Glow-worm F.9 Fault Code
The F.9 fault means the Glow-worm PCB cannot confirm the fan is running correctly, or the air pressure switch (APS) has not triggered during the fan proving sequence. As a safety measure, ignition is blocked until flue clearance is confirmed.
What is the Glow-worm F.9 fault code?
The Glow-worm F.9 fault is triggered when the control board does not receive the correct signal from the air pressure switch within the expected time after the fan starts. The APS is a small diaphragm switch that confirms flue gases can escape safely before ignition is permitted — if it does not close, the PCB will not allow the boiler to fire. The fault can originate at the fan motor, the APS itself, the rubber hose connecting them, a blocked flue terminal, or a blocked condensate trap. A Gas Safe engineer is required — do not attempt to bypass the APS proving sequence.
What causes the F.9 fault?
The fan motor degrades over time and may run too slowly to generate sufficient flue draught for the APS to trigger. On Glow-worm boilers over 8 years old, fan motor wear is a common cause of F.9.
The APS diaphragm can crack or the contacts can fail. The APS shows F.9 even when the fan is running correctly at the right speed.
A bird nest, debris, or partial obstruction at the flue terminal restricts airflow and trips F.9 even if the fan and APS are both functioning.
The short rubber hose between the APS and fan assembly can split, crack, or work loose. Without this connection, the APS cannot detect fan pressure — F.9 results regardless of fan condition.
A blocked condensate trap creates backpressure in the flue gas path, mimicking a restricted flue and preventing the APS from triggering correctly.
What you can safely check yourself
If the flue terminal is safely visible from ground level, inspect for obvious blockages — bird nests, debris, or vegetation. Do not reach inside the flue terminal.
In cold weather, pour warm (not boiling) water over the external condensate pipe. A frozen or blocked condensate pipe commonly causes F.9 in winter. Attempt one reset after thawing.
If F.9 returns on the first startup attempt, call a Gas Safe engineer. Do not reset repeatedly — the fault will not clear without component diagnosis.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Test fan motor speed and current draw — replace motor if below specification
- Inspect flue terminal and accessible flue run for blockages
- Test air pressure switch operation with a manometer
- Inspect APS hose for splits, cracks, or loose connections
- Clear or replace blocked condensate trap
- Check PCB fan speed sensing circuit if all components test normal
- Commission and test boiler after repair
Fan faults are common across South London's older rental housing stock. Many properties in Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley run Glow-worm boilers 8–14 years old — within the fan motor wear window. Annual servicing includes a fan speed check and APS test, which can identify F.9 causes before a lockout occurs.
How much does a F.9 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Glow-worm boiler showing F.9
In winter, check and thaw the external condensate pipe with warm water first.
If safely visible, check the flue terminal for obvious blockages.
Hold the reset button for 3–5 seconds.
Listen for the fan starting within 5 seconds of reset.
If F.9 returns within the first heating cycle, call a Gas Safe engineer.
Fan motor replacement is a straightforward repair costing £200–£380 including parts and labour. On any Glow-worm boiler under 10 years old, it is unambiguously worth doing. Glow-worm fan assemblies share components with the Vaillant Group range, so parts are readily available.
Common questions about the Glow-worm F.9 fault
What does a Glow-worm F.9 fault code mean?
Can I fix a Glow-worm F.9 fault myself?
Why does my Glow-worm show F.9 in cold weather?
How much does a Glow-worm F.9 repair cost in London?
Is a Glow-worm F.9 fault dangerous?
Other Glow-worm fault codes
Need a Glow-worm engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
