Boiler shuts down with F.20, casing and flow pipe very hot, pump may be noisy

Critical safety: Overheat conditions can damage the heat exchanger. Turn off power and let the boiler cool completely before touching any components.

Basic checks:

• Verify that all radiator and isolation valves on the flow and return are open.
• Bleed air from radiators and the boiler's automatic air vent (if accessible) to remove air locks.
• Listen for the pump when the boiler calls for heat: if silent or just humming, the pump may be seized.
• With power off, remove the pump head cover and gently turn the pump shaft with a screwdriver (if design allows) to free it.
• Check the system pressure on the Vaillant pressure gauge; top up via the filling loop to the recommended cold pressure (typically around 1.0–1.5 bar).
• Reset the boiler according to the manual and observe operation. If F.20 recurs quickly, internal blockages or a failed pump are likely.

Professional tasks: Cleaning or replacing the plate heat exchanger, replacing the circulation pump, or checking PCB pump control should be done by a qualified heating engineer.