Glow-worm F28 Fault: Ignition Unsuccessful — Causes & What To Do

F28 indicates ignition was unsuccessful when starting up — the boiler attempted to ignite but no flame was established, so it locked out. You'll usually have no heating and no hot water.

The most common causes, in order of how often we see them on call-outs:

• • No gas supply — the gas valve at the meter or boiler may be off, a prepayment meter may have run out, or the supply may be interrupted. Check other gas appliances first.
• • Frozen condensate pipe — in cold weather the condensate discharge pipe outside can freeze, blocking the flue and preventing ignition. F28 (often with F1 or F4) is a classic frozen-condensate code on a Glow-worm.
• • Faulty igniter or spark electrode — the component that creates the spark to light the burner can fail, so no flame is established.
• • Faulty flame detection electrode — the electrode that confirms the flame is present can fail, so the boiler shuts down even if ignition starts.
• • Gas valve fault — the gas valve may not open to let gas through, so there's nothing to ignite.

You can try one reset after checking the gas supply and (in winter) the condensate pipe. If the condensate is frozen, thawing it and resetting can clear F28. If gas supply is fine and the condensate isn't frozen, a reset won't help — an ignition component or the gas valve needs a Gas Safe engineer.

• • Check other gas appliances — if your hob or gas fire also won't light, the issue is your gas supply (meter, prepayment credit, or a supply fault), not the boiler. Contact your gas supplier.
• • Check the condensate pipe outside — in freezing weather, look for ice at the pipe end or where it dips. If frozen, warm it gently with a hot water bottle or warm (not boiling) water poured over the frozen section, then reset the boiler.
• • Check the pressure gauge — if it's below 0.6 bar, repressurise to 1.2 bar via the filling loop, then reset.
• • If gas supply is fine, the condensate isn't frozen, and pressure is normal, the fault is an ignition component or gas valve — book a Gas Safe engineer. Don't keep resetting.

The igniter, flame electrode and gas valve sit inside the boiler casing and involve gas. Any work inside the casing is gas work that must be done by a Gas Safe Registered engineer. Never open the boiler casing yourself.

• • Confirm gas supply at the boiler and check the meter/regulator.
• • Test the ignition spark and inspect the spark electrode.
• • Test flame rectification — whether the boiler detects the flame once lit.
• • Check the condensate route and flue for blockages.
• • Test the gas valve opens and lets gas through correctly.
• • Diagnose the PCB ignition and gas valve signals, replace the faulty part, and re-commission with a combustion 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.