F9
Saunier Duval Internal Relay / Output Failure

Thermostat shows F9 and heating demand is ignored by the boiler

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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

Stuck or burned relay contacts in thermostat, Damaged low-voltage switching transistor, Overload due to incorrect 230 V wiring on low-voltage contacts, Surge damage from mains spikes

How to Fix / Troubleshooting

Safety: Turn off power to both boiler and thermostat. If 230 V has been miswired, there is a risk of electric shock.

  • Check wiring type: Confirm whether your Saunier Duval smart thermostat is wired as a low-voltage bus device or as a simple on/off relay. Never connect 230 V to bus terminals.
  • Test boiler directly: Temporarily bridge the boiler's room thermostat terminals (if present) with a short wire to see if the boiler starts heating. If it does, the thermostat output is faulty.
  • Inspect for burning: Remove the thermostat from its base and look for burn marks or melted plastic near the relay area.

If the relay is stuck or burned, the thermostat must be replaced; relay replacement on the PCB is not recommended for end users.

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Repair Difficulty

Hard 4/5

Required Part

Saunier Duval Smart Thermostat Relay Output
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