Display shows F11 and water remains hard even after regeneration

Safety first: No immediate hydraulic disassembly is required; keep covers closed while testing.

• Test water hardness: Use a hardness test kit on raw water and softened water. Compare results to the programmed hardness value.
• Verify settings: Ensure the raw water hardness and regeneration capacity are correctly programmed in the controller.
• Check brine strength: Confirm that there is adequate salt in the brine tank and that the water level is correct (not excessively high or low). A weak brine solution will not fully regenerate the resin.
• Run an extra regeneration: Manually initiate one or two additional regenerations to see if water hardness improves.
• Inspect resin bed: If hardness remains high, the resin may be exhausted or iron-fouled. Plan to replace the Viessmann ion-exchange resin or perform a resin cleaning procedure using an approved cleaner.