Vaillant Chronic Pressure Loss

Boiler losing pressure daily, no visible leaks on radiators

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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

Leaking expansion vessel diaphragm, Pressure relief valve passing, Hidden leak in pipework under floors, Micro-leak on boiler hydraulic block

How to Fix / Troubleshooting

Safety: Do not repeatedly overfill the system above 2 bar.

  • Check PRV discharge pipe: Look outside at the copper pressure relief pipe. If it drips when the boiler runs, the PRV is passing or expansion vessel is faulty.
  • Mark gauge: Note the pressure when cold and again after 24 hours with the boiler off. A steady drop indicates a leak.
  • Inspect visible pipework: Check around radiators, valves, and joints for staining or damp patches.
  • Engineer tests: A technician can test and recharge or replace the expansion vessel, replace the PRV, and pressure-test the system for hidden leaks.
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Repair Difficulty

Medium 3/5

Required Part

Expansion Vessel
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