitodens 100-W Combi Boiler?