F.22Glow-worm (also shown as F9)
Glow-worm F.22: low water pressure in the boiler (F9 on older models)
Checked against Glow-worm's own documentation: Glow-worm: Energy range fault codes and 4 more sources. Last reviewed October 2026. How we check
You can usually fix this
The water pressure in your heating system is too low, so the boiler has shut down to protect itself. Glow-worm says this can usually be fixed by repressurising. Older Glow-worm boilers show the same problem as F9.
Recommended pressure when cold for Glow-worm boilers: 1.0 to 1.5 bar.
What you can safely do
- Turn the boiler off and let it cool.
- Find the filling loop and open its valves slowly.
- Stop when the gauge reads 1 to 1.5 bar, as Glow-worm recommends, then close the valves and check the gauge again.
Call a Gas Safe engineer if
- The pressure keeps dropping.
- You notice anything else unusual, such as water leaking.
Find a Gas Safe engineer near you
Opens the official Gas Safe Register search.
Applies to Energy range, Betacom A range. Codes can differ between models, so check your manual.