Display shows F.22, boiler will not fire, and the pressure gauge reads very low or zero.

Safety: Ensure the system is cool before adding water. Do not over-pressurize the system.

Refill procedure:

• Locate the filling loop (flexible hose or integrated filling device) beneath the Vaillant boiler.
• Check the current pressure on the gauge; if below approximately 0.8 bar, slowly open the filling valves.
• Watch the gauge and close the valves when pressure reaches the recommended cold pressure (usually 1.0–1.5 bar; check the data plate/manual).
• Bleed air from radiators, starting from the lowest to the highest, then recheck pressure and top up if needed.
• Reset the boiler and check for normal operation.

If F.22 returns:

• Inspect visible pipework, radiators, and the boiler underside for leaks.
• If pressure drops repeatedly, a hidden leak or faulty expansion vessel may be present.
• If pressure is normal but F.22 still displays, the water pressure sensor on the hydraulic block may be faulty and require replacement.

Call a professional if you suspect internal leaks, expansion vessel failure, or if you are unsure how to safely use the filling loop.