F11 error and the unit aborts regeneration during brine fill or rinse

Safety: Disconnect power. Be prepared for water discharge when opening drain connections.

Steps:

• Inspect drain hose: Follow the hose from the softener to the drain. Straighten any kinks and ensure the hose rises and falls according to Vaillant installation guidelines (no excessive vertical lift).
• Check air gap / trap: If an air gap fitting is used, remove and clean it. Clear any slime or debris that could restrict flow.
• Flush the drain: Run water from another source into the drain to confirm it is not blocked. If the household drain is slow, have it cleared.
• Check internal drain valve: With the unit isolated, remove the drain elbow or fitting on the control head and inspect for resin beads or debris. Clean and reassemble with new O‑rings if necessary.
• Test regeneration: Restore power and water, then start a manual regeneration. Observe the drain discharge during backwash and rinse to ensure strong, steady flow.