Boiler shows F8, burner stops, pump may continue running

Safety: The boiler has overheated. Allow it to cool fully before attempting any reset. Do not cover or obstruct ventilation openings.

• Check system pressure: Ensure pressure is at the recommended level. Top up via the filling loop if necessary, then bleed radiators.
• Open valves: Confirm all isolation valves under the boiler and on the heating circuit are open.
• Bleed air: Bleed air from the boiler's automatic air vent (if accessible) and from high points in the system.
• Reset overheat stat: Some Viessmann models have a manual reset button on the overheat thermostat. Press gently once after cooling.
• Check pump operation: Listen/feel for the internal pump running. If silent or very hot, the pump may be seized.
• If F8 recurs: A technician should check for sludge in the heat exchanger, verify pump performance, and flush the system if necessary.