Viessmann
Expansion Vessel Failure / Overpressure
Boiler pressure rises rapidly when heating, then discharges 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
Ruptured expansion vessel diaphragm, loss of pre-charge, blocked connection to vessel, faulty pressure relief valve
How to Fix / Troubleshooting
Safety: Hot water may discharge from the pressure relief pipe; keep clear of the outlet.
- Observe pressure: Note cold and hot pressures. If pressure climbs above 3 bar and PRV discharges, expansion is inadequate.
- Check filling loop: Ensure the filling loop valves are fully closed and not letting by.
- Expansion vessel recharge (technician): A professional should isolate and drain the boiler, then check and recharge the expansion vessel with air/nitrogen to the specified pressure.
- Replace vessel/PRV: If the vessel will not hold charge or the PRV continues to drip, replace these components.
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Repair Difficulty
Medium
3/5
Required Part
Expansion Vessel
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