James Elston

James Elston

Published April 9, 2024

With a renewed focus on sustainability and environmentally friendly products, the best 40kW combi boiler is now hydrogen-blend-ready.

These combi boilers can produce hot water on demand and heating to meet your household’s needs while aligning with your sustainable goals. A larger home requires a bigger capacity boiler, and a 40kW boiler can meet the increased demand.

If you are unfamiliar with combi boilers, Eco Happy recommends reading our extensive guide on what combi boilers are.

This article highlights the best 40kW boilers on the market that are energy efficient, reliable, and good value for money.

4 Affordable 40kW Combi Boilers

Here are the most affordable 40kW combi boilers on the market:

1. Baxi 800 Platinum Plus 40kW Combi Boiler – Best for homes with 2+ bathrooms

The Baxi 800 Platinum Plus stands out with its efficiency and durable construction.


  • Efficiency rating: 93%
  • Band rating: A-rated
  • Flow rate: 16.4 litres per minute (L/min)
  • Central heating output: 34.4kW
  • Hot water max output: 40kW
  • Condensing: Yes
  • LPG compatible: No
  • Warranty: 10 Years
  • Price: £1,307
  • Total cost, including installation: £2,107 – £2,607


Made from brass components, including hydraulics, these condensing combi boilers are perfect for households with higher central heating and domestic hot water needs. Another handy feature is the compact design, which fits snugly into kitchen cupboards.

Notable features of this boiler include an OpenTherm interface, a 24-hour timer and an exceptional flow rate. Its versatility is enhanced by compatibility with multi-fit flue kits, IFOS, OWOS weather sensors and convenient front access to all components. Additionally, it ensures year-round efficiency with its built-in frost thermostat.

2. Ideal Vogue Max 40kW Combi Boiler – Best value for money

The Ideal Vogue Max C40 combi gas boiler is designed for wall-mounted installation.


  • Efficiency rating: 93%
  • Band rating: A-rated
  • Flow rate: 16.4 L/min
  • Central heating output: 32kW
  • Hot water max output: 40kW
  • Condensing: Yes
  • LPG compatible: Yes
  • Warranty: 10 years
  • Price: £1,323
  • Total cost, including installation: £2,123 – £2,623


These combi boilers have a heating output of 32kW and a domestic hot water output of 40kW. This makes it suitable for homes with two bathrooms and 15 radiators. The stylish compact design has rotary control dials and a damped opening control cover.

This boiler comes with a warranty period of 10 years, which can be extended to 12 years. Its most useful features include a stainless steel heat exchanger, ensuring durability and longevity.

It has built-in frost protection to ensure performance during colder months. The Ideal system filter protects this combi boiler from oxide buildup, ensuring its longevity.

Check out our page on Ideal boilers to learn more about the brand.

3. ATAG i40C Combi Boiler – Best warranty period

The ATAG i40C Combi Boiler offers advanced features, like communication for smart technology integration and an Ideal Weather Compensation kit for automated temperature adjustments.


  • Efficiency rating: 94%
  • Band rating: A-rated
  • Flow rate: 16.2 L/min
  • Central heating output: 31.2kW
  • Hot water max output: 42kW
  • Condensing: Yes
  • LPG compatible: Yes
  • Warranty: Up to 14 years
  • Price: £1,300
  • Total cost, including installation: £2,100 – £2,750


With its dimensions of 700 mm high, 440 mm wide, and 366 mm deep, the ATAG i40C Combi Boiler is designed to save space. It comes with a stainless steel heat exchanger with a lifetime replacement guarantee, ensuring durability. 

This boiler is compatible with hot water systems and the ATAG iSteel twin coil cylinder range. The LCD provides precise boiler diagnosis by displaying fault codes. This boiler guarantees reliability with a 10-year warranty (to 14 years) and an anti-cycling function for optimal performance.

4. Viessmann Vitodens 050-W – Most versatile

Viessmann boilers are some of the most reliable, efficient, and cost-effective products on the market, and this boiler is no different. For homes with up to 20 radiators and one to two bathrooms, the Viessmann Vitodens 050-W 40kW Combi Boiler is the ideal choice. 


  • Efficiency rating: 93%
  • Band rating: A-rated
  • Flow rate: 14.1 L/min
  • Central heating output: 32kW
  • Hot water max output: 34.2kW
  • Condensing: Yes
  • LPG compatible: Yes
  • Warranty: 3 years, can be extended to 10 years
  • Price: £916
  • Total cost, including installation: £1,716 – £2,216


This incredible combi boiler can run on a blend containing 20% hydrogen and 80% natural gas, making it future-ready. Additionally, its compact size allows for installation within a kitchen cupboard.

The Vitodens 050-W has a steel Inox Radial heat exchanger and an impressive modulation range of up to 1:8. These combi boilers have built-in WiFi and digital user interfaces with LCD screens.

One key benefit of installing this boiler is upgrading to the ViCare thermostat, which further increases control over your heating needs.

2 Top Premium 40kW Combi Boilers

If combi boiler cost (for purchasing and installation) is not a concern, then you should consider a premium boiler.

Premium boilers might be expensive but are worth the investment. They are designed to be eco-friendly and cost less to run. The following premium boilers are our top picks:

1. Viessmann Vitodens 200-W 40kW Combi Boiler – Most energy efficient

The Viessmann Vitodens 200-W is ideal for homes with four to six bedrooms.


  • Efficiency rating: 94%
  • Band rating: A-rated
  • Flow rate: 14 L/min
  • Central heating output: 35kW
  • Hot water max output: 34.9kW
  • Condensing: Yes
  • LPG compatible: Yes
  • Warranty: 3 years standard; 5 years with Viessmann trained installer; 10 years on the heat exchanger
  • Price: From £1,516
  • Total cost, including installation: £2,450 – £2,950


The Viessmann Vitodens 200-W 40kW Combi Boiler is known for its technology, which includes a “Lambda combustion control system” that ensures combustion. Its versatility is showcased by its ability to modulate up to 19:1 and compatibility with fuel sources, including hydrogen.

The user-friendly features include an LCD touchscreen control panel and compatibility with technology. Additionally, the boiler offers a 5″ colour touchscreen control with weather compensation and an “Energy cockpit “, making it appealing to those seeking an option that prioritises optimal combustion and future fuel source compatibility.

2. Worcester Bosch Greenstar 8000 Style 40kW Combi Boiler – Best technology features

The Worcester Bosch Greenstar 8000 Style 40kW Combi Boiler provides an energy-efficient solution for homes with two bathrooms. To see how Worcester Bosch boilers stack up against Viessman boilers, check out our Worcester Bosch Vs Viessmann boilers guide. 


  • Efficiency rating: 94%
  • Band rating: A-rated
  • Flow rate: 16.4L/min
  • Central heating output: 35kW
  • Hot water max output:40.8kW
  • Condensing: Yes
  • LPG compatible: Yes
  • Warranty: 5 years, extended warranty of up to 12 years
  • Price: £1,625
  • Total cost, including installation: £2,425 – £2,925


The Worcester Bosch 8000 measures 440 mm wide, 824.95 mm high, and 364.6 mm deep. The compact design allows for wall-mounted installation.

Notable features include the Intelligent Filling System that prevents the boiler from losing pressure. Its increased modulation ratio and innovative technology compatibility are easy to use, allowing you to fine-tune your heating needs using your smartphone.

A notable feature includes the Bosch EasyControl smart thermostat; it is compatible with Google Assistant and Amazon’s Alexa.

How We Chose The Best 40kW Combi Boilers

Here are the resources we used to create our list of the best 40kW combi boilers.

Independent testing bodies

Independent testing bodies are an excellent source of knowledge as they evaluate boilers based on the same criteria we use to determine the best 40kW combi boilers. They carry out rigorous tests based on boiler efficiency, reliability, and overall performance, and then publish the impartial results. 

This data is often more reliable than the information provided by manufacturers who, of course, are likely to be biased. 

Customer reviews

We also spent a lot of time scouring review sites like Trustpilot and compiling lists of four and five-star reviews of 40kW combi boilers. This allowed us to see which boilers receive the most positive ratings and reviews, and which ones were being reviewed poorly. 

We also focused on reviews that contained key terms related to boiler performance like “power output” and “energy efficiency rating”. 

Why Choose A 40kW Combi Boiler?

The size of a boiler is a factor in meeting a larger home’s central heating and high hot water demand. 40kW combi boilers can provide heat for mid-size to larger homes with multiple bathrooms.

A combi boiler that is too small can lead to inadequate heating. This increases energy consumption, leading to higher operating costs. To learn more about the factors that influence your choice, refer to our guide on selecting the right boiler size

Who Needs A 40kW Boiler?

40kW combi boilers are ideal for:

  • Larger homes: If you have a home with four to five bedrooms, two or more bathrooms, and spacious living areas, a 40kW combi boiler will meet your demands of increased domestic hot water usage.
  • High heating demand: Homes with 20 radiators in colder climates benefit from the higher output provided by a 40kW boiler.  
  • Extensive heating zones: If your house has heating zones, a 40kW combination boiler can effectively meet the heating requirements of each area within your home.

Advantages Of A 40kW Combi Boiler 

There are several benefits to installing a 40kW combi boiler in your home.

  • High heating output: Combi boilers of this size offer a higher heating capacity, making them well-suited for larger properties with higher heating requirements. 
  • Efficient hot water supply: The average flow rate for 40kW combi boilers is between 14L and 16.4L per minute at 35 degrees Celsius, providing hot water within five seconds.
  • Versatile heating adaptability: A 40kW combination boiler can adapt to most modern heating systems, such as radiator setups and underfloor heating. 
  • Compact design: Despite its output, modern 40kW combination boilers often come in sleek, compact designs that fit perfectly into kitchen cupboards or wall mounting installations. 

Top Features To Look For

There are various features to consider before purchasing the best 40kW combi boiler for your needs:

Property size

The size of your home plays a role in determining the heating capacity required. Consider the number of people in the house, the amount of radiators, and how many bathrooms you have to ensure you choose the best boiler for a large house

Energy efficiency ratings

The ErP (Energy-related Products) Directive typically measures boiler energy efficiency ratings. The ErP rating scale ranges from A to G, with A+++ being the most efficient and G being the least efficient.

The percentage represents how the boiler converts energy into heat over a year. For example, an A-rated combination boiler can convert 92% of consumed energy into heat throughout the year, resulting in minimal energy waste, reduced costs and environmental friendliness.

Smart controls

Most 40kW combi boilers come equipped with smart controls that enable homeowners to control and monitor their heating systems using mobile apps or smart thermostats. This feature offers flexibility and can optimise heating schedules according to preferences.

Hydrogen-ready capability

In line with the growing focus on clean energy sources, most combi boilers are designed to be hydrogen-blend-ready. This means they can adapt to using 20% hydrogen as a fuel source with natural gas.

Water flow rate

The flow rate of a boiler refers to how much hot water it delivers to taps within a minute. It’s important to note that this depends on the main water supply to your home. Unlike regular boilers that store hot water in a separate hot water cylinder, combi boilers draw water directly from the mains supply. If the mains flow rate is lower than the boiler capacity, it won’t be able to achieve its specified level.

Boiler capacity

The capacity of a boiler (output rating) and its flow rate are closely connected. The output rating determines how much water the boiler can heat, while the flow rate determines how much water can be pushed through for use. 


Having a warranty offers peace of mind. This can indicate the manufacturer’s confidence in their product’s durability. For added protection, it may be worth considering warranty options, like an extended warranty on parts and labour.

Comparing The Best 40kW Combi Boilers: What Sets Our Top Picks Apart 

Our top pick for the best 40kW combi boiler is the Worcester Bosch Greenstar 8000 Style combi boiler.

  1. Worcester Bosch Greenstar 8000 Style 40kW Combi Boiler: ErP: 94%; Warranty: Up to 12 years; Price: £1,625
  2. ATAG i40C Combi Boiler: ErP: 94%; Warranty: Up to 14 years; Price: £1,300
  3. Viessmann Vitodens 200-W: ErP: 94%; Warranty: 3 years; Price: From £1,516
  4. Viessmann Vitodens 050-W: ErP: 93%; Warranty: 3 years (can be extended to 10 years); Price: £916
  5. Baxi 800 Platinum Plus 40kW: ErP: 93%; Warranty: 10 years; Price: £1,307
  6. Ideal Vogue Max 40kW: ErP: 93%; Warranty 10 years; Price £1,323

Installation Requirements

To ensure your combi boiler functions at its peak, it is crucial to have a Gas Safe registered engineer complete the combi boiler installation. Incorrect pressure, airlocks and gas or water leaks can compromise the boiler system. These issues can be prevented if the combi boiler installation is done correctly.

Qualified heating engineers follow safety protocols to minimise the risk of gas leaks, carbon monoxide emissions and other safety hazards. This is crucial for ensuring the safety of your home.

Many manufacturers require professional installation to maintain warranty coverage. By choosing a certified installer, you can ensure that you meet these requirements and safeguard your investment in the boiler.

Qualified heating engineers have the expertise to seamlessly integrate a boiler with heating components like smart thermostats or weather sensors. This integration optimises the functionality of the system.

Servicing Requirements

One key advantage that combi boilers have is that they do not require a hot water cylinder. This ensures minimal maintenance. However, all boilers must be serviced annually, whether you have a traditional system boiler or a combi boiler.

Regularly servicing your boiler is essential for prolonging its lifespan. Routine maintenance helps address issues before they escalate, prevents wear and tear, and promotes the system’s longevity.

Plus, manufacturers often require servicing to uphold warranty coverage. Failure to adhere to these maintenance guidelines may void the warranty. This leaves you responsible for repair costs that would otherwise have been covered.

During servicing, components are cleaned, and checks are done for limescale buildup. Combustion optimisation takes place—all contributing factors to lower energy bills.

While professional combi boiler installation and regular servicing may involve costs incurred by homeowners, they ultimately lead to long-term savings. A working boiler can save on energy bills and prevent breakdowns.


What is the most reliable combi boiler?

The Viessmann Vitodens 100-W is the most reliable combi boiler, boasting a 98.9% reliability rating. The Worcester Bosch Greenstar 8000 Lifestyle 50 is just as durable and reliable and is a top pick for many homeowners.

How much does a 40kW boiler cost per hour?

A standard system boiler can cost £2.79-£4.66 per kWh to run, while an oil boiler typically costs 11.66p, and combi boilers cost £2.82 per kWh to run.

What is the most powerful combi boiler in the UK?

The 35kW Vaillant ecoTEC 835 Exclusive with Green iQ is considered the most powerful combi boiler on the market. The Green iQ technology optimises running cycles to keep consumption low. The modulation range is 1:10, which ensures low fuel bills.

Final Thoughts

The best 40kW combi boiler is the Worcester Bosch Greenstar 8000 Style. Not only does it have a high ErP rating, but the technological advantages are incredibly easy to use. The ATAG i40C Combi Boiler deserves an honourable mention since you get more for an affordable price.

Each home is different. Before deciding, consider the property size and the features you would like. Contact Eco Happy today for a personalised quote or if you require assistance assessing your needs.

James Elston

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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