Saunier Duval Internal Water Leak

Boiler leaking water from underneath the casing

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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 domestic hot water plate heat exchanger, Dripping automatic air vent, Faulty pressure relief (safety) valve, Perished O-rings on hydraulic block

How to Fix / Troubleshooting

Safety first: Turn off power to the boiler. If the leak is heavy, isolate the cold water supply and central heating flow/return valves if fitted.

Steps to check:

  • Remove the front panel and visually inspect inside with a torch to locate the source of the leak (top, side, or bottom of the hydraulic group).
  • Check the pressure gauge: if pressure is high (>3 bar) and water is discharging via a pipe outside, the safety valve may be opening due to expansion vessel issues.
  • Inspect the automatic air vent on top of the pump or heat exchanger for signs of water; tighten or replace if leaking from the cap.
  • Look for staining or wetness around the plate heat exchanger and its connections.
  • Place a tray or towel to catch drips and prevent water reaching electrical components.
  • For any component replacement (plate heat exchanger, safety valve, O-rings), contact a qualified engineer.

Warning: Do not continue to operate a leaking boiler. Water can damage the PCB and create electrical hazards.

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Repair Difficulty

Hard 4/5

Required Part

Plate Heat Exchanger / Safety Valve / Automatic Air Vent
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