Viessmann
Chronic pressure loss / leak or expansion fault
System pressure drops to zero over days or weeks, requiring frequent topping up
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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
Hidden leak in pipework or underfloor heating, Faulty expansion vessel losing charge, Slightly leaking pressure relief valve, Micro-leaks at radiator valves
How to Fix / Troubleshooting
Safety: Do not repeatedly top up without investigating; oxygenated water can damage the system.
- Check visible leaks: Inspect all radiators, valves, and accessible pipes for damp patches or staining.
- Monitor PRV discharge: Place a container under the pressure relief valve discharge (if accessible) to see if it drips during operation.
- Mark pressure: Note the pressure when cold and after heating. Large increases suggest expansion vessel issues.
- Professional leak detection: A technician can test the expansion vessel pre-charge and use leak detection methods to locate hidden leaks.
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Repair Difficulty
Hard
4/5
Required Part
Expansion Vessel
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