Error S.11

Safety precautions:

1. If you smell gas, leave the property immediately and call your gas emergency number—do not operate any electrical switches or the boiler. 2. Before opening any covers or touching internal components switch off the boiler at the electrical isolator and the main switch and isolate mains power. If you are not competent with electrical/gas appliances do not remove panels—call a Gas Safe/registered engineer. 3. Do not block the flue or attempt improvised repairs to the flue terminal or fan.

Initial homeowner checks you can safely do:

1. Note any other displayed codes, and the exact circumstances when S.11 appears (e.g., only when you turn on hot water). Record the date/time. 2. Reset the boiler once: use the fault clearance/reset key (press and hold the fault-clearance or tick button for one second as per the manual) or follow the display reset sequence. Wait and see if the boiler attempts to run and whether the fault returns. 3. Check mains power and that the boiler is powered on. 4. Check the gas supply: ensure the gas meter/isolator is on and other gas appliances light. 5. Inspect the external flue terminal (outside): look for visible blockages such as bird nest, leaves, frost or debris. Clear only external obstructions you can safely reach without ladders. 6. Check external condensate pipe (in cold weather it can freeze): if frozen, thaw carefully (do not pour boiling water) and clear any blockage.

Specific diagnostic steps (observational and safe tests):

1. While someone calls for hot water (turn a tap on), listen at the boiler for the fan attempting to run. If you hear the fan spin, note whether it runs quietly or noisily and whether the display still throws S.11. If the fan runs but fault persists, the problem may be fan speed sensing or control electronics. 2. If no fan noise is heard during a DHW call and you have already checked power and reset, suspect a failed fan, wiring, connector, or PCB. 3. If you are competent with basic electrical isolation and the boiler is powered off at the isolator, you may remove the outer service flap (only if the manual allows and you are comfortable doing so) to visually check for obvious loose connectors or burned wiring at the fan harness. Do not touch internal parts with power connected. If you find loose or obviously damaged wires, do not attempt to repair unless you are a qualified engineer—record what you saw for the engineer.

What homeowners must not do:

1. Do not attempt to replace the fan motor, PCB, or any sealed combustion components unless you are a Gas Safe/registered engineer. 2. Do not block or modify the flue, or bypass safety devices. 3. Do not operate the boiler repeatedly if it locks out—this can hide a developing safety fault.

When to call a professional and what to tell them:

1. Call a Gas Safe/registered heating engineer if the fault persists after the reset and external checks, if the fan does not run when hot water is called, if you discover damaged wiring, or if you notice flue or combustion issues. 2. Provide the engineer with the boiler model, serial number, the exact fault code (S.11), any other displayed codes, what checks you performed (reset, flue inspection, condensate thaw), and whether the fan made any noise during testing. 3. The engineer will carry out powered diagnostics, test fan motor and tachometer, inspect wiring and connectors, test the control board and pressure/airflow switches, and replace faulty components or clear flue/airway obstructions. They will also perform a combustion and safety check after repair.

Other recommendations:

1. If the boiler is under warranty or covered by a service plan, contact Vaillant or your installer/servicer. 2. Keep a record of maintenance and faults; annual servicing by a competent engineer reduces the risk of these faults. 3. Do not ask an unqualified person to work on the boiler. Safety and correct combustion are critical—use a Gas Safe or equivalent registered professional for repairs.