Main
Expansion Vessel Failure or PRV Operation
Boiler pressure rises to the red zone when heating is on and may discharge water outside
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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
Flat or ruptured expansion vessel diaphragm, Blocked vessel connection, Overfilled system, Faulty pressure relief valve
How to Fix / Troubleshooting
Safety: High pressure can cause leaks and PRV discharge. Allow the system to cool before venting water.
- Reduce pressure: Bleed a radiator or use a drain point to bring pressure down to about 1.2 bar when cold.
- Observe pressure rise: Run the heating and watch the gauge. If it climbs rapidly above 2.5 bar, the expansion vessel is likely not functioning.
- Engineer actions:
- Check and recharge the Main expansion vessel via the Schrader valve to the specified pre-charge (typically ~0.75–1.0 bar).
- Clear any blockage in the vessel connection pipe.
- Replace the expansion vessel if the diaphragm is ruptured.
- Replace the PRV if it has been lifted and now drips continuously.
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Repair Difficulty
Medium
3/5
Required Part
Expansion Vessel
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