When homeowners, landlords, tenants and heating engineers all give you glowing reviews and you’ve been a Which? Best Buy for 10 years, you know you’re doing something right. Worcester Bosch makes some of the UK’s best-loved boilers – their reliability is seemingly unparalleled, but this year they decided to replace their bestselling boiler, the Greenstar i, with a new model.
The new Greenstar 4000 combi boiler and system boiler range launched on 17 May 2021. It aims to emulate all the greatness of the great Greenstar i condensing boiler, but to refine and improve on it by adding new tech, a compact size and sleek design into the mix. So have they made the nation’s favourite boiler better? Let’s take a look at the main features and differences in our review of both the Greenstar 4000 combi boiler and the Greenstar 4000 system boiler range.
Our verdict: Worcester Bosch Greenstar 4000
No time to read the full review? Here’s our verdict in a nutshell: 4.8/5
The Greenstar 4000 has managed to uphold the key selling points of the top-selling Greenstar i – its reliability and it’s an investment that will last years – and improve it on many other fronts. It’s quieter, it’s more compact and now, thanks to a sleek design rehaul, it looks as good as it performs. It’s also now compatible with third-party smart home controllers Hive, Nest, and more. In our opinion it’s the best Worcester Bosch boiler out there and if you’re in the market for a new boiler, we’d highly recommend checking out the 4000 combi version or any of the 4000 system boilers.
How the Greenstar 4000 scored in 4 key areas:
Reliability: 5/5
You can’t fault Worcester Bosch boilers on reliability. They are trusted by all UK heating engineers to heat your home, whatever size and configuration your home is.
Performance: 5/5
The Worcester Bosch Greenstar 4000 has been given an A-rating (at an impressive 94%) for energy efficiency. It caters for a wider audience with 25kW and 30kW combi boiler options, and a range of system options for larger properties. The 4000 also provides a higher flow rate to your taps and shower than the previous Greenstar i, of 10.2 litres a minute.
Warranty: 5/5
Worcester Bosch excel when it comes to their ‘10-year guarantee’. This is one of the reasons their boilers are so well-loved. You usually get this when you agree to have their recommended filter installed – your installer can help with this.
Price: 4/5
It’s true that you get what you pay for. While the Worcester Bosch Greenstar 4000 isn’t cheap, if you value long-term reliability, ease of use, smart home compatible tech and super efficiency, you’ll really see the benefit in paying a little more for it. Alternatively, if you’re looking for a cheaper option, take a look at the Greenstar 2000.
Our in-depth verdict
OK, let’s take a deep-dive into the Greenstar 4000 and everything it has to offer. We’ll dig into our 4 testing areas in more detail and explain why there’s a new best-selling Worcester Bosch Boiler in town…
Main upgrades
The 4000 is Worcester Bosch’s mid-range model, designed to sit between the existing 2000 and 8000 series boilers. It’s available in a combi boiler version and a system boiler, as well as multiple sizes, including 25kW and 30kW.
Worcester Bosch say the main upgrades from the Greenstar i are as follows:
- Quieter operation
- Compatible with all up-to-date Worcester Bosch controls and third-party controllers such as Hive, Nest, tado°, etc
- Perfect for small and medium-sized properties
- Removable side panels for easier access
- Energy efficiency rating of 94% making it A-rated
- 310mm depth so it fits more snugly into kitchen cabinets
Reliability: 5/5
A British company through and through, Worcester Bosch has built up an incredible amount of consumer and professional trust in its products over the almost 60 years it’s been trading. The company started off as a small Worcestershire-based business called Worcester in 1962. In 1992 it was acquired by Robert Bosch and became known as Worcester Bosch.
Known as one of the UK boiler market leaders, Worcester Bosch has an average trust rating of 4.6 on Trustpilot, with over 23,000 reviews, which is testament to their level of reliability, efficiency and aftercare.
The Worcester Bosch Greenstar i boiler has been a Which? Best Buy for 10 years, receiving an overall rating of 79% in 2020. The feedback included 5-star ratings across the board for build quality and availability of parts and spares (being a UK-based manufacturer, this is no surprise).
Performance: 5/5
The boiler’s performance will be different for homes using the Combi and the System boiler, so let’s look at them separately.
Worcester Bosch 4000 combi boiler
Combi boilers are a great choice for smaller homes, such as flats, small apartments or small houses.
The Greenstar 4000 combi boiler range is perfectly adequate for these types of homes and it’s available in a choice of two power outputs:
25kW – for small homes
The 25kW combi boiler has a heat output of 25kW and a flow rate of 10.5 l/min.
30kW – for mid-sized homes
The 30kW has an additional 5kW of power at 30kW and its flow rate is 12.3 l/min.
Worcester Bosch Greenstar 4000 system boiler: 12-24kW
If you have a larger house with multiple bathrooms and hot water outlets, you probably have a system boiler fitted already, with a separate hot water storage cylinder.
The 4000 system boiler range is available in a range of power outputs:
- 12kW
- 15kW
- 18kW
- 21kW
- 24kW
Efficiency
The Greenstar 4000 scores a whopping 94% for efficiency and therefore has an A-rating (if they gave A*s, it would be right up there).
Features and upgrades
SimpleSwitch
‘SimpleSwitch’ is Worcester Bosch’s offer for previous Greenstar i, i Junior or Si boiler customers wanting to upgrade to the 4000 with limited disruption. It essentially means the 4000 is compatible and will fit within their existing boiler space.
Upgraded exterior
As the 4000 is the middle child in the 2000-8000 range, the exterior is very similar to its bigger and smaller siblings. The casings have curved edges and the round digital display is a thoroughly modern touch.
User-friendly
The LCD display upgrade and push button controls are also a huge improvement in terms of user-friendliness. You can connect the 4000 wirelessly to Bosch EasyControl, their smart thermostat, which means you can control the boiler remotely from your smartphone or tablet. If you don’t have a Bosch EasyControl smart thermostat, you might want to talk about including this in your new boiler package with your boiler installer as this is a game-changer for intelligently managing your home’s heating from wherever you are. The 4000 also works well with the MT20 mechanical timer and the Comfort+ thermostat systems.
Easy-access side panels
In line with the design of the Greenstar 2000 and 8000, the 4000 has handy side panels which allow engineers to service and repair your boiler easier than with the Greenstar i. This is great news for regular maintenance and if anything goes wrong with the system, including problems with your radiators and pipes in the winter.
Compatible with existing Worcester Bosch tech
Just like the Greenstar i, the 4000 is compatible with Worcestor Bosch’s intelligent filling system, which uses software to maintain the 4000’s pressure at its optimum level. If the system registers a drop in pressure, it automatically refills. This is a brilliant piece of automation that means fewer interruptions, dramas and unnecessary engineer call-outs. (Note: this feature is available with the combi boiler but not the system boiler.)
The 4000 series is also compatible with the Worcester Greenstar System filter system (and third-party filters). UK building regulations now insist that every new boiler must be installed with a system filter.
Warranty: 5/5
Worcester Bosch Greenstar 4000 guarantee
Most boiler manufacturers offer a warranty as standard. This is an agreement that they’ll investigate any problem with the boiler, but it’s not a promise that they’ll fix or repair the boiler for free.
A Worcester Bosch guarantee takes things one step further. It’s an unconditional agreement that, if there’s a problem with the boiler during the guarantee period, they’ll come to your home and either repair or replace the boiler within a reasonable timeframe.
With the 4000 series, if your installation meets all the stipulations of the agreement, you’ll get Worcester Bosch’s industry-leading 10-year guarantee. (You may, however, have to pay a little extra to have their proprietary filter fitted to benefit from the increased cover.)
Price point: 4/5
Worcester Bosch is one of the best respected boiler brands in the UK, with the nation’s favourite boiler to their name. So if you’re looking for the cheapest boiler on the market, you won’t find it here. But, if you’re looking for value for money over time, then you can’t go far wrong with the 4000 from Worcester Bosch. The 4000 is competitively priced for an excellent quality mid-range boiler accompanied by the second-to-none aftercare you receive from Worcester Bosch.
The Greenstar i vs the Greenstar 4000 – how do they compare?
Let’s recap on the differences between the best-selling Greenstar i and the new and improved Greenstar 4000.
Comparison – combi boiler range
| Make | Worcester Bosch | Worcester Bosch | Worcester Bosch | Worcester Bosch |
| Model | Greenstar 25i Combi Gas Boiler | Greenstar 4000 25kW Combi Gas Boiler | Greenstar 30i Combi Gas Boiler | Greenstar 4000 30kW Combi Gas Boiler |
| Image |  |
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 |
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| Condensing Boiler | Yes | Yes | Yes | Yes |
| LPG Compatible | Yes | Yes | Yes | Yes |
| Standard Guarantee | 5 years | Up to 2 years | 5 years | Up to 10 years |
| Central Heating Output | 25kW | 25kW | 30kW | 30kW |
| Mounting | Yes | Yes | Yes | Yes |
| Flow Rate | 9 l/min | 10.2 l/min | 12.3 l/min | 12.3 l/min |
| Dimensions (mm) (HxWxD) |
710 x 400 x 330 | 710 x 400 x 310 | 710 x 400 x 330 | 710 x 400 x 310 |
comparison – system boiler range – available in 12kW, 15kW, 18kW, 21kW and 24kW.
| Make | Worcester Bosch | Worcester Bosch | Worcester Bosch | Worcester Bosch |
| Model | Greenstar 21i System Gas Boiler | Greenstar 4000 21kW System Gas Boiler | Greenstar 24i System Gas Boiler | Greenstar 4000 24kW Combi Gas Boiler |
| Image |  |
 |
 |
 |
| Condensing Boiler | Yes | Yes | Yes | Yes |
| LPG Compatible | Yes | Yes | Yes | Yes |
| Standard Guarantee | Up to 7 years | Up to 10 years | Up to 7 years | Up to 10 years |
| Central Heating Output | 21.6kW | 21kW | 24.6kW | 24kW |
| Mounting | Wall Mounted | Wall Mounted | Wall Mounted | Wall Mounted |
| Flow Rate | N/A | N/A | N/A | N/A |
| Dimensions (mm) (HxWxD) |
710 x 400 x 330 | 710 x 400 x 310 | 710 x 400 x 330 | 710 x 400 x 310 |
So, to recap, the Worcester Bosch 4000 basically offers all the features the Greenstar i range was well-loved for, but adds some major improvements:
- Quieter operation
- More contemporary, sleeker design
- Slightly higher hot water output for the combi boiler – 10.2 litres per minute (The extra output means hot water arrives to your outlets faster). The system boiler has slightly lower hot water output
- 310mm depth so it’ll now fit more snugly into a kitchen cabinet (the Greenstar i was 330mm deep)
- Tech compatibility updates for smart thermostats and smart homes
Our final verdict on the Worcester Bosch Greenstar 4000
With a fair few upgrades, including quieter operation, easier installation and improved flow rate on the 25kW combi boiler, plus a more compact, sleeker design and some tech compatibility upgrades, we believe that this Worcester Bosch Greenstar 4000 would be a sound investment for any homeowner or landlord.
