Glow-worm F.28 Fault Code
The F.28 fault is the most common Glow-worm lockout — the boiler attempted to ignite multiple times but could not establish or confirm a stable flame. It shares the same ignition chain as the Vaillant ecoTEC F.28, Worcester Bosch E1, and Ideal F1, due to Glow-worm's Vaillant Group engineering.
What is the Glow-worm F.28 fault code?
The Glow-worm F.28 fault is triggered when the PCB initiates a series of ignition attempts and the ionisation probe does not confirm a stable flame within the permitted window. Because Glow-worm boilers share the Vaillant Group ignition chain, the diagnosis follows the same sequence as a Vaillant F.28: gas supply check first, then electrode, ionisation probe, ignition lead, and gas valve. The boiler typically makes three attempts before displaying F.28 and locking out. A Gas Safe engineer is required to diagnose and repair.
What causes the F.28 fault?
A closed meter valve, empty prepayment meter, or work on the local gas network starves the boiler. Always test another gas appliance before calling an engineer for F.28.
The ceramic insulator cracks or the electrode tip corrodes over time, preventing a consistent spark from reaching the burner. Common on Glow-worm boilers over 7 years old.
Combustion deposits on the probe prevent it detecting the flame. The PCB receives no flame signal and locks out — even if ignition was momentarily successful. Accelerated by South London hard water.
A partially blocked or slow-opening gas valve provides insufficient gas at the burner during the ignition window. The spark fires but the flame cannot establish.
In winter, a frozen or blocked condensate pipe creates backpressure in the combustion chamber, disrupting ignition. This is the first thing to check during cold weather F.28 lockouts.
Cracked insulation on the high-voltage lead causes the spark to arc to ground. The electrode never receives the spark — the boiler cycles through ignition attempts without any spark reaching the burner.
What you can safely check yourself
Turn on a gas hob or fire. If nothing works, the supply is the problem — call your gas supplier or check your prepayment meter.
Ensure the emergency control valve at the meter is fully open (handle in line with the pipe). If it was accidentally knocked closed, open it and reset.
Hold the reset button for 3–5 seconds. Allow up to 60 seconds for the ignition sequence. If F.28 returns, do not reset again — call a Gas Safe engineer.
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 ignition electrode spark output and replace if degraded
- Clean or replace the ionisation probe
- Inspect and replace the ignition lead if cracking or arcing
- Measure gas inlet and manifold pressure
- Test gas valve opening speed and replace if sluggish
- Clean the burner if carbon deposits are present
- Check condensate trap and pipe for blockage
- Check flue for backpressure that could prevent ignition
Hard water in South London — Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley — accelerates combustion deposits on the ionisation probe and burner. The calcium-heavy Thames Water supply also contributes to limescale inside the burner assembly on Glow-worm boilers that miss annual services. F.28 is the most frequently booked Glow-worm fault across Reion's South London callouts, mirroring its Vaillant parent brand.
How much does a F.28 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.28
Confirm gas supply is working by testing another gas appliance.
Check the meter emergency control valve is fully open.
In winter, check whether the condensate pipe is frozen — thaw with warm water if needed.
Hold the reset button for 3–5 seconds.
Wait for the full ignition sequence — up to 60 seconds.
If F.28 returns, do not reset again. Call a Gas Safe engineer.
Gas valve replacement is among the more expensive Glow-worm repairs. On a Flexicom or Energy model under 10 years old with no other faults, it is worth doing. Because Glow-worm shares Vaillant Group gas valves, parts availability is good. On a boiler over 13 years old with additional age-related concerns, Reion will give a clear repair-vs-replace comparison at the diagnostic visit.
Common questions about the Glow-worm F.28 fault
What does a Glow-worm F.28 fault code mean?
Can a Glow-worm F.28 fault fix itself?
How long does a Glow-worm F.28 repair take in London?
Is the Glow-worm F.28 fault the same as a gas leak?
Why does my Glow-worm F.28 fault keep coming back?
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.
