Glow-Worm F28 Fault Code: Ignition Failure Explained
The F28 code means your Glow-Worm boiler attempted to light the burner, failed to detect a flame after three ignition attempts, and has locked itself out as a safety precaution. Because unburned gas could accumulate if the boiler kept trying indefinitely, the control system shuts everything down and displays F28 to let you know intervention is needed. The fault points to a breakdown somewhere in the ignition chain — gas supply, spark generation, flame detection, or the burner itself — and the boiler will not restart until the underlying problem is resolved.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Glow-worm F28 fault code mean?
The F28 code means your Glow-Worm boiler attempted to light the burner, failed to detect a flame after three ignition attempts, and has locked itself out as a safety precaution. Because unburned gas could accumulate if the boiler kept trying indefinitely, the control system shuts everything down and displays F28 to let you know intervention is needed. The fault points to a breakdown somewhere in the ignition chain — gas supply, spark generation, flame detection, or the burner itself — and the boiler will not restart until the underlying problem is resolved.
Common causes
- Interrupted or absent gas supply Common
If the gas supply to the property is off, restricted, or a prepayment meter has run out of credit, the burner simply has nothing to ignite. This is the single most common reason for an F28 callout and is worth ruling out immediately by checking whether other gas appliances in the home — hob, gas fire — are working normally.
- Low or insufficient gas pressure Common
Even when gas is flowing, pressure below the boiler's minimum threshold can prevent a reliable flame from establishing. Older installations can develop air pockets in the gas main over time, and a newly fitted boiler may expose inadequate pipe capacity that wasn't apparent with the previous appliance.
- Worn or corroded ignition electrodes and leads Common
The electrodes and high-tension leads that create the ignition spark deteriorate with age, particularly in the damp conditions common in UK boiler cupboards. Loose connections, hairline cracks in the ceramic electrode body, or corroded lead terminals all reduce spark intensity to the point where reliable ignition fails. This is a frequent cause in boilers more than five to seven years old.
- Faulty spark generator Sometimes
The spark generator is the component that supplies high voltage to the ignition electrodes. If it fails partially or completely, either no spark or a very weak spark is produced. Unlike electrode wear, spark generator failure tends to be more sudden rather than gradual.
- Frozen condensate pipe Sometimes
During cold spells, the condensate discharge pipe — often routed externally on UK installations — can freeze solid, causing condensate to back up inside the boiler. The boiler detects the blockage and locks out. Because this is weather-related, it tends to cluster in January and February and affects many homes simultaneously.
- Blocked or dirty burner Sometimes
Fine debris, dust, or combustion deposits can partially block the burner jets over time, restricting gas flow enough that a consistent flame cannot be established. If gas supply and ignition components check out, a dirty burner is the next logical suspect.
- Faulty gas valve Rare
The gas valve controls the precise flow of gas into the burner. A valve that sticks, fails to open fully, or receives incorrect signals from the PCB will prevent the right amount of gas from reaching the burner during ignition. Gas valve failures are less common but do occur, particularly in older units.
- PCB fault or wiring failure Rare
The printed circuit board orchestrates the entire ignition sequence. Damaged wiring between the PCB and key components such as the gas valve or ignition unit, or a failing PCB itself, can disrupt signals at a critical moment and produce an F28. This is typically a diagnosis of exclusion after other causes have been ruled out.
How to fix it
- Check that other gas appliances are working DIY safe
Turn on a gas hob ring or gas fire. If none of your gas appliances work, the problem lies with the supply to your property rather than the boiler. Contact your gas network operator (Cadent, SGN, Wales & West, or Northern Gas Networks depending on your region) or check your prepayment meter for credit. Do not attempt to reset the boiler until gas supply is confirmed.
- Check and top up boiler pressure if needed DIY safe
Look at the pressure gauge on the boiler — it should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, repressurise the system using the filling loop (two valves, usually braided hoses under the boiler). Open both valves slowly, watch the gauge, and close both once the needle reaches approximately 1.2 bar. Never exceed 2 bar.
- Thaw a frozen condensate pipe if appropriate DIY safe
If the weather is at or below freezing and the condensate pipe (typically a white or grey plastic pipe exiting through an external wall) feels solid, it is likely frozen. Pour warm — not boiling — water along the frozen section using a watering can, or hold a hot water bottle against it. Work from the boiler end outward. Once thawed, reset the boiler and consider lagging the pipe to prevent recurrence.
- Reset the boiler DIY safe
Once you have checked the gas supply, pressure, and condensate pipe, attempt a reset. On most Glow-Worm models this involves pressing and holding the reset button (marked with a flame or reset symbol on the control panel) for around three seconds, then releasing and allowing the boiler to go through its start-up sequence. If the boiler fires successfully, monitor it for the next hour. If F28 returns within a short time, do not keep resetting — repeated failed ignition attempts are not safe and further resets will not resolve a component fault.
- Call a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
If the boiler locks out again after one or two resets, or if you were unable to identify an obvious cause such as gas supply loss or a frozen pipe, a Gas Safe registered engineer must attend. They will carry out a systematic check of gas inlet pressure, ignition electrode condition and gap, lead continuity, spark generator output, gas valve operation, and PCB signals. Attempting to inspect or adjust the gas valve, electrodes, burner, or PCB yourself is unsafe and illegal without Gas Safe registration.
- Allow the engineer to replace faulty ignition components as required Gas Safe engineer
If the electrodes or leads are found to be worn, cracked, or corroded, the engineer will replace them. Importantly, before fitting new parts they should check for internal water leaks — moisture is a common cause of electrode failure, and fitting new components into a damp environment without fixing the leak simply delays the problem.
- Gas valve or PCB repair and replacement if required Gas Safe engineer
A confirmed gas valve fault will almost always require a full replacement rather than a repair — valves are safety-critical and a repaired valve is not considered reliable. PCB replacement is similarly an engineer-only task requiring careful component matching and testing. Both repairs should be followed by a full ignition and flue-gas analysis check before the boiler is returned to service.
Parts you may need
- Ignition electrodes (pair) · from £25
- Ignition leads / HT leads · from £20
- Spark generator / ignition unit · from £55
- Gas valve · from £120
- Flame sensor / ionisation probe · from £18
- PCB (printed circuit board) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix a Glow-Worm F28 fault myself?
There are a small number of checks a homeowner can safely carry out: confirming the gas supply is live, topping up system pressure using the filling loop, thawing a frozen condensate pipe, and resetting the boiler once or twice. Beyond that, all diagnosis and repair work on the gas valve, ignition components, burner, and PCB must be done by a Gas Safe registered engineer. This is both a legal requirement and a genuine safety matter — unburned gas accumulation is the reason the boiler locked out in the first place.
How much does it cost to fix a Glow-Worm F28 fault in the UK?
For most people the repair falls between £120 and £350, which covers a Gas Safe engineer's visit and the most commonly replaced parts such as ignition electrodes, leads, or a spark generator. A gas valve replacement sits at the higher end of that range. PCB replacement is significantly more expensive — typically £350 to £500 including labour — but is a less common outcome. If your boiler is within its warranty period and has been serviced annually by a Gas Safe engineer, contact Glow-Worm before booking an independent engineer, as parts costs may be covered.
Why does my Glow-Worm keep showing F28 after I reset it?
A reset clears the lockout but does not fix the underlying fault. If F28 returns quickly after a reset it means the ignition sequence is failing again for the same reason — a faulty component, insufficient gas pressure, or a blocked burner. Repeatedly resetting the boiler is not a solution and can be unsafe. After two or three failed resets you should stop and call a Gas Safe registered engineer to properly diagnose and repair the fault.
Could a frozen condensate pipe cause an F28 on a Glow-Worm boiler?
Yes, and this is a surprisingly common cause during cold snaps in the UK. When the external condensate pipe freezes, acidic condensate backs up inside the boiler and triggers a lockout — which can display as F28 on Glow-Worm models. The fix is to gently thaw the pipe using warm water or a hot water bottle, then reset the boiler. To prevent it happening again, insulate the external section of the pipe with lagging foam, which is inexpensive and straightforward to fit yourself.