Quick answer
F28 on a Glow-worm boiler means an ignition failure: the boiler tried to fire up but no flame was detected at start-up, so it locks out as a safety measure.
You can safely check your gas supply, boiler pressure and (in winter) a frozen condensate pipe, then reset the boiler once.
If F28 returns after a single reset, the fault is internal and gas-side, so book a Gas Safe registered engineer rather than repeatedly resetting.
What does F28 mean on a Glow-worm boiler?
An F28 fault code on Glow-worm combi and system boilers (such as the Energy, Ultimate and Easicom ranges) signals an ignition failure. In plain terms: the boiler began its start-up sequence, asked for ignition, but the flame-sensing electrode never confirmed a flame within the permitted time.
As a safety measure the boiler "locks out" and stops, so it won't keep releasing gas when nothing is lighting.
This is the boiler doing exactly what it should. F28 is a protective shutdown, not a sign that the appliance is unsafe to be near. But it does mean you have no heating or hot water until the underlying cause is found and cleared.
Important: Ignition involves the gas valve, the gas supply and the sealed combustion circuit. Beyond the simple checks below, F28 is Gas Safe work only. Never remove the boiler casing, touch the gas valve, ignition electrodes or flue, or attempt to "force" ignition yourself. If you ever smell gas, call the National Gas Emergency line on 0800 111 999.
Common causes of F28
Because F28 simply means "no flame proven", several different problems can trigger it:
- No gas reaching the boiler — a closed gas isolation valve, an empty prepayment meter, or a wider supply interruption.
- A frozen condensate pipe — extremely common in cold snaps; the boiler can't drain its acidic condensate and locks out (often shows alongside ignition faults).
- Low or lost gas pressure at the meter or governor.
- Faulty ignition electrodes or leads that aren't producing a spark.
- A failing flame-sensing electrode that can't "see" the flame even when one is present.
- A faulty gas valve or PCB that isn't opening or controlling ignition correctly.
Only the first two are things a homeowner can sensibly check. The rest are internal, gas-side faults that must be diagnosed by a registered engineer.
| Possible cause | What to check | Call an engineer? |
|---|---|---|
| No gas reaching the boiler | Do other gas appliances light? Is the prepayment meter in credit and the isolation valve open? | No — homeowner check |
| Frozen condensate pipe (winter) | Thaw the external white pipe with warm (not boiling) water, then reset | No — homeowner check |
| Low / lost gas pressure | Note the symptom; gas-side pressure is not a DIY check | Yes — Gas Safe engineer |
| Faulty ignition or flame-sensing electrodes | Internal — do not open the casing | Yes — Gas Safe engineer |
| Faulty gas valve or PCB | Internal — do not open the casing | Yes — Gas Safe engineer |
Safe checks you can do yourself
Before you call anyone, run through these homeowner-safe checks. None of them involve opening the boiler or touching anything gas-side.
- Check your other gas appliances. Does your gas hob or gas fire light normally? If nothing gas-powered works, the problem is your gas supply, not the boiler. Check whether your prepayment meter has credit, and confirm the gas isolation valve (the lever on the pipe below the boiler) is in line with the pipe (open). If you have a wider outage, contact your supplier.
- Check the boiler pressure gauge. Normal cold pressure is roughly 1–1.5 bar, rising to around 2 bar when hot. Below about 1 bar is low. Pressure problems usually throw a different code, but it's worth ruling out while you're there. You can safely top up using the filling loop if your boiler has one.
- In winter, check for a frozen condensate pipe. Look for the white plastic pipe leaving the boiler and running to an outside drain. If it's frozen, you can thaw it by pouring warm (not boiling) water along the exposed external section, or holding a covered hot water bottle against it. Once thawed, reset the boiler.
- Reset the boiler once. Press and hold the reset button on the front panel as described in your user guide. Give it a couple of minutes to attempt ignition.
Reset once — then stop. Manufacturers and Gas Safe engineers advise resetting a locked-out boiler no more than once. If F28 returns, repeatedly resetting can release unburnt gas into the combustion chamber and is unsafe. A second lockout is your signal to book an engineer.
When to call a Gas Safe registered engineer
If the boiler relocks to F28 after one reset — or if your other gas appliances work fine and the condensate isn't frozen — the fault is internal and gas-side. That means it's time for a professional.
A Gas Safe registered engineer can legally open the boiler to inspect and test the ignition electrodes, flame-sensing electrode, gas valve, PCB and gas pressure.
Anyone working on gas appliances in the UK must be on the Gas Safe Register — this is the legal replacement for the old CORGI scheme, which ended in 2009. Always ask to see the engineer's Gas Safe ID card.
Likely repair costs in 2026
Costs vary by region, parts and how long the diagnosis takes. The figures below are indicative UK ranges for 2026, not quotes.
| Job | Typical indicative cost |
|---|---|
| Diagnostic / call-out visit | £70–£120 |
| Replace ignition or flame-sensing electrode (parts + labour) | £100–£200 |
| Replace gas valve | £250–£450 |
| Replace PCB (control board) | £300–£550 |
| Clear / re-route a chronically freezing condensate pipe | £80–£200 |
This is where a boiler cover policy can pay for itself: with cover in place, an F28 lockout usually means one phone call and no separate repair bill. Without it, a single gas valve or PCB replacement can cost more than a year's premium.
Use our tool to compare boiler insurance and service plans and find a plan that fits your boiler and your budget.
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Compare boiler coverFrequently asked questions
Can I fix an F28 fault myself?
You can do the safe checks above — gas supply, pressure, frozen condensate and a single reset. If F28 returns after that, the cause is internal (electrodes, gas valve or PCB) and must be repaired by a Gas Safe registered engineer. Never open the boiler casing yourself.
Why does F28 keep coming back?
A recurring F28 usually points to a hardware fault that won't clear with a reset — most often failing ignition or flame-sensing electrodes, a faulty gas valve, or a PCB problem. It can also recur each cold night if a condensate pipe keeps freezing. Either way, book an engineer to diagnose it properly.
Is it safe to keep using the boiler with an F28 lockout?
The lockout itself is a safety feature, so the boiler isn't running while it shows F28. The risk comes from repeatedly resetting it, which can release unburnt gas. Reset once at most, and if it relocks, leave it off and call an engineer. If you smell gas at any point, call 0800 111 999.
Does boiler cover include F28 ignition faults?
Most boiler breakdown policies cover ignition and gas-valve faults as standard repairs, subject to your boiler's age and the policy terms. Always check the exclusions and any age limit before buying. You can weigh up whether cover is worth it for your situation.
F28 isn't usually a sign of a dangerous boiler — it's the appliance refusing to run safely until ignition is sorted. Do the simple checks, reset once, and if it relocks, get a registered engineer in. For more on protecting yourself against repair bills, see our guides to the best boiler cover and cheaper cover options.
Information, not advice. Boiler Cover UK is an independent comparison site and earns commission from a selected panel of providers, not the whole market. Always use a Gas Safe registered engineer for any work on your boiler.