F12 displayed and the unit reports resin capacity error or cannot calculate remaining capacity

Safety: No hydraulic work is required for basic reprogramming.

Actions:

• Verify water hardness: Test the raw water hardness using a test kit or refer to the water supplier's data. Enter the correct value into the Vaillant control.
• Check resin volume setting: Ensure the programmed resin volume matches the installed cylinder size specified for your Vaillant model.
• Factory reset: If allowed by the manual, perform a factory reset of the control and re‑enter all parameters (time, hardness, capacity, regeneration type).
• Monitor operation: After reset, observe the unit over several days to confirm that capacity is calculated correctly and F12 does not reappear.
• PCB replacement: If F12 persists despite correct programming, the control PCB memory may be faulty and require replacement by a technician.