Glow-worm F3 Fault: Fan Unable to Reach Ignition Speed — Causes & What To Do

F3 indicates the fan is unable to reach ignition speed, so the boiler can't confirm flue gases will be safely expelled. It locks out before ignition as a safety measure — a boiler can't fire unless it's certain the flue is being cleared.

The most common causes, in order of how often we see them:

• • Failed fan — the fan motor can fail or slow with age, so it can't reach the speed needed for ignition.
• • Fan wiring fault — a loose, damaged or corroded connection between the fan and the PCB interrupts the speed signal.
• • Air pressure switch fault — the switch that confirms the fan is producing enough draft can fail or stick, so the boiler won't fire.
• • Flue blockage or restriction — a blocked or restricted flue can prevent the fan reaching the required speed or pressure.

You can try one reset — occasionally a transient fan signal will clear. But fan faults don't usually self-correct, and the boiler won't fire unless it's certain the flue is being cleared. If F3 persists after one reset, book a Gas Safe engineer.

• • Check the pressure gauge — confirm it's in the healthy 1.0–1.5 bar range, then try one reset.
• • Listen for the fan — when the boiler tries to fire, you should hear the fan spin up before ignition. If you don't hear it, or it's noisy or scratching, the fan may have failed.
• • Check the flue terminal outside is clear of debris, nests or snow — a blocked flue can affect fan operation.
• • If F3 persists after one reset, the fault is the fan, its wiring, the air pressure switch, or the flue — book a Gas Safe engineer.

The fan, its wiring and the air pressure switch sit inside the boiler casing, and the flue involves gas safety. Replacing the fan, repairing its wiring, or any work inside the casing is gas work that must be done by a Gas Safe Registered engineer.

• • Test the fan — confirm it runs and reaches ignition speed.
• • Inspect the fan wiring and connections for damage or corrosion.
• • Test the air pressure switch — confirm it detects the fan's draft correctly.
• • Check the flue for blockage or restriction.
• • Replace the fan, wiring or air pressure switch as needed, then re-commission with a flue check.

Our Gas Safe Registered (594547) engineers repair every Glow-worm model — Betacom, Ultracom, Flexicom and Energy — across Surrey, Berkshire, Buckinghamshire and West London, and carry common sensors, fans and ignition parts on the van. Most Glow-worm fault-code repairs are completed in a single visit, with any part quoted and agreed before fitting.