A1
Worcester Bosch Boiler
Severity: ModerateWhat Does This Error Mean?
A1 means central heating pressure is below the boiler's minimum threshold. Repressurise via the filling loop until the gauge reads 1.0–1.5 bar. The boiler will resume normal operation.
Affected Models
- Worcester Greenstar 4000
- Worcester Greenstar 8000
- Worcester Greenstar i
- Worcester Greenstar CDi
- Worcester Greenstar Si
Common Causes
- System pressure dropped below 0.8 bar (most common cause)
- Air locked in radiators reducing measured pressure
- Slow leak somewhere in the heating system
- Failed pressure sensor
- Expansion vessel needs recharging
How to Fix It
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Check the pressure gauge.
The pressure gauge is on the front of the boiler. A cold reading below 0.8 bar will produce A1. Aim for 1.0–1.5 bar when cold.
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Locate the filling loop.
On most Worcester Bosch combi boilers, the filling loop is a flexible silver braided hose under the boiler with two black or grey valves. Older models use a removable filling key — check your boiler manual.
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Repressurise to 1.2 bar.
Open both filling loop valves slowly. Watch the pressure gauge and close both valves when it reads about 1.2 bar. Do not exceed 1.5 bar. Reset the boiler.
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Bleed any air from radiators.
If pressure drops as soon as heating starts, there is air in the system. Bleed each radiator with a bleed key. Top up the pressure again if needed after bleeding.
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Investigate persistent pressure loss.
If you need to repressurise more often than every few months, there is a leak. Look around radiator valves, pipework joints, and under the boiler. Persistent leaks need a heating engineer.
When to Call a Professional
Repressurising is a user task on combi boilers with an external filling loop. If pressure drops repeatedly, a heating engineer should find and repair the leak. Pressure sensor and expansion vessel work require an engineer.