Furnace shows E11, often related to domestic hot water (DHW) demand on combi models.

Safety: Isolate power and close the cold water inlet valve before working on DHW components.

• Check DHW demand: Open a hot tap fully and observe the furnace. If E11 appears and the burner does not light, suspect the DHW flow sensor.
• Clean inlet filter: Close the cold inlet valve under the furnace. Remove the inlet filter (usually a small mesh filter at the cold inlet connection) and rinse under running water. Refit and open the valve slowly.
• Inspect DHW flow sensor: The sensor/turbine assembly is typically located on the cold inlet manifold. Check for leaks and ensure the electrical connector is secure.
• Descale (if hard water): Limescale can jam the turbine. A technician can remove the sensor body and soak it in descaling solution, then flush thoroughly.
• Reset and test: Restore power, open a hot tap, and check operation. Persistent E11 requires sensor replacement by a professional.