Vaillant
Failed Expansion Vessel
Boiler pressure rises rapidly when heating is on and discharges through pressure relief valve
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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
Loss of air charge in expansion vessel, Punctured expansion vessel diaphragm, Undersized vessel for system volume
How to Fix / Troubleshooting
Safety: Hot water may discharge from the PRV pipe. Do not cap or block this pipe.
- Observe pressure behaviour: Note cold pressure (e.g. 1.0 bar) and hot pressure during heating. If it climbs above 3 bar and PRV discharges, the expansion vessel is likely faulty.
- Temporary mitigation: Run heating at a lower temperature to reduce expansion until an engineer attends.
- Engineer service: A technician can isolate, drain, and recharge the vessel with air/nitrogen to the correct pre-charge, or replace the vessel if the diaphragm has failed. The PRV may also need replacement if it has been passing.
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Repair Difficulty
Hard
4/5
Required Part
Expansion Vessel
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