WORCESTER BOSCH GREENSTAR 21i CLASSIC SYSTEM
The Worcester Bosch Greenstar 21i Classic System was a reliable and efficient wall-mounted system boiler from the Greenstar i range and remains installed in many smaller to medium homes today. While it is no longer manufactured, this page explains its specification, who it was designed for and your options for servicing, repairs or replacement.
We continue to service and repair this model regularly across Buckinghamshire, Berkshire, West London and surrounding counties.
Need help with your Greenstar 21i Classic System? Contact our Worcester Bosch accredited and Gas Safe engineers.
What is the Greenstar 21i Classic System?
The Greenstar 21i Classic System is a wall-mounted gas system boiler from Worcester Bosch’s Greenstar i range. Sitting between the 18i and 24i in the range, it offered a 21.6kW output making it well suited to small to medium homes with an existing hot water cylinder.
As a system boiler, it works alongside a separate hot water cylinder to store and deliver hot water on demand. This makes it better suited than a combi boiler for homes that require hot water at multiple outlets at the same time, without needing a cold water storage tank in the loft.
The 21i Classic System was part of Worcester Bosch’s most popular wall-mounted system boiler range, consistently winning the Which? Best Buy award and widely regarded for its reliability and A-rated efficiency.
> Explore the full Worcester Bosch Greenstar range
Greenstar 21i Classic System: Key Specifications
✔ Boiler type: System boiler
✔ Central heating output: 21.6kW
✔ Efficiency rating: A rated / 94%
✔ Fuel type: Natural gas or LPG
✔ Dimensions: 710mm x 400mm x 330mm
✔ Frost protection: Yes
✔ Solar compatible: Yes, compatible with Greenskies panels
✔ Smart control compatible: Yes, including Bosch EasyControl
✔ Suitable for: Small to medium homes, up to 15 radiators, with hot water cylinder
Who is this boiler suitable for?
The 21i Classic System was designed for small to medium homes with an existing hot water cylinder and moderate heating demand. It is best suited to properties with:
- 2 to 3 bedrooms
- 1 bathroom
- Up to 15 radiators
- An existing hot water cylinder
- No loft cold water tank required
For smaller homes with lower demand, the 18i Classic System was the more appropriate choice. For homes needing more output, the 24i Classic System offered the next step up. If you already have a 21i Classic System installed and are unsure whether to service, repair or replace it, our engineers can assess your system and give you honest advice.
Servicing and repairing your Greenstar 21i Classic System
Although the 21i Classic System is no longer manufactured, it can still be serviced and repaired. Parts remain widely available for this model and our Gas Safe engineers carry out full services and fault diagnosis across Buckinghamshire, Berkshire, West London and surrounding counties.
Regular servicing is important for any boiler, but particularly for older models. An annual service helps to:
- Maintain efficiency and keep running costs down
- Identify worn components before they cause a breakdown
- Keep your boiler operating safely
- Satisfy any remaining warranty or insurance requirements
If your 21i Classic System has developed a fault, our engineers carry a wide range of Worcester Bosch parts and aim to resolve most repairs in a single visit.
When to consider replacing your 21i Classic System
If your boiler is over 12 to 15 years old or experiencing repeated breakdowns, replacement is often the more cost-effective long-term decision. The 21i Classic System has been replaced by the Worcester Bosch Greenstar 4000 System, which offers meaningful improvements across warranty and controls.
The Greenstar 4000 System is the natural modern equivalent for homes currently running a 21i Classic System. It delivers a longer warranty and full compatibility with the Bosch EasyControl smart thermostat.
Greenstar 21i Classic System vs Greenstar 4000 System
Efficiency: 94% vs up to 94%
Warranty: 5 to 7 years vs up to 10 years
Controls: Basic display vs smart control ready
ErP rating: A rated vs A rated
Status: Discontinued vs current model
> Learn more about the Greenstar 4000
Why choose Heppelthwaite – Red Van Plumbers?
Fewer than 5% of UK installers achieve Worcester Bosch Platinum Accredited status. As Platinum Accredited Installers, we install, service and repair the full Greenstar range, including legacy i range system boilers, across Buckinghamshire, Berkshire, West London and surrounding counties.
- Platinum Accredited Worcester Bosch Installers
- Family-run business established 1948
- Gas Safe registered engineers
- Free fixed quotations with no hidden costs
- Up to 10-year warranty available on new Greenstar 4000 installations
> Request a free quote or call 01628 533 550
FAQs – Worcester Bosch Greenstar 21i Classic System
Is the Worcester Bosch Greenstar 21i Classic System still available to buy?
No. The 21i Classic System is no longer manufactured. The modern equivalent is the Worcester Bosch Greenstar 4000 System boiler, which offers an improved warranty and smart control compatibility.
Can the Greenstar 21i Classic System still be serviced?
Yes. Parts remain widely available and our Gas Safe engineers service and repair all Greenstar i system models across Buckinghamshire, Berkshire and West London. Annual servicing is recommended to maintain efficiency and safety.
What replaced the Greenstar 21i Classic System?
The Greenstar 21i Classic System was replaced by the Worcester Bosch Greenstar 4000 system boiler range. The 4000 offers smart control compatibility and up to a 10-year warranty when installed by a Platinum Accredited Installer.
What is the difference between the 21i Classic System and the 18i Classic System?
Both are system boilers from the same Greenstar i range. The key difference is the output. The 18i Classic System delivers 18kW and is suited to homes with up to 15 radiators, while the 21i delivers 21.6kW making it better suited to homes with slightly greater heating demand.
What size home is the Greenstar 21i Classic System suitable for?
The 21i Classic System is suited to small to medium homes with 2 to 3 bedrooms, 1 bathroom and up to 15 radiators. It works alongside a separate hot water cylinder, so loft space for a cold water tank is not required.
How long should a Greenstar 21i Classic System last?
With regular annual servicing, a well-maintained 21i Classic System can last between 12 and 15 years. If your boiler is approaching or has exceeded this age, it may be worth considering a boiler replacement to avoid repeated repair costs.
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