Vaillant
System Water Leak / Pressure Relief Valve Discharge
Furnace/boiler leaking water from underneath the casing or from relief pipe 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
Failed expansion vessel (loss of air charge), Faulty pressure relief valve (PRV) not sealing, Corroded heat exchanger or hydraulic block, Loose compression fittings on Vaillant manifold
How to Fix / Troubleshooting
Safety: Turn off power. If leak is significant, close the system isolation valves and, if safe, the cold water feed. Water may be hot.
User checks:
- Observe whether water is dripping from the pressure relief valve discharge pipe outside when the system heats up. If so, the expansion vessel may be flat.
- Check the system pressure gauge; if it rises above 2.5–3 bar when hot, the expansion vessel likely needs recharging or replacement.
- Inspect visible joints under the boiler for slow drips; gently tighten compression nuts if clearly loose (do not overtighten).
Professional tasks:
- Check and recharge the internal expansion vessel via the Schrader valve to the specified pre-charge, or replace if the diaphragm has failed.
- Replace a leaking PRV; once it has discharged debris, it often will not reseal.
- Investigate and repair any leaks from the primary heat exchanger or hydraulic block.
Do not ignore leaks; they can damage the boiler and electrics.
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Repair Difficulty
Hard
4/5
Required Part
Expansion Vessel
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