F.22 displayed; water heater will not fire, sometimes accompanied by gurgling noises

Safety: Switch off power and close gas. Do not operate the heater without adequate water flow; this can damage the heat exchanger.

• Check water supply: Ensure the cold water isolation valve to the heater is fully open. Confirm adequate mains pressure at a nearby tap.
• Clean inlet filter: Isolate water, relieve pressure, and remove the cold inlet filter/strainer on the Vaillant water heater. Rinse debris under running water and refit.
• Inspect flow sensor: Locate the flow sensor/flow turbine assembly. Remove and check for scale or debris preventing movement. Clean carefully or replace if worn or stuck.
• Bleed air: Open a hot tap fully and allow water to run to purge air from the system. Some models have dedicated bleed points; follow the manual.
• Restore and test: Turn water back on, check for leaks, restore power and gas, then demand hot water. The F.22 code should clear automatically if flow is restored.
• Persistent F.22: If flow is clearly present but the unit still reports F.22, the flow sensor or main PCB may be faulty. Have a Vaillant technician test and replace the defective component.