Furnace shows F8, unit may attempt to start but shuts down quickly, sometimes accompanied by gurgling noises

Safety first: Allow the system to cool before adding water. Do not over-pressurize the system.

• Check system pressure: Look at the pressure gauge on the front of the furnace. If it is below the recommended cold pressure (often around 1.0 bar), the system needs topping up.
• Top up the system: Locate the filling loop (flexible hose or built-in filling device). Open the valves slowly and watch the gauge rise to the target pressure (typically 1.0–1.5 bar). Close the valves securely afterwards.
• Bleed radiators: Bleed air from radiators and the pump. Recheck and adjust pressure as needed after bleeding.
• Inspect for leaks: Check visible pipework, radiator valves, and the underside of the furnace for signs of water leakage. Persistent pressure loss indicates a leak that must be repaired.
• Sensor check: If pressure is correct but F8 persists, the water pressure sensor or its wiring may be faulty. A technician can test and replace the sensor with a Saunier Duval pressure transducer.