Defro
Expansion Vessel / Safety Valve Issue
Boiler pressure rises excessively during heating and safety valve discharges water
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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
Failed or uncharged expansion vessel, Blocked connection to expansion vessel, Faulty safety valve, Overfilled system
How to Fix / Troubleshooting
Safety: Hot water discharge can cause burns. Keep clear of safety valve outlet during operation.
- Check the cold system pressure; if it is already high (>1.8–2.0 bar), drain some water to the recommended level.
- Test the expansion vessel air charge (technician): isolate and drain pressure, then measure air side with a gauge; recharge to specified pressure if low.
- Ensure the pipe between boiler and expansion vessel is not blocked.
- If the safety valve drips constantly even at normal pressure, replace it.
Expansion vessel checks and charging are best performed by a heating professional.
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Repair Difficulty
Hard
4/5
Required Part
Expansion Vessel
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