Worcester
The Worcester Greenstar 12i System is a 12.85 kW condensing system boiler with an ErP A-rating for heating, suitable for wall-hung installation in UK homes.
The Worcester Greenstar 12i System is a wall-hung condensing system boiler designed for central heating applications in UK domestic properties. This model, identified by GC number 41-311-67 for its natural gas variant, offers a maximum central heating output of 12.85 kW. It is also available in LPG configurations and various output ratings within the Greenstar i System family, providing flexibility for different installation requirements. This page details its technical specifications, operational parameters, and common fault diagnostics.
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Full technical specifications extracted from the manufacturer brochure. Empty sections are hidden.
Diagrammed on Fig. 21 with dimensions from left case edge.
ARTIFICIALLY SOFTENED WATER MUST NOT BE USED TO FILL THE CENTRAL HEATING SYSTEM. The pH value of the system water must be less than 8 or the appliance guarantee will be invalidated. The addition of sealing agents to the system water is not permitted. Do not use lubricants or sealing compounds for condensate connection.
The Worcester Greenstar 12i System is a wall-hung condensing system boiler, providing a maximum central heating output of 12.85 kW with a minimum modulated output of 3.45 kW. This appliance achieves a seasonal efficiency of 90.1% and holds an ErP A-rating for heating, indicating its performance in energy conversion. It operates on either natural gas or LPG, with specific variants available for each fuel type, such as the natural gas 12i System (GC 41-311-67) and the LPG 12i System (GC 41-311-69).
Physical dimensions are 700mm height, 400mm width, and 330mm depth, with a dry weight of 37.5 kg. The boiler features a 3-speed fixed-speed pump with an available head of 19.6 kPa. Flue options include horizontal, vertical, concentric, and plume management systems, supporting sizes like 60/100mm and 80/125mm, with maximum horizontal lengths up to 13 metres. Electrical protection is rated IPX4D, and the unit includes safety devices such as overheat and frost protection. NOx emissions are rated Class 5 at 39 mg/kWh.
Every fault code listed in the manufacturer brochure, with likely causes and engineer actions. Each code has a permalink — paste a URL like https://www.boilertag.com/boilers/worcester/15i-system-natural-gas#fault-no power at control board to jump straight to the entry.
| Code | Meaning | Details |
|---|---|---|
| No power at control board | No power at control board Show causes & actionsLikely causes
Engineer action Otherwise replace control board. | Details |
| Boiler not operating during central heating demand (HW ok) | Boiler not operating during central heating demand (HW ok) Show causes & actionsLikely causes
| Details |
| Boiler operating without live demand at ST10-LR (from external room thermostat timer) | Boiler operating without live demand at ST10-LR (from external room thermostat timer) Show causes & actionsLikely causes
| Details |
| Boiler not operating during hot water demand (CH ok) | Boiler not operating during hot water demand (CH ok) Show causes & actionsLikely causes
| Details |
| Boiler not operating during any demand. | Boiler not operating during any demand. Show causes & actionsLikely causes
| Details |
| Ignition lockoutLockout | Ignition lockout Show causes & actionsLikely causes
Engineer action Otherwise replace control board Reset procedure Flashing (reset required) | Details |
| Flue overheatLockout | Flue overheat Show causes & actionsLikely causes
Engineer action Otherwise replace control board Reset procedure Flashing (reset required) | Details |
| Heat exchanger overheatLockout | Heat exchanger overheat Show causes & actionsLikely causes
Engineer action Otherwise replace control board Reset procedure Flashing (reset required) | Details |
| Volatile lockout - Fan does not run | Volatile lockout - Fan does not run Show causes & actionsLikely causes
Engineer action Check condition and continuity of leads/sensors Replace control board | Details |
| Internal faultLockout | Internal fault Show causes & actionsEngineer action Replace control board Reset procedure Flashing (reset required) | Details |
| Not a fault code | Service mode selected to minimum Show causes & actionsEngineer action press service button to return to normal | Details |
| Not a fault code | Service mode selected to maximum Show causes & actionsEngineer action press service button to return to normal | Details |
Patterns that group across the manufacturer's fault-code table. Each entry links to the full code listing further down the page.
Engineer: Otherwise replace control board
Triggered by fault code Ignition lockout.
Engineer: Otherwise replace control board
Triggered by fault code Flue overheat.
Engineer: Otherwise replace control board
Triggered by fault code Heat exchanger overheat.
Engineer: Replace control board
Triggered by fault code Internal fault.
Engineer: Otherwise replace control board.
Triggered by fault code No power at control board.
Diagnostic checklists from engineer-submitted Worcester faults, with step-by-step checks and safety notes.
Frequent pressure drops on a Worcester Bosch Greenstar boiler typically indicate a leak within the sealed system, a faulty pressure relief…
The Worcester Greenstar CDi is a condensing combination boiler.
The Worcester Greenstar 12i System displaying a "Heat exchanger overheat" fault code indicates the boiler's safety systems have activated…
The Worcester Greenstar 12i System displaying an 'Ignition lockout' fault indicates that the boiler has attempted to ignite but failed to…
Common questions from engineers and homeowners about the Worcester 15i System (Natural Gas).
Parts referenced in the manufacturer brochure for routine service and repair.
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Specifications on this page are extracted from the manufacturer brochure. Always cross-check against the latest datasheet before commissioning or quoting. Last updated 28 June 2026.