Match your home size, bathroom count, and shower flow to the ideal Worcester Bosch Greenstar model.
Independent Engineering Guide: Boiler Size Calculator is an independent sizing resource and is NOT affiliated with, authorized, endorsed, or sponsored by Worcester, Bosch Group or its subsidiaries.
Worcester Bosch is the UK's leading boiler brand. Sizing depends on matching your DHW flow demand (for Greenstar Combi units) or system heat loss (for Greenstar System/Regular units).
| Model Range | Boiler Type | Available Outputs | DHW Flow Rate | Modulation Turndown | Standard Warranty | Optimal Property Archetype |
|---|---|---|---|---|---|---|
| Greenstar 2000 | Combi | 24 kW, 30 kW | 9.8 – 12.3 L/min | 1:6 (4.0 kW min) | 5–7 Years | 1-2 Bed Flats & Small Terraced Homes |
| Greenstar 4000 | Combi & System | 25 kW, 30 kW | 10.2 – 12.3 L/min | 1:10 (3.0 kW min) | 7–10 Years | Medium 2–3 Bed Semi-Detached |
| Greenstar 8000 Life/Style | Combi & System | 30, 35, 40, 45, 50 kW | 12.3 – 20.3 L/min | 1:10 (3.0 kW min) | 8–12 Years | Large 4–5 Bed Homes with Multiple Showers |
The maximum kW rating of your boiler only matters on the 3 or 4 coldest days of winter. For the remaining 95% of the heating season, your home requires just 2 kW to 6 kW of continuous heat. A high modulation turndown ratio (1:7 to 1:10) allows the burner to throttle down to a tiny flame without shutting off, preventing short-cycling and saving money.
Rated: 30 kW Maximum → 10 kW Minimum Limit. Because the house only needs 4.5 kW, the 10 kW burner overheats the water in 2 minutes and cuts out repeatedly.
Rated: 30 kW Maximum → 3.0 kW Minimum Limit. Burner modulates smoothly to match the exact demand, sustaining condensing mode without stopping.
The Worcester Greenstar 4000 and 8000 series feature a 1:10 modulation ratio, allowing a 30 kW boiler to throttle down to 3.0 kW to eliminate short-cycling.
Complete breakdown of Worcester Greenstar models by kW, flow rate, and warranty.
| Model Name | Boiler Type | Heating / DHW Output | Hot Water Flow Rate | Manufacturer Guarantee |
|---|---|---|---|---|
| Greenstar 2000 25kW / 30kW | Combi | 20 kW Heat / 25–30 kW DHW | 10.2 – 12.3 L/min | Up to 7 Years Guarantee |
| Greenstar 4000 25kW / 30kW | Combi & System | 24 kW Heat / 25–30 kW DHW | 10.2 – 12.3 L/min | Up to 10 Years Guarantee |
| Greenstar 8000 Life 30kW – 50kW | Combi, System, Regular | 35 kW Heat / 30–50 kW DHW | 12.3 – 20.3 L/min | Up to 12 Years Guarantee |
| Greenstar CDi Highflow 44kW / 50kW | Floor-Standing Storage Combi | 30 kW Heat / 44–50 kW DHW | 20.0 – 25.0 L/min | Up to 10 Years Guarantee |
Guarantee valid when installed with a Worcester Bosch Greenstar System Filter.
The 2 variables that move the worcester bosch boiler result most, and how Worcester Bosch Technical Engineering Manuals & ErP expects each one to be established.
Engineered for high water turbulence and optimal heat transfer with easy-clean front access.
Communicates digitally via Bosch EMS protocol to optimize condensing modulation.
The mistake below costs more than any other on a worcester bosch boiler job — specification mismatches that surface only after the unit is installed and commissioned.
Consequence: The 2000 series has a narrower 1:3 turndown, cycling more than the 4000 series on mild days.
Put these 2 questions to your heating engineer before you approve a quoted output — each one checks the specification against Worcester Bosch Technical Engineering Manuals & ErP.
Worcester Bosch has been the UK’s most popular boiler manufacturer for decades. Understanding the differences between the 2000, 4000, and 8000 series ensures perfect home matching.
Formulations benchmarked against Worcester Bosch Technical Engineering Manuals & ErP (Worcester Bosch UK & Gas Safe Register) for Worcester Bosch Boiler Size Calculator.
2 questions we are asked most about worcester bosch boiler sizing, answered against Worcester Bosch UK & Gas Safe Register guidance.
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