James Elston

Published April 9, 2024

A 40kW system boiler is particularly suitable for larger households with several rooms and approximately 20 radiators that require domestic hot water and heating. When deciding which boiler output best suits your needs, you should consider factors such as the size of your property, the number of bedrooms and the number of bathrooms.

This article will explore why selecting the best 40kW system boiler is essential and provide insights into the top-rated 40kW boilers that can help you make an informed decision.

Top 7 40kW System Boilers Of 2024

Let’s review the top-rated 40kW system boilers for 2024.

1. ATAG I40S 40kW System Gas Boiler

Based in the Netherlands, ATAG manufactures high-quality boilers made with stainless steel and brass components and precision engineering techniques. All of ATAG’s boilers are rated A for efficiency. 


  • Efficiency rating: 94%
  • Band rating: A rated
  • Central heating output: 38.8kW
  • Warranty: Up to 14 years
  • Price: £1,500
  • Cost including installation: £2,300 – £2,800


The ATAG I40S 40kW System Gas Boiler is an efficient heating solution for large homes with space and high hot water demand. 

This system boiler, measuring 700 x 440 x 355 mm, can be easily wall-mounted and offers an output of 38.8kW, making it suitable for homes with greater heating needs. There are also smaller output options starting from 15kW, providing flexibility to meet different needs.

One outstanding feature of the ATAG I40S is its performance, which enhances the comfort of the living area. Its user-friendly controls further contribute to its appeal by giving homeowners control over the boiler’s operations.

The unique iCon heat exchanger comes with a lifetime replacement guarantee, adding a level of confidence in the reliability of the unit. It also features quality steel and brass components, ensuring its strength and durability.

It’s important to note that the ATAG I40S may require space for a water cylinder. The limitation on the size of the tank might impact the amount of water, so it could be necessary to add insulation to the tank to avoid wasting heat and energy.

2. Vaillant ecoTEC plus 637 System Gas Boiler

Vaillant is based in Germany and has been manufacturing heating and cooling products since 1874. Its extensive range of boilers is known for its innovative technology and durable stainless steel heat exchangers. 


  • Efficiency rating: 94%
  • Band rating: A rated
  • Central heating output: 37.6kW
  • Warranty: 7 years
  • Price: £1,537
  • Cost including installation: £2,337 – £2,837


For homes with five to six bedrooms, the Vaillant ecoTEC Plus is a great choice as it offers a powerful heating solution.

Vaillant ecoTEC’s A-rated efficiency and two-speed automatic pump contribute to energy savings and minimising heat loss.

The stainless steel heat exchanger ensures durability. This system boiler also seamlessly blends in with the home as a floor-standing unit.

One disadvantage of the ecoTec boiler is that it will automatically shut down if it experiences air duct leaks, rendering it useless until fixed.

Additionally, potential buyers might be dissuaded by the warranty. While a seven-year warranty is standard, you can get an extended warranty of 10 years at a cost.

3. Mistral Sealed System CS4 41kW Oil Boiler

Mistral is a specialist oil-fired boiler manufacturer that produces a range of condensing and non-condensing boilers. Based in the UK, its products are highly regarded for their durability and quiet operation. 


  • Efficiency rating: 93%
  • Band rating: A rated
  • Heating output: 41kW
  • Warranty: 2 years
  • Price: £3,200
  • Cost including installation: £4,500 – £5,000


The Mistral Sealed System CS4’s output of 41kW is ideal for homes with higher heating demands and hot water needs. This boiler offers flexibility, with options for condensing and non-condensing models as well as indoor and outdoor units.

The boiler features include a circulating pump, a 12-litre expansion vessel and a safety valve. One potential drawback, however, is that it only comes with a two-year warranty with no option for extended warranties. 

This particular oil boiler from Mistral is designed to be placed under kitchen worktops, making it a suitable choice for smaller homes.

Since it is a flooring unit, this oil boiler requires additional space for a hot water cylinder. 

4. Alpha E-Tec 38kW

Alpha has a wide range of boilers including system, regular, and combi boilers. Alpha boilers tend to be fairly affordable and are known for their longevity. 


  • Efficiency rating: 93%
  • Band rating: A rated
  • Central heating output: 32kW
  • Warranty: 13 years
  • Price: £1,033
  • Cost including installation: £1,833 – £2,370


The E-Tec’s stainless steel heat exchanger and high-efficiency Grundfos pump align with heating standards. It also includes a tech-modulating remote called ‘Climatic’ that enhances control and synchronisation with the Tec range, providing an easy-to-use experience.

However, it only offers three outputs and relies on strong main pressure. This means households with high hot water demands or those looking to connect power showers may face some restrictions.

On the other hand, the Alpha E Tec 38kW comes with integrated features like a filling loop and control panel, which make it a reliable and efficient option for those seeking a system boiler.

5. Viessmann Vitodens 100-W 35kW

Viessmann are another high-quality boiler brand based in Germany. Viessmann boilers are becoming increasingly popular due to the diversity of the range and the company’s focus on green technology. 


  • Efficiency rating: 98%
  • Band rating: A rated
  • Central heating output: 32kW
  • Warranty: 5 years; extended Warranty up to 12 years
  • Price: From £1,500
  • Cost including installation: £2,300 – £2,800


This system boiler’s design includes hydrogen capabilities and built-in WiFi catering. An LCD touchscreen enhances user experience, allowing navigation and control. With its 1:6 modulation ratio, it can also adjust according to the home’s heating demands.

This boiler is known for its durability thanks to features like the MatriX Plus burner, steel MatriX surface, and impressive lifespan of 15 to 20 years.

However, it can only deliver water to one faucet at a time and there may be potential pressure issues if you use multiple faucets simultaneously.

Overall, the Viessmann Vitodens 100-W 35kW is a technologically advanced option for those who need a high-capacity system boiler.

6. Intergas Compact HRE SB 40

Intergas boilers are known for their versatility and the company has become famous for integrating cutting-edge tech into their products. 


  • Efficiency rating: 92%
  • Band rating: A rated
  • Central heating output: 42.5kW
  • Warranty: 7 years
  • Price: From £1,015
  • Cost including installation: £1,935 – £2,435


The Intergas Compact HRE SB 40 is a condensing and wall-mounted system boiler with an output rating of 42.5kW. If you want to learn more about condensing boilers, check out our comprehensive guide. 

One of its standout features is its versatility – you can heat water by adding a storage cylinder. The warranty on the heat exchanger (10 years) and parts (two years) provides added peace of mind regarding its durability.

Another advantage is that it doesn’t have an air pressure switch, which adds to its adaptability as well as the choice between horizontal and vertical flue options.

The Intergas Compact HRE SB 40 works well for properties with moderate hot water and heating demands. It can accommodate both on/off thermostats and programmable room thermostats, making it flexible regarding temperature control. Additionally, it has built-in boiler frost protection, which proves useful during winter months.

7. Worcester Bosch Greenstar 8000 Life

Worcester Bosch is one of the most well-known and well recommended boiler brands in the world. Its manufacturing is of the highest quality and the performance of its boilers is praised by customers and engineers alike. 


  • Efficiency rating: 94%
  • Band rating: A rated
  • Central heating output: 35kW
  • Warranty: Up to 12 years
  • Price: £1,200
  • Cost including installation: £2,000 – £2,500


It has a 94% Energy-related Product (ErP) efficiency rating and is suitable to supply hot water to multiple bathrooms. Its compact design allows for wall hanging and it can be easily controlled with smart technology controls. The Greenstar smart filling system ensures that pressure is maintained at a practical level for users.

However, the Worcester Bosch Greenstar 8000 Life comes with a hefty price tag and installation costs, which might not be ideal for budget buyers. Additionally, its larger size may not be suitable for smaller spaces, and the thermostat only supports two zones, which could be limiting.

Despite these considerations, the Worcester Bosch Greenstar 8000 Life stands out as a technologically advanced option if you prioritise performance and convenience for your heating system.

How We Chose The Best 40kW System Boilers

Here’s how we determined which boilers should appear on our list.

Customer reviews

To create our list, we assessed boilers based on a set of criteria such as energy efficiency, versatility, and pricing. One of the ways we evaluated this was by finding customer reviews of boilers on sites like Trustpilot. 

We looked out for reviews containing keywords that related to our criteria and then checked the ratings customers were giving boilers for specific specs and features. This allowed us to establish plenty of data on a boiler’s overall performance. 

Independent testing bodies

We also analysed the test results from independent bodies like Which? to see how different boilers compared to each other. If a boiler was highly rated by customers and performed well in these impartial tests, then it had a good chance of being included in the list. 

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

Our top pick for a system boiler would be the Viessmann Vitodens 100 W. It stands out for its energy efficiency, affordability and advanced technology.

Another option to consider is the Worcester Bosch Greenstar 8000 Life. It offers smart technology controls and has a high-efficiency rating, making it a valuable investment.

The Intergas Compact HRE SB 40 is also worth considering if you’re looking for a budget-friendly system boiler.

  1. Viessmann Vitodens 100-W 35kW: ErP of 98%; heat output of 32kW; 5-year warranty (option to extend to 12 years)
  2. Worcester Bosch Greenstar 8000 Life: ErP of 94%; heat output of 35kW; warranty up to 12 years
  3. Intergas Compact HRE SB 40: ErP of 92%; heat output of 42.4kW; 7-year warranty.

Is A 40kW System Boiler Right For You?

The ideal boiler for your home will depend on the size of your property and how much heating and hot water you need. A 40kW system boiler offers unique benefits:

  • Efficient heating for larger areas: With its ability to power an extensive network of radiators effectively, a 40kW system boiler ensures consistent warmth throughout every corner of your home.
  • Energy efficiency: These boilers optimise energy, promoting sustainability and cost-effectiveness, resulting in lower energy bills.
  • Rapid and responsive heating: Thanks to its capacity, your home reaches its desired temperature promptly.
  • No need for a cold water tank: Unlike other boiler systems, system boilers don’t require a cold water tank since they directly draw water from the main supply.

How Much Does A System Boiler Cost?

A 40kW system boiler can start at £1,000. However, this price doesn’t include the installation of a hot water cylinder. 

Typically, installation prices average around £3,000. However, keep in mind that final costs can vary based on the type of boiler, its complexity, and where it needs to be installed.

Some factors can impact the expenses of installing system boilers:

  • Installation complexity: The complexity of your existing heating system in your home can affect the installation costs.
  • Location and access: The geographical aspects and how easily accessible the installation site is.
  • Property size: For instance, if you have heated rooms, you might require additional radiators.

The installer rates, fuel type, boiler brand, size of pipework and radiators, heating controls, and labour costs can also affect the final cost of installation.

What To Consider When Choosing The Best 40kW System Boiler 

When selecting a system boiler for your home, it’s important to consider the following points:

  • Output size and type: A boiler’s kilowatt (kW) rating determines the water’s flow rate. If you have a five-bedroom house with 20 radiators, the Worcester Bosch 8000 Style in the 40kW size would be suitable.
  • Efficiency rating: The Worcester Bosch 8000 has an efficiency rating of 94%, making it highly energy efficient. This means that around 6% of its energy is wasted.
  • Warranty length: Opting for a manufacturer-backed warranty can offer protection against boiler problems and save you money on repairs. A comprehensive warranty should cover issues like leaks and faulty heat exchangers that may arise due to manufacturing faults.

Read reviews and compare brands and models of 40kW system boilers, considering their pros and cons. It’s also essential to align your budget with the boiler you are considering and factor in installation costs.

Take the next step in your home heating journey and get a personalised quote or advice from Eco Happy today.


How long does it take to install a system boiler?

This largely depends on how complex the installation is. For example, if it’s just a like-for-like swap then it may only take around four to six hours to install a system boiler. However, if it’s a complete system upgrade and your boiler will be installed in a different location to your old one, it could take one to two days. 

Why is a system boiler’s efficiency rating important?

The more efficient a system boiler it is, the more energy is converted into usable heat and the less energy is wasted. This means less energy is required to heat your home and supply hot water which can reduce your energy bills and your home’s carbon footprint. 

Which system boiler is most efficient?

Modern condensing boilers are generally the most efficient to run, resulting in lower energy and heating costs.

Final Thoughts

Selecting a 40kW system boiler is more than making a decision – it’s an investment in the comfort and efficiency of your home. It’s crucial to consider the advantages of choosing the system boiler, including warmth, energy efficiency and prompt response times. Contact us at Eco Happy. We’ll be happy to assist you with a quote.

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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