The 5 best 40kW system boilers for larger homes across Kent and the South East are the
- ATAG i40S
- Intergas Compact HRE SB 40
- Vaillant ecoTEC Plus 637
- Worcester Bosch Greenstar 8000 Life 35
- Alpha E-Tec 38kW System
If you live anywhere across Kent postcodes such as Maidstone, Tunbridge Wells, Ashford or Canterbury, or South East postcodes including Guildford, Tonbridge and Brighton, one of these five models is very likely to be the right match for a bigger household.
Below, our engineers break down the running costs, warranty terms and independent ratings for each boiler, so you can see exactly which one fits your property rather than choosing based on the kilowatt figure alone.
5 Best 40kW System Boilers: A Quick Overview
The best 40kW system boilers combine strong central heating output with reliable hot water storage and a warranty that genuinely protects your investment, and the table below shows how the top five compare against each other.
| Boiler | Efficiency | Boiler Only Cost | Installed Cost | Warranty | Best For | Which? / Trustpilot Rating |
| ATAG i40S | 94% ErP A | £1,500 | £2,300 to £2,800 | 10 years, up to 14 years | Longest warranty seekers, large homes | 4.8/5 Trustpilot from nearly 4,000 reviews |
| Intergas Compact HRE SB 40 | 92% ErP A | £1,125 to £1,600 | £1,900 to £2,435 | 7 years, 10 on heat exchanger | Budget conscious households, simple reliable design | Endorsed by Energy Saving Trust |
| Vaillant ecoTEC Plus 637 | 94% ErP A | £1,390 to £1,500 | £2,300 to £3,300 | 5 years, up to 7 with Advance Installer | Quiet running, compact installations | 4.6/5 Trustpilot |
| Worcester Bosch Greenstar 8000 Life 35 | 94% ErP A | £1,300 to £1,550 | £2,110 to £2,750 | Up to 10 years | Homeowners wanting proven UK brand reliability | Which? Recommended Provider, 4.5/5 Trustpilot |
| Alpha E-Tec 38kW System | 93% ErP A | £1,000 | £1,800 to £2,300 | 10 years | Best value for larger homes, landlords | 4.6/5 Trustpilot |
Prices are UK guide figures for 2026 and can vary depending on your property, cylinder choice and where in Kent or the South East you are based.
If you are weighing up whether a 40kW system boiler is genuinely the right size for your home, it is worth getting a proper survey rather than guessing from a spec sheet. Guscott Heating installs and replaces boilers across Kent and South East postcodes, including Maidstone, Tunbridge Wells, Ashford, Canterbury, Sevenoaks, Tonbridge, Dartford, Medway and Brighton, and every boiler quote is free with no obligation to book.
Our Gas Safe registered engineers can measure your actual hot water and heating demand on site and tell you honestly whether a 40kW system boiler is the correct fit, or whether a different size or boiler type would serve your household better.
5 Best 40kW System Boilers: Recommended By Our Engineers
Each of these five boilers earns its place on this list for a genuine reason, whether that is warranty length, running cost, reliability or brand support, so here is a proper breakdown of what you actually get with each one.
1. ATAG i40S: Best For Longest Warranty And Large Households
The ATAG i40S stands out as the only system boiler on the UK market with an output of exactly 40kW, and it backs that up with the longest warranty and some of the strongest customer satisfaction scores of any boiler brand available.
ATAG has designed and manufactured boilers for nearly seven decades, and the i40S reflects that experience with genuinely dependable engineering built for households with heavy heating and hot water demand.
| Specification | Detail |
| Features | Lifetime guaranteed heat exchanger, iSteel cylinder compatible, built-in ErP pump |
| Efficiency | 94% ErP A |
| Estimated unit cost | £1,500 |
| Installed cost | £2,300 to £2,800 |
| Running costs | £1,230 to £1,360 per year |
| Best for | Large homes, longest warranty seekers |
| Warranty | 10 years standard, up to 14 years |
| Which? rating | Not independently tested by Which? |
| Trustpilot rating | 4.8 out of 5 |
Features and specs
The i40S measures a compact 440mm wide by 700mm high by 355mm deep, which is smaller than several rivals despite its high output. It includes a stainless steel iCon heat exchanger backed by a lifetime guarantee against corrosion, simple user friendly controls that make day to day operation straightforward, and full compatibility with ATAG’s own iSteel hot water cylinders for a matched system.
Efficiency
This model carries a 94 percent ErP A efficiency rating, among the strongest figures available in the 40kW system boiler category.
Unit cost
The boiler alone typically costs around £1,500, positioning it in the middle of this comparison for unit price.
Installation cost
Fully fitted, including a compatible cylinder where needed, homeowners across Kent postcodes such as ME14, ME15 and TN1 to TN13 can expect to pay between £2,300 and £2,800, though the final figure depends on cylinder size and existing pipework.
Running costs
For a large four to five bedroom household using around 16,000 kWh of gas a year, based on the current Ofgem gas price cap rate, annual running costs typically land between £1,230 and £1,360, though insulation levels and how well the heating system is balanced play a significant part in the final figure.
Warranty
Standard cover runs to 10 years, extending to as much as 14 years when the right conditions are met at installation, the longest term available in this comparison.
Best for
Large households with two or more bathrooms, heavy simultaneous hot water use, and homeowners who want the longest possible warranty backed by a lifetime heat exchanger guarantee.
Which? and Trustpilot ratings
ATAG is not independently tested by Which?, but it holds an outstanding 4.8 out of 5 rating on Trustpilot from nearly 4,000 customer reviews, one of the highest scores of any boiler brand in the UK.
Should You Choose The ATAG i40S?
Choose the ATAG i40S if:
- You want the only genuinely 40kW system boiler on the UK market rather than a close alternative
- Long term warranty cover matters more to you than choosing the most recognisable household name
- Your home has two or more bathrooms with heavy simultaneous hot water demand
- You want a lifetime guarantee on the heat exchanger for real long term peace of mind
Do not choose the ATAG i40S if:
- You want the widest possible network of local installers, since ATAG’s partner network is smaller than Worcester Bosch’s
- You are set on choosing one of the most widely recognised household boiler brands
- You want independent Which? test data to compare against other models
2. Intergas Compact HRE SB 40: Best For Budget Conscious Households
The Intergas Compact HRE SB 40 is the most affordable genuinely high output system boiler in this comparison, and it earns its place by proving that dependable performance for larger homes does not always require a premium price tag. Intergas is a Dutch heating brand known for building simple, reliable boilers, and the Compact HRE SB has even been endorsed by the Energy Saving Trust for its efficiency credentials.
| Specification | Detail |
| Features | Only 4 moving parts, choice of vertical or horizontal flue, frost protection |
| Efficiency | 92% ErP A |
| Estimated unit cost | £1,125 to £1,600 |
| Installed cost | £1,900 to £2,435 |
| Running costs | £1,280 to £1,410 per year |
| Best for | Budget conscious larger homes |
| Warranty | 7 years, 10 on heat exchanger |
| Which? rating | Not independently tested by Which? |
| Trustpilot rating | Energy Saving Trust endorsed |
Features and specs
This model measures 450mm wide by 710mm high by 240mm deep, the slimmest depth of any boiler on this list, which makes it a good fit for tighter airing cupboards in older South East properties.
A genuinely useful feature is that it has only four moving parts, which significantly lowers the risk of faults developing over the boiler’s lifetime, and it offers a choice between vertical and horizontal flues for more flexible installation. It also includes built-in frost protection, useful if the boiler ends up fitted in a garage or loft space.
Efficiency
This model carries a 92 percent ErP A efficiency rating, slightly behind the premium brands on this list but still comfortably within A rated territory, and it delivers a strong 42.5kW output.
Unit cost
The unit typically costs between £1,125 and £1,600, making it the most budget friendly option in this comparison.
Installation cost
Fully installed pricing across Kent and South East postcodes generally runs from £1,900 to £2,435, a meaningful saving compared with the other boilers reviewed here.
Running costs
With a slightly lower efficiency rating, annual running costs for a similar sized household typically sit a touch higher at around £1,280 to £1,410, though the real world difference tends to be modest for most homes.
Warranty
Standard cover runs to 7 years, with a 10 year warranty specifically on the heat exchanger, which is shorter overall than several rivals but still solid for the price point.
Best for
Budget conscious homeowners and landlords who want a simple, reliable system boiler with fewer components that could potentially fail, without paying premium brand prices.
Which? and Trustpilot ratings
Intergas is not independently tested by Which?, but it holds a solid Trustpilot reputation and carries an Energy Saving Trust endorsement, which reflects strong efficiency credentials from an independent body.
Should You Choose The Intergas Compact HRE SB 40?
Choose the Intergas Compact HRE SB 40 if:
- Budget is your main concern but you still need a genuinely high output system boiler
- You like the idea of a simpler design with fewer moving parts that could develop faults
- Your installation needs flexibility between a vertical or horizontal flue
- The boiler will be fitted somewhere cold, like a garage or loft, where frost protection matters
Do not choose the Intergas Compact HRE SB 40 if:
- You want the longest possible warranty available in this category
- Brand recognition and a large UK installer network are important factors for you
- You want the highest efficiency rating available among 40kW system boilers
3. Vaillant ecoTEC Plus 637: Best For Quiet Running And Compact Installations
The Vaillant ecoTEC Plus 637 appeals to homeowners who want proven German engineering along with genuinely quiet day to day operation. Vaillant has built a strong reputation across the UK for boilers that installers describe as straightforward to service, and the ecoTEC Plus 637 brings that reliability into a compact case that suits properties where cupboard space is limited.
| Specification | Detail |
| Features | Quiet Mark certified, flame light detection, vSmart app control |
| Efficiency | 94% ErP A |
| Estimated unit cost | £1,390 to £1,500 |
| Installed cost | £2,300 to £3,300 |
| Running costs | £1,230 to £1,360 per year |
| Best for | Quiet running, compact installations |
| Warranty | 5 years, up to 7 years |
| Which? rating | Not independently tested by Which? |
| Trustpilot rating | 4.6 out of 5 |
Features and specs
This model measures 440mm wide by 720mm high by 406mm deep and includes a durable stainless steel heat exchanger designed to boost efficiency and resist corrosion over the boiler’s lifetime. A standout feature is its flame light detection system, which allows for quieter and more efficient combustion than older boiler designs, and it holds Quiet Mark accreditation confirming it runs among the quietest boilers independently tested in the UK.
The Vaillant vSmart app also allows full control of heating and hot water from a smartphone, wherever you are.
Efficiency
This model carries a 94 percent ErP A efficiency rating, putting it level with the ATAG and Worcester Bosch models in this comparison, with a central heating output of 37.6kW.
Unit cost
Retailers typically list this boiler between £1,390 and £1,500, though some listings show a wider range up to £3,000 depending on specification and included controls.
Installation cost
Fully fitted, expect to pay between £2,300 and £3,300 across Kent and South East postcodes, reflecting both the unit price and installation complexity.
Running costs
Thanks to the strong efficiency rating, running costs for a large household typically sit between £1,230 and £1,360 a year, comparable to the ATAG and Worcester Bosch models.
Warranty
Standard cover runs to 5 years, extending to 7 years when installed by a Vaillant Advance Installer, which is notably shorter than the maximum available on the ATAG or Alpha models.
Best for
Households that want a genuinely quiet boiler backed by independent Quiet Mark certification, and homeowners with limited cupboard space who still need strong central heating output.
Which? and Trustpilot ratings
Vaillant holds a 4.6 out of 5 rating on Trustpilot, with customers regularly praising quiet operation and dependable performance.
Should You Choose The Vaillant ecoTEC Plus 637?
Choose the Vaillant ecoTEC Plus 637 if:
- You want a genuinely quiet boiler backed by independent Quiet Mark certification
- Your installation space is tight and needs a more compact system boiler
- You value smartphone control of your heating and hot water through the vSmart app
- You are happy to use a Vaillant Advance Installer to unlock the full 7 year warranty
Do not choose the Vaillant ecoTEC Plus 637 if:
- You want a longer warranty without needing a specific accredited installer tier
- Budget is a major factor, since the price range here is broader and can run higher
- You want the widest choice of flue installation options, since Intergas offers more flexibility
4. Worcester Bosch Greenstar 8000 Life 35: Best For Proven UK Brand Reliability
The Worcester Bosch Greenstar 8000 Life 35 is the safest overall pick for homeowners who want a widely trusted brand with the largest installer network in the UK. Worcester Bosch has held Which? Recommended Provider status for years running, and the 8000 range brings that dependable reputation into the system boiler category with genuinely strong engineering.
| Specification | Detail |
| Features | 1:10 modulation, cycling prevention, removable panels for easier servicing |
| Efficiency | 94% ErP A |
| Estimated unit cost | £1,300 to £1,550 |
| Installed cost | £2,110 to £2,750 |
| Running costs | £1,220 to £1,350 per year |
| Best for | Proven UK brand reliability, large installer network |
| Warranty | Up to 10 years |
| Which? rating | Recommended Provider |
| Trustpilot rating | 4.5 out of 5 |
Features and specs
The Greenstar 8000 Life includes a stainless steel heat exchanger, a 1 to 10 modulation ratio that lets the boiler scale its output down gently on milder days rather than short cycling, and an on and off cycling prevention feature that helps keep the boiler running efficiently for longer. The front, top and side panels are all removable, which makes servicing considerably easier for engineers and can help keep long term maintenance costs down.
Efficiency
This model carries a 94 percent ErP A efficiency rating with a central heating output of 35kW, among the strongest figures in this comparison.
Unit cost
The unit typically costs between £1,300 and £1,550, sitting in the mid-range for this category.
Installation cost
Fully installed pricing across Kent and South East postcodes generally runs from £2,110 to £2,750, depending on cylinder choice and existing pipework.
Running costs
Thanks to the strong efficiency rating, running costs for a large household typically come in at £1,220 to £1,350 annually, comparable to the ATAG and Vaillant models.
Warranty
Standard cover runs to 10 years, one of the longer terms available among mainstream household brands in this category.
Best for
Homeowners who want dependable heating without unnecessary noise, and anyone who values a huge UK installer network for future servicing and parts availability.
Which? and Trustpilot ratings
Worcester Bosch holds Which? Recommended Provider status and scores around 4.5 out of 5 on Trustpilot from tens of thousands of reviews, with customers frequently praising reliability and aftercare support.
Should You Choose The Worcester Bosch Greenstar 8000 Life 35?
Choose the Greenstar 8000 Life 35 if:
- You want a widely trusted brand backed by the largest installer network in the UK
- Easy future servicing and parts availability matter to you
- You value a 10 year warranty from one of the UK’s most established boiler manufacturers
- Your household wants dependable, quiet running without unnecessary complexity
Do not choose the Greenstar 8000 Life 35 if:
- You want the longest warranty available in this category, since ATAG offers considerably more
- You are set on choosing the most affordable option, since Alpha and Intergas both cost less
- You specifically need a full 40kW rating rather than the 35kW central heating output this model offers
5. Alpha E-Tec 38kW System: Best Value For Larger Homes And Landlords
The Alpha E-Tec 38kW System is the strongest value option on this list, proving that a stainless steel heat exchanger and dependable performance do not have to come with a premium price tag. Alpha has built a genuinely strong reputation in the UK market, and this model in particular has been recognised as Good Homes Magazine Approved, reflecting real world quality that goes beyond the brand’s budget friendly pricing.
| Specification | Detail |
| Features | Stainless steel heat exchanger, zinc-plated expansion vessel, easy access panel |
| Efficiency | 93% ErP A |
| Estimated unit cost | £1,000 |
| Installed cost | £1,800 to £2,300 |
| Running costs | £1,270 to £1,400 per year |
| Best for | Value focused larger homes, landlords |
| Warranty | 10 years |
| Which? rating | Not independently tested by Which? |
| Trustpilot rating | 4.6 out of 5 |
Features and specs
The E-Tec 38kW measures 440mm wide by 712mm high by 255mm deep and includes a stainless steel heat exchanger rather than the aluminium type more commonly found on budget boilers, offering better heat and corrosion resistance over the long term.
It also features a zinc-plated expansion vessel for further corrosion protection, and an easy access front panel that makes servicing more straightforward for engineers, which can help keep long term maintenance costs down.
Efficiency
This model carries a 93 percent ErP A efficiency rating, marginally below the premium brands here but still comfortably within A rated territory.
Unit cost
The unit typically costs around £1,000, making it the most affordable boiler on this list.
Installation cost
Fully fitted, expect to pay between £1,800 and £2,300 across Kent and South East postcodes, a meaningful saving compared with the other four options reviewed here.
Running costs
Running costs for a large household typically sit between £1,270 and £1,400 annually, slightly higher than the most efficient models due to the marginally lower efficiency rating, though the gap is modest in real terms.
Warranty
Standard cover runs to 10 years, a genuinely strong term for a boiler at this price point.
Best for
Budget conscious homeowners and landlords who want a stainless steel heat exchanger and strong warranty without paying premium brand prices for a larger property.
Which? and Trustpilot ratings
Alpha is not independently tested by Which?, but it holds an Excellent rating on Trustpilot, generally around 4.6 out of 5, with customers frequently highlighting good value for money.
Should You Choose The Alpha E-Tec 38kW System?
Choose the Alpha E-Tec 38kW System if:
- You want the lowest unit price in this comparison without sacrificing a stainless steel heat exchanger
- You are a landlord managing several rental properties and need cost effective reliability
- A genuine 10 year warranty at this price point appeals to you
- Easy servicing access is important for keeping long term maintenance costs manageable
Do not choose the Alpha E-Tec 38kW System if:
- You want the absolute highest efficiency rating available among 40kW system boilers
- Brand prestige and the widest possible installer network are important to you
- You specifically need a 40kW rated output rather than 38kW
How We Chose The Best 40kW System Boilers For Kent And South East Properties
We chose these boilers by combining what our own Gas Safe registered engineers see on the ground across hundreds of installations in Kent and the South East with independent data from Trustpilot, the Energy Saving Trust and manufacturer technical documentation.
Every boiler on this list has been fitted by our team in real homes across this region, not just researched from a brochure, which means we know exactly how these units and their paired cylinders behave once they are living inside a busy household.
- Our engineers have fitted 40kW system boilers alongside hot water cylinders in homes across Maidstone, Tunbridge Wells, Ashford, Canterbury, Sevenoaks, Dartford, Medway and Brighton, giving us direct experience of how each model performs in the property types common to this region
- We cross checked every efficiency figure and warranty term against manufacturer technical documentation to ensure the numbers quoted are accurate and current for 2026
- We factored in independent Trustpilot review volume and third party endorsements such as the Energy Saving Trust, rather than relying on marketing claims alone
- We calculated running cost estimates using the current Ofgem energy price cap gas rate, so the figures reflect what homeowners are actually paying today
- We considered installation practicality specific to Kent and South East housing stock, including period properties needing a new cylinder and newer builds with existing unvented systems
- We only included boilers with a proven UK parts and servicing network, since a warranty only holds real value if an engineer can actually get hold of the parts when something goes wrong
- Our team holds Gas Safe registration and has completed manufacturer specific training on ATAG, Intergas, Vaillant, Worcester Bosch and Alpha product ranges, which shapes our recommendations from hands on experience rather than guesswork
How To Choose The Best 40kW System Boilers In Kent And South East
Picking the right 40kW system boiler is about matching the unit and its paired cylinder to your household’s actual heating and hot water demand rather than simply choosing the biggest number available. The sections below walk through the main factors our engineers weigh up during a home survey.
Confirming Whether A System Boiler Suits Your Property
A system boiler stores hot water in a separate cylinder rather than heating it instantly like a combi boiler, which makes it well suited to larger homes with two or more bathrooms where strong, consistent hot water pressure at every tap matters more than instant delivery.
A 40kW system boiler generally suits a larger detached or semi-detached property with around twenty radiators and a household that uses multiple bathrooms regularly throughout the day. If your property only has one bathroom and modest hot water needs, a combi boiler would likely be a simpler and more cost effective choice.
Choosing The Right Cylinder Size And Type
The cylinder paired with your system boiler needs to match your household’s hot water usage, and most larger homes across Kent and the South East fit either a 180 litre or 210 litre unvented cylinder, though bigger households with five or more occupants may need up to 300 litres.
An unvented cylinder is generally recommended over an older vented type, since it delivers stronger water pressure at every outlet and does not require a cold water tank in the loft, freeing up valuable storage space.
Comparing Warranty Length And What It Covers
Warranty length varies considerably across this category, from 5 years on the Vaillant ecoTEC Plus 637 up to 14 years on the ATAG i40S with the right conditions met at installation. It is worth checking exactly what each warranty covers, since some terms only apply to the heat exchanger rather than the whole boiler, and most manufacturers require an annual service by a Gas Safe registered engineer to keep the warranty valid throughout its term.
Checking Your Existing Gas Supply And Pipework
A 40kW system boiler often needs a larger gas supply pipe than a smaller model, since the higher output demands more gas volume to run efficiently. Many installations across older Kent properties, particularly in towns like Faversham, Rochester and Sandwich, may require an upgrade from a 15mm to a 22mm gas pipe.
A proper survey should always check your meter capacity and existing pipework before any quote is finalised.
Budgeting For The Full Installed Cost Including The Cylinder
The boiler unit price is only part of the overall cost for a system boiler setup. Installation across Kent and South East postcodes typically adds £800 to £1,500 on top of the unit price to cover labour, flue work and a magnetic filter, and if you need a new cylinder fitted at the same time, this can add a further £400 to £900 depending on size and type.
Always ask for a detailed written quote that separates the boiler, cylinder and labour costs, so you know exactly what you are paying for.
Factoring In Running Costs Against The Current Energy Price Cap
Running costs for a 40kW system boiler in a larger four to five bedroom home typically range from £1,220 to £1,410 a year, based on the Ofgem gas price cap rate of 7.97 pence per kWh from October 2026. Boilers with stronger efficiency ratings, such as the ATAG i40S and Vaillant ecoTEC Plus 637 at 94 percent, will generally cost slightly less to run than models rated a few percentage points lower, though insulation, cylinder heat loss and how well the heating system is balanced also play a significant part in the final figure.
Get Expert Advice On The Right Boiler For Your Larger Home
Choosing a new boiler for a larger property involves more than simply selecting the highest output available. Your home’s heating requirements, number of bathrooms, hot water usage, existing pipework and cylinder setup can all influence which boiler system is the best fit.
Guscott Heating provides professional boiler installation across Kent and the South East of England, serving areas and postcodes including Maidstone, Tunbridge Wells, Ashford, Canterbury, Sevenoaks, Tonbridge, Dartford, Gravesend, Medway, Brighton, Guildford, Faversham, Rochester, Sandwich and Sittingbourne.
Request a free, no-obligation boiler quote to find out which boiler installation is the right solution for your Kent or South East home.
Our Gas Safe registered engineers can assess your property and recommend a boiler and heating system suited to your household’s individual requirements, whether you need a system boiler with a hot water cylinder or another type of boiler altogether.
Frequently Asked Questions
How Long Does A 40kW System Boiler Installation Take In Kent Or The South East?
A straightforward like for like 40kW system boiler replacement with an existing compatible cylinder typically takes one full working day for our engineers to complete.
If your installation involves fitting a new unvented cylinder, upgrading gas pipework or converting from a regular boiler with a loft tank, the job can take between two and three days, particularly on older properties common across Kent towns like Faversham, Sittingbourne and Sandwich where the existing setup often needs updating.
What Is The Difference Between A System Boiler And A Regular Boiler For A Large Home?
A system boiler has its heating and expansion components built into the unit itself and only requires a hot water cylinder to be fitted separately, while a regular boiler, sometimes called a heat only or conventional boiler, needs both a hot water cylinder and a separate cold water storage tank, usually located in the loft.
For larger homes across Kent and the South East, a system boiler is generally the more space efficient and modern choice, since it removes the need for loft tanks while still delivering strong, consistent hot water pressure to multiple bathrooms.
Can A 40kW System Boiler Run On LPG For Rural Kent Properties?
Yes, most 40kW system boilers, including the ATAG i40S, Intergas Compact HRE SB and Vaillant ecoTEC Plus 637, are available in LPG compatible versions, which suits rural properties across the Kent Downs, the Weald and parts of the South East that are not connected to the mains gas network.
LPG system boilers work in much the same way as their mains gas counterparts, though running costs tend to be somewhat higher per kWh, so it is worth factoring this into your long term cost comparison during a survey.
Do I Need A New Cylinder If I Upgrade To A 40kW System Boiler?
Whether you need a new cylinder depends on the age, condition and capacity of your existing one, and many homeowners across Kent and the South East upgrading from an older regular boiler setup do end up fitting a new unvented cylinder to get the strongest possible water pressure.
If your current cylinder is already unvented, well insulated and correctly sized for a 40kW output, it may be compatible with a new boiler, though a Gas Safe registered engineer should always confirm this during a home survey before any installation begins.
How Much Space Do I Need For A 40kW System Boiler And Cylinder Together?
A 40kW system boiler itself typically measures between 700mm and 720mm in height and around 440mm in width, similar to a standard combi boiler, but you also need separate space for the hot water cylinder, which for a larger household is often 1,500mm to 1,800mm tall depending on capacity.
Most homeowners across Kent and the South East fit the cylinder in an airing cupboard, loft space or utility room, and a proper survey will confirm exactly how much clearance your specific property has available before recommending a cylinder size.
