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

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.