Boiler shows F11 and locks out, often with rapid temperature rise

Safety first: Turn off power and allow the boiler to cool completely. Overheated components can cause burns.

Steps to check:

• Check system pressure and top up if low (typically 1.2–1.5 bar cold).
• Bleed air from radiators and any automatic air vents near the boiler.
• Listen for the circulation pump when the boiler calls for heat; if silent or excessively noisy, it may be stuck.
• On some Saunier Duval models, the manual reset overheat thermostat can be reset by pressing the button on the sensor body (consult manual for location).
• If F11 recurs, the pump performance or primary heat exchanger may be compromised and require professional cleaning or replacement.

Warning: Do not repeatedly reset an overheating boiler without finding the cause. This can be dangerous and damage the appliance.