Worcester Bosch Greenstar Si Problems — Faults & Fixes
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The Worcester Bosch Greenstar Si remains one of the UK's most reliable compact combi boilers, but like any heating system approaching two decades in service, faults do develop. Low pressure warnings, ignition failures, and diverter valve issues are the most common problems engineers encounter on these units. Most Si owners are surprised to learn that many faults can be reset at home, whilst others require a qualified Gas Safe engineer — knowing which is which can save you both time and money. This guide walks through the most frequent issues, what the fault codes mean, and whether you're looking at a simple fix or a job for a professional. If your boiler isn't firing or you're losing pressure repeatedly, don't ignore it — a breakdown in winter leaves your home without heating, and some problems worsen if left unaddressed.
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Most likely cause & what to check
The Worcester Bosch Greenstar 25Si and 30Si are compact combi boilers designed to fit inside a standard kitchen cupboard. They were extremely popular throughout the 2000s and 2010s before being replaced by the Greenstar 4000 range. Many are still in service and performing well. Manuals are at worcester-bosch.co.uk/support.
Fault code 227 (low pressure): the most common fault. Repressurise using the filling loop — on the Si this is a flexible silver braided hose with two valves, located under the boiler inside the cupboard. Fill to 1.2 bar and press reset.
Fault code EA (ignition failure): reset once. If it returns, check gas supply at other appliances. Recurring EA on the Si is often an ignition electrode or lead fault — straightforward for a Gas Safe engineer to diagnose and replace.
Fault code E5 (high temperature — fan/flue): indicates the flue fan may be failing. This is a Gas Safe engineer job. The Si's fan is accessible but should only be replaced by a registered engineer.
The Si diverter valve: like the CDi, the Si combi uses a motorised diverter valve. Failure is less common than on older models but still occurs on units over 8–10 years old. A stuck diverter valve will cause either no hot water or no heating, with the boiler otherwise appearing to fire normally.
The Greenstar Si is no longer manufactured but parts are still widely available through Wolseley, Plumb Center, and Screwfix Trade. Any Gas Safe engineer familiar with Worcester Bosch products can service it.
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Frequently asked questions
How do I know if my Greenstar Si has a problem?
Your boiler will display a fault code on the control panel — commonly 227 (low pressure), EA (won't ignite), or E5 (overheating). You may also notice no hot water, no heating, or the boiler shutting down unexpectedly. Always check the digital display first before calling an engineer.
Can I fix a Greenstar Si fault code myself?
Low pressure (code 227) can often be resolved by topping up via the filling loop — a straightforward job you can do in minutes. However, ignition failures, fan problems, and diverter valve faults must be diagnosed and fixed by a Gas Safe registered engineer, as they involve gas and electrical components.
Is the Greenstar Si still worth repairing, or should I replace it?
At 10–15 years old, the Si is still a solid investment if parts are available and the fault is a straightforward component replacement. However, if the heat exchanger has failed or repairs exceed £400–500, replacement with a modern combi may be more cost-effective in the long run.
Why does my Greenstar Si lose pressure so quickly?
Rapid pressure loss usually indicates a leak in the heating circuit — check pipework, radiator valves, and the boiler itself for moisture or drips. A slow leak inside the boiler (such as in the heat exchanger) is harder to spot but must be addressed promptly, as continued operation risks damage.
Where can I get parts for an old Greenstar Si?
Despite being discontinued, spare parts are widely stocked by Wolseley, Plumb Center, and Screwfix Trade. Your Gas Safe engineer can order components directly, or you can contact Worcester Bosch support for guidance on availability and part numbers.
How often should a Greenstar Si be serviced?
Annual servicing by a Gas Safe engineer keeps your boiler safe, maintains efficiency, and catches small issues before they become costly faults. Many faults like ignition electrode wear develop gradually, so regular checks are money well spent.