Boiler displays F22, with no heating or hot water, and the pressure gauge reads low or zero.

Safety: Allow the system to cool before topping up. Do not over-pressurise above manufacturer limits (typically 1.5–2.0 bar cold).

• Check pressure gauge: Check the boiler's pressure gauge. If below approximately 0.8 bar, the boiler will lock out with F22.
• Top up via filling loop:
  • Locate the filling loop (usually a braided hose with two valves).
  • Open both valves slowly until the gauge rises to approximately 1.2–1.5 bar.
  • Close both valves fully and ensure there is no continuous filling.
• Bleed radiators: After topping up, bleed radiators to remove air, then recheck the pressure and top up again if needed.
• Check for leaks: Inspect visible pipework, radiators, and the boiler's underside for signs of water leaks.
• Engineer tasks:
  • Test and replace the water pressure sensor if the pressure is correct but F22 persists.
  • Locate and repair hidden leaks in the system.

Do not leave the filling loop open permanently; this can over-pressurise the system.