Water heater locks out with F.20; unit overheats and shuts down, sometimes with very hot water at outlets

Safety: Turn off power and gas. Allow the appliance to cool completely before touching internal components. Hot surfaces can cause burns.

• Check system pressure and flow (if applicable): On models with a primary circuit, verify that system pressure is within Vaillant’s specified range and that all isolation valves are open.
• Bleed air: Bleed air from the pump and high points in the system. Airlocks can cause local overheating.
• Inspect the pump (if fitted): Listen for pump operation when there is demand. If the pump is silent or seized, it may need freeing or replacement.
• Check for scale/blockage: In hard water areas, the plate or primary heat exchanger can become scaled, restricting flow and causing overheat. Inspect for signs of kettling (whistling/boiling sounds). Descale or replace the heat exchanger as needed.
• Verify NTC readings: Compare flow and return NTC readings to actual temperatures. Replace any sensor that is significantly inaccurate.
• Reset: After resolving the cause, reset the appliance according to the manual. Monitor operation to ensure temperatures remain stable.
• If F.20 recurs: Do not repeatedly reset without finding the cause. Contact a qualified Vaillant engineer to check combustion, gas rate, and circulation thoroughly.