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Which is the Best 40kW System Boiler?

Becky Mckay
By: Becky Mckay
Updated: 6th July 2022

Best 40kW System Boiler

The best 40kW system boilers are powerful heating systems.

They can meet the heating demand of large homes with around 20 radiators. And, thanks to storing hot water in a cylinder, can send hot water to 2 or more bathrooms.

Only one system boiler has an output of exactly 40 kW and that’s the ATAG IS40. So if you're specifically looking for a 40 kW system boiler, your decision is pretty straightforward.

Fortunately, an output rating between 35 kW and 42 kW could also be suitable and this gives us more system boilers to compare:

  • ATAG i40S
  • Intergas Compact HRE SB 40
  • Vaillant ecoTEC Plus 637
  • Worcester Bosch Greenstar 8000 Style 35

How do system boilers work?

A system boiler is suited to larger properties with 2 or more bathrooms. That’s because they store domestic hot water for bath, taps and showers in a cylinder so can meet higher demand.

They’re connected to the mains water supply, which will give you strong water pressure. And means you can remove the tanks in your loft if you’re replacing a regular boiler.

Is a 40kW system boiler right for my home?

40 kW system boilers are powerful heating systems.

They’re best suited to homes with around 20 radiators and a high demand for central heating.

Before investing in a 40 kW system boiler, you should get expert advice from a fully-qualified heating engineer. Using Boiler Guide, you can get free quotes from up to 3 Gas Safe registered engineers in your area by simply completing our quick online form. They'll be in touch to arrange a quote and will take a look at your property and recommend a type of boiler and size based on the heating and hot water demands of your home.

If a lower output would be better for your heating and hot water demands, we have more comparison articles:

Best 40kW system boilers

As we mentioned, there's currently only one 40kW system boiler available, the ATAG i40S, making it the best 40kW system boiler by default. So, for that reason, we're comparing the best system boilers with outputs of 35-42 kW which could all suit similar property types and sizes.

Best 35-42 kW System Boilers Central Heating Output Rating Space Heating Efficiency Rating ErP Rating Warranty
ATAG i40S 38.8 kW 94% A 10 years
Intergas Compact HRE SB 42.5 kW 92% A 7 years
Vaillant ecoTEC Plus 637 37.6 kW 94% A 5 years
Worcester Bosch Greenstar 8000 Life 35 kW 94% A 10 years


ATAG are well known for making lengthy 10 year warranties available on all of their boilers which shows how much faith they have in their products. The i40S system boiler is highly efficient and the simple controls make it very easy to operate.

  • Built-in ErP pump which helps to maximise efficiency
  • User-friendly controls
  • The iCON heat exchanger has a lifetime guarantee
  • Compatible with ATAG iSteel hot water tanks


Intergas Compact HRE SB 40

Intergas Compact HRE SB 40
Intergas offer the most affordable option when it comes to finding a powerful system boiler but it can still compete with the more premium models and has even been endorsed by the Energy Saving Trust.

  • Only 4 moving parts which greatly reduces the risk of any faults developing
  • Choice between horizontal and vertical flues for a more flexible installation
  • Built-in boiler frost protection to keep the unit running through cold winter months (particularly important when the boiler is installed in a garage or loft)
  • A 10 year warranty on the heat exchanger


Vaillant ecoTEC Plus 637

Vaillant ecoTEC Plus 637
The Vaillant ecoTEC range is made up of modern stylish boilers that boast a stainless steel heat exchanger which helps to boost efficiency and are well covered by an extensive warranty.

  • Stainless steel heat exchanger for increased efficiency
  • Compact dimensions make it possible for the boiler to be installed into a small cupboard
  • Quiet Mark accreditation shows that you won't have to worry about operating noises
  • Frost protection to prevent water in the pipes from freezing during the winter
  • With the Vaillant vSmart app you can control your central heating and domestic hot water from wherever you are

Worcester Bosch Greenstar 8000 Style

Worcester Bosch Greenstar 8000 Style 35

Worcester Bosch have been an industry leading boiler manufacturer for a number of years and with the Greenstar 8000 Lifestyle range, they've further cemented their place.

  • Greenstar 8000 Lifestyle boilers are available in black as well as the conventional white
  • Modulation ratio of 1:10 which means that the boiler won't use more energy than it needs to for increased efficiency
  • An on/off cycling prevention feature helps to keep the boiler running for longer
  • Front, top and side panels are all removable which makes life much easier for a heating engineer and will save you money when it comes to servicing and any necessary maintenance

How much do 40 kW system boilers cost?

A powerful 40kW system boiler is likely to cost at least £1,000. That’s before you add need for a hot water cylinder and installation.

The Intergas Compact HRE SB is often the most affordable but all of the system boilers in this comparison have a similar price. Although the price tag of the ATAG i40S hasn't been made available.

Best 40kW System Boilers Potential Cost (before installation)
Intergas Compact HRE SB £1,125 – £1,600
Vaillant ecoTEC Plus 637 £1,390 – £1,500
Worcester Bosch Greenstar 8000 Life £1,300 – £1,550
Worcester Bosch Greenstar 8000 Style £1,300 – £1,550

Best system boilers for off-grid homes

The system boilers we've looked at in this article all run on natural gas but not every property is connected to the gas network. In those cases, you will need to store fuel onsite and that means turning to liquified petroleum gas (LPG) or oil. Fortunately, all of these natural gas system boilers run on LPG.

Best 40kW System Boiler LPG Compatible?
ATAG i40S Yes
Intergas Compact HRE SB Yes
Vaillant ecoTEC Plus 637 Yes
Worcester Bosch Greenstar 8000 Style Yes

Should you prefer an oil boiler, then there are plenty of options available that are capable of meeting the heating and hot water demands as a gas-fired boiler.

Best 35-42 kW System Oil Boilers Output Rating Space Heating Efficiency Rating ErP Rating Warranty
Firebird EnviroBlue SystemPac C35 35 kW 94.4% A 2 years
Grant VortexBlue 36 kW 94.5% A 2 years
Mistral Sealed System CS4 41 kW 93% A 2 years
Navien LCB700 36 kW 93% A 10 years
Viessmann Vitorondens 200-T 42 kW 97% A 10 years

If you plan to have an oil boiler installed, you’ll need to hire an OFTEC registered heating engineer to carry out the installation. We can connect you with OFTEC or Gas Safe registered engineers in your area who will provide you with free quotes. All you need to do is answer a few quick questions.

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Becky Mckay

About the author

Becky Mckay

Becky is one of our home heating and renewable energy experts and has a wealth of experience writing about the world of heating.

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