F3
Saunier Duval Overheat Thermostat Trip / Safety Thermostat Open

Furnace displays F3 and stops, often after running for a short time; casing may feel very hot

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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

Blocked primary heat exchanger reducing water flow,Failed or tripped manual reset overheat thermostat,Seized or air-locked circulation pump,Closed or blocked system valves causing no circulation

How to Fix / Troubleshooting

Safety first: Turn off power and gas. The appliance may be very hot; wait until it cools before touching internal parts. Never bypass an overheat thermostat.

  • Check system pressure and air: Verify that the heating system pressure is within the recommended range (typically 1.0–1.5 bar when cold). Bleed radiators and the pump to remove trapped air that can stop circulation.
  • Inspect circulation pump: Confirm that the pump is free to turn. On many Saunier Duval units, you can remove the pump vent plug and gently turn the shaft with a screwdriver. If seized, the pump may need replacement.
  • Reset overheat thermostat: Locate the manual reset overheat thermostat on the flow pipe or heat exchanger. Press the reset button if present. If it trips repeatedly, there is an underlying circulation or scaling problem that must be corrected.
  • Check valves and filters: Ensure all isolation valves on the flow and return are fully open. Clean any system filters or strainers in the return line or hydraulic block.
  • Descale if necessary: If the primary heat exchanger is heavily scaled (common in hard water areas), it can cause localized overheating. This requires chemical descaling or heat exchanger replacement by a qualified technician.
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Repair Difficulty

Hard 4/5

Required Part

Overheat Thermostat
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