Boiler displays E25, noisy boiling or kettling sounds from the heat exchanger
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.
Possible Causes
How to Fix / Troubleshooting
Safety first: Turn off the boiler and allow it to cool. Do not ignore kettling noises as they indicate overheating.
User checks:
- Bleed radiators and top up system pressure.
- Ensure all radiator valves are open to allow circulation.
Engineer checks:
- Check pump operation and speed; free a stuck pump shaft if necessary.
- Assess system water quality; if dirty, perform a chemical flush or powerflush.
- Descale or replace the primary heat exchanger if heavily scaled.
- Check for correct bypass operation and system balancing.
Addressing the underlying restriction will prevent repeated E25 lockouts and extend boiler life.
Repair Difficulty
Required Part
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