UK landlords searching for the best 40kW system boilers need powerful, reliable heating solutions that handle large rental properties with multiple bathrooms, high hot water demand, and existing unvented cylinders while supporting EPC compliance and minimising maintenance callouts.
The 5 Best 40kW System Boilers: At a Glance
These top 40kW (or closest high-output) system boilers are ideal for landlords in 2026, offering excellent modulation, long warranties to reduce repair risks in tenanted homes, A-rated efficiency for better EPC ratings, and straightforward servicing via national Gas Safe networks. They pair perfectly with unvented cylinders for simultaneous hot water delivery across multiple outlets.
| Model | Efficiency (ErP) | Typical Unit Cost | Installed Cost (approx.) | Warranty (accredited install) | Trustpilot Rating | Key Landlord Benefit |
| ATAG i40S | 94% | £1,450–£1,650 | £2,900–£3,700 | Up to 14 years | 4.6/5 | Longest warranty & heat exchanger guarantee |
| Vaillant ecoTEC Plus 637 | 94% | £1,400–£1,600 | £2,800–£3,600 | Up to 10 years | 4.5/5 | Quiet operation & easy servicing |
| Worcester Bosch Greenstar 8000 Life System | 94% | £1,500–£1,800 | £3,100–£4,000 | Up to 12 years | 4.6/5 | Strongest UK service network |
| Intergas Compact HRE SB 40 | 92-94% | £1,100–£1,400 | £2,600–£3,400 | Up to 10 years (heat exchanger) | 4.4/5 | Only 4 moving parts for low maintenance |
| Alpha E-Tec 38kW System | 93% | £950–£1,200 | £2,500–£3,300 | Up to 10 years | 4.7/5 | Excellent value & corrosion resistance |
Notes: Prices are 2026 indicative averages for high-output system boilers (25-40kW range, suitable for large properties). Installed costs include standard Gas Safe fitting, flue, and controls. All are wall-mounted condensing system boilers compatible with unvented cylinders. Actual output may be 37-42kW depending on model variant.
If you’re a UK landlord managing larger rental properties requiring high-output heating, request a new boiler quote from Boilers 2 Go for competitive pricing, expert sizing advice, and professional installation that supports EPC improvements and tenant satisfaction.
1. ATAG i40S
The ATAG i40S stands out among the best 40kW system boilers for landlords thanks to its near-exact 40kW output, exceptional warranty, and reliable performance in high-demand rental settings.
Specifications and Features
- Output: 38.8kW (ideal for 15-20+ radiators)
- Compact dimensions with front access for servicing
- Stainless steel heat exchanger with lifetime guarantee option
- Compatible with smart controls and weather compensation
- LPG compatible
Efficiency
94% ErP A-rated condensing technology for strong EPC scores.
Estimated Cost of the Boiler Unit
£1,450–£1,650.
Costs with Installation
£2,900–£3,700, including cylinder compatibility checks.
Running Costs
Low fuel consumption through high modulation reduces tenant energy bills and void-period expenses.
Which? Trustpilot Ratings
Strong brand 4.6/5 Trustpilot; frequently praised for build quality and long-term reliability.
2. Vaillant ecoTEC Plus 637
The Vaillant ecoTEC Plus 637 excels in the best 40kW system boilers category for its quiet operation and installer-friendly design, making it perfect for noise-sensitive rental flats or HMOs.
Specifications and Features
- Output: 37.6kW (handles large properties well)
- Built-in pump and expansion vessel
- IoniDetect combustion monitoring
- App-compatible controls
- Frost protection
Efficiency
94% ErP A-rated.
Estimated Cost of the Boiler Unit
£1,400–£1,600.
Costs with Installation
£2,800–£3,600.
Running Costs
Efficient modulation minimises cycling and gas usage.
Which? Trustpilot Ratings
4.5/5 Trustpilot; Which? recommended for performance.
3. Worcester Bosch Greenstar 8000 Life System
Worcester Bosch Greenstar 8000 Life System offers premium reliability among the best 40kW system boilers, backed by the UK’s largest service network — essential for landlords with multiple properties.
Specifications and Features
- High-output variants up to 40kW
- Full-colour touchscreen diagnostics
- Pre-plumbing jig for faster installs
- Smart thermostat ready
Efficiency
94% ErP A-rated.
Estimated Cost of the Boiler Unit
£1,500–£1,800.
Costs with Installation
£3,100–£4,000.
Running Costs
Precise control delivers consistent savings.
Which? Trustpilot Ratings
4.6/5 Trustpilot with high volume of positive reviews.
4. Intergas Compact HRE SB 40
The Intergas Compact HRE SB 40 provides low-maintenance simplicity in the best 40kW system boilers range, appealing to landlords seeking reduced service frequency.
Specifications and Features
- Output: 42.5kW
- Only 4 moving parts
- Stainless steel components
- Flexible flue options
Efficiency
Up to 94% ErP A-rated.
Estimated Cost of the Boiler Unit
£1,100–£1,400.
Costs with Installation
£2,600–£3,400.
Running Costs
Durable design keeps long-term costs low.
Which? Trustpilot Ratings
Solid 4.4/5 Trustpilot.
5. Alpha E-Tec 38kW System
The Alpha E-Tec offers great value among the best 40kW system boilers with robust corrosion resistance for reliable performance in rental environments.
Specifications and Features
- Output: 38kW
- Zinc-plated expansion vessel
- Stainless steel heat exchanger
- Easy front-panel access
Efficiency
93% ErP A-rated.
Estimated Cost of the Boiler Unit
£950–£1,200.
Costs with Installation
£2,500–£3,300.
Running Costs
Competitive efficiency for everyday use.
Which? Trustpilot Ratings
Impressive 4.7/5 Trustpilot.
How to choose the Best 40kw system boilers
Choosing the best 40kW system boilers for landlords rental properties requires balancing output for property size, warranty length to protect against tenant-related breakdowns, service network coverage, and cylinder compatibility for multi-bathroom setups.
Choose the ATAG i40S if you want the longest warranty and premium heat exchanger protection for large, high-demand rentals.
Don’t choose ATAG if parts availability in your region is limited.
Choose the Vaillant ecoTEC Plus 637 if quiet performance and straightforward servicing matter most in occupied tenancies.
Don’t choose Vaillant if extended warranties are a higher priority than noise reduction.
Choose the Worcester Bosch Greenstar 8000 Life System if nationwide engineer support and proven reliability are essential for your portfolio.
Don’t choose Worcester Bosch if budget constraints require the lowest upfront cost.
Choose the Intergas Compact HRE SB 40 if minimal moving parts and low maintenance are key to reducing repair callouts.
Don’t choose Intergas if you prefer the most established premium brands.
Choose the Alpha E-Tec if you need strong value with excellent corrosion resistance on a tighter budget.
Don’t choose Alpha if the absolute highest modulation ratio is required.
Need a New Boiler Quote?
For UK landlords upgrading heating in larger rental properties, installing one of the best 40kW system boilers improves reliability, EPC ratings, and tenant comfort. Request a new boiler quote from Boilers 2 Go today for expert advice on sizing, compliant installations, and competitive pricing tailored to private rented sector needs.
FAQs
Why choose a system boiler over a combi for larger rental properties?
System boilers work with unvented cylinders to provide better hot water flow rates when multiple taps or showers run simultaneously, making them suitable for family-sized or HMO rentals with high simultaneous demand.
How does boiler output sizing affect EPC compliance for landlords?
A correctly sized 40kW system boiler improves EPC ratings by matching property heat loss accurately, avoiding inefficient oversizing that could lower efficiency scores and risk failing the minimum EPC C standard by 2030.
Are 40kW system boilers hydrogen-ready for future-proofing rentals?
Many top models (including ATAG, Vaillant, and Worcester Bosch variants) are 20% hydrogen blend ready, offering landlords a pathway to lower carbon compliance without full replacement when hydrogen networks expand.
What cylinder compatibility should landlords check before installing a 40kW system boiler?
Ensure the existing unvented cylinder is sized adequately (e.g., 200-300 litres for large properties) and rated for high flow rates; mismatched cylinders can limit performance or require costly upgrades.
How often must landlords service high-output system boilers in rentals?
Annual Gas Safe safety checks and servicing are legally required for all gas appliances in rented properties; this also maintains warranty validity and prevents breakdowns that could lead to tenant disputes or void periods.
I’m Penny North, a home energy heating expert. My mission is to demystify new boilers and complex heating systems to help you achieve a warm, cosy home with lower energy bills and a smaller environmental footprint.