Worcester Bosch
System Water Leak / Expansion Vessel Failure
Boiler losing pressure regularly, pressure gauge drops to zero and boiler locks out.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Leaking radiator valves or pipework, Leaking pressure relief valve, Failed internal expansion vessel diaphragm, Micro-leak on boiler components
How to Fix / Troubleshooting
Safety first: Turn off power before working on the boiler. Water may be hot; use caution.
User checks:
- Inspect visible radiators, valves, and pipework for signs of water staining or damp patches.
- Check the pressure relief valve discharge pipe outside for dripping water when the system is hot.
- Top up the system to 1.0–1.5 bar and monitor how quickly pressure drops.
Engineer actions:
- Test the expansion vessel pre-charge by isolating and draining system pressure, then measuring air charge; re-pressurise or replace vessel if diaphragm has failed.
- Replace a leaking pressure relief valve if it continues to discharge after pressure normalisation.
- Use leak detection methods (dye, pressure test) to locate hidden leaks in the system or within the boiler casing.
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Repair Difficulty
Medium
3/5
Required Part
Expansion Vessel
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