Display shows F5 and no water is delivered through the softener

Safety first: Turn off power. Slowly close the inlet valve and relieve pressure by opening a downstream tap.

• Verify valves: Confirm that the inlet, outlet, and bypass valves are in the correct positions (service position, not bypass or closed).
• Check pre-filter: If a Viessmann pre-filter is installed before the softener, isolate it and remove the filter bowl. Clean or replace the filter cartridge if clogged.
• Inspect inlet strainer: Many Viessmann softeners have a small mesh strainer at the inlet. Remove and rinse it under running water to clear debris.
• Check for internal blockage: If external components are clear, the control valve may be blocked by debris or scale. Remove the valve head according to the service manual and inspect internal ports for obstructions.
• Restore service: Reassemble all components, open valves slowly to avoid water hammer, and check for normal flow. If F5 persists, the control head may need professional servicing.