Safety: Be prepared to shut off the main water supply if a leak is suspected.

Steps:

Check for actual flow: With all taps closed, look at the Vaillant display. If it shows flow, confirm by listening for water movement or checking the water meter at the property.
Inspect for leaks: Walk the property and check visible pipework, toilets, and appliances for drips or running water. Repair any leaks found.
Test softener bypass: Place the softener in bypass mode. If the display still shows flow, the issue is likely in the flow meter or control electronics, not the valve stack.
Check flow meter: Remove and inspect the flow meter for debris or mechanical damage. A partially broken turbine can cause false pulses. Replace the flow meter if damaged.
Review leak detection settings: Some models allow configuration of leak thresholds. Ensure these are set according to Vaillant guidelines and local usage patterns.

Note: Do not ignore F08; continuous unnoticed flow can cause significant water damage and high bills.

F08 displayed and the unit reports excessive water usage or continuous flow when no taps are open

Possible Causes