Calculate required BTU/hr capacity and select the ideal Weil-McLain residential gas, oil, or steam boiler for your property.
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.
Weil-McLain cast iron hydronic boilers publish Net AHRI ratings accounting for a 1.15 piping and pickup factor. Always match your calculated room heat loss to Net AHRI ratings, not Gross Input.
Popular Weil-McLain gas, oil, and steam boilers matched to heating loads and property sizes.
| Model Family | Boiler Type & Fuel | Heating Capacity (BTU/hr) | Efficiency Rating | Ideal Application |
|---|---|---|---|---|
| AquaBalance (Series 2) | High-Efficiency Combi Gas | 80,000 to 155,000 BTU/hr | 95% AFUE Condensing | Modern homes, baseboards, domestic hot water |
| Ultra Series 3 | Modulating Condensing Gas | 80,000 to 399,000 BTU/hr | 94% AFUE Condensing | Multi-zone hydronic radiant and baseboard heating |
| CGa / CGi Series | Atmospheric Cast Iron Gas | 38,000 to 280,000 BTU/hr | 84% AFUE Chimney Vent | Direct replacement of legacy cast-iron boilers |
| EG / PEG Series | Cast Iron Residential Steam | 75,000 to 299,000 BTU/hr | 83% AFUE Steam Draft | Steam heating systems sized to connected radiator EDR |
| Ultra Oil / WTGO | Cast Iron Residential Oil | 95,000 to 260,000 BTU/hr | 86% AFUE Oil-Fired | Off-grid rural heating oil hydronic systems |
Independent sizing guide. Weil-McLain is a registered trademark of The Marley-Wylain Company.
Core hardware and load parameters governing model selection, benchmarked against AHRI / I-B-R Rating Standards & ASME Section IV.
Cast-iron boilers have a ~15% difference between gross heating capacity and net radiation rating. Sizing must be based on Net AHRI output.
Weil-McLain steam boilers (EG/PEG) require strict Hartford Loop and drop header piping to prevent water carryover and wet steam hammer.
Common installer miscalculations when specifying Weil-McLain Boiler Size Calculator: Residential BTU Guide equipment — specification mismatches that surface only after the unit is installed and commissioned.
Consequence: A boiler sized only to building heat loss cannot produce enough steam to reach distant radiators.
Consequence: Return temperatures above 130°F prevent condensing, lowering AFUE from 95% to 84%.
Review these 2 essential questions with your heating installer to ensure your Weil-McLain Boiler Size Calculator: Residential BTU Guide quote meets AHRI / I-B-R Rating Standards & ASME Section IV.
Weil-McLain is North America's leading manufacturer of cast-iron and high-efficiency modulating hydronic boilers. Sizing Weil-McLain heating equipment requires distinguishing between hot water boilers (sized to Manual J heat loss) and steam boilers (strictly sized to connected radiator EDR).
Formulations benchmarked directly against published British & European standards: AHRI / I-B-R Rating Standards & ASME Section IV (Air-Conditioning, Heating, and Refrigeration Institute (AHRI) & ASME) for Weil-McLain Boiler Size Calculator: Residential BTU Guide. Maintained by the Boiler Size Calculator Technical Editorial Desk.
Direct engineering guidance for common weil-mclain boiler size calculator: residential btu guide questions, referenced against Air-Conditioning, Heating, and Refrigeration Institute (AHRI) & ASME.
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