F28 Boiler Fault Code: What It Means on Every Brand

F28 means the same on all five brands that display it — Vaillant, Glow-worm, Saunier Duval, Heatline and Protherm share control electronics — and it is an ignition lockout: the boiler tried to light, usually three times, and never detected a flame. Its sibling F29 is flame loss after a successful light-off. In winter the overwhelming cause is a frozen condensate pipe; otherwise gas supply, a blocked flue, or a worn ignition electrode. One reset after checking gas and condensate; then an engineer.

Lockout — engineer likely 5 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F28 means	Severity	DIY?
GWGlow-worm	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. Full guide →	Lockout — engineer likely	Some checks
HHeatline	The F28 code on a Heatline boiler signals an ignition lockout. Full guide →	Lockout — engineer likely	No
PProthermshown as F.28	The F.28 code appears on Protherm boilers — including the Gepard and Panther ranges — when the burner fails to light on its first attempt. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F28 code means your Saunier Duval boiler has tried to light the burner three times in a row and failed every time. Full guide →	Lockout — engineer likely	No
VVaillantshown as F.28	F.28 is an ignition lockout fault. Full guide →	Lockout — engineer likely	No

F28 brand by brand

Glow-worm F28

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.

Causes, fixes & costs for Glow-worm F28 → · All Glow-worm codes

Heatline F28

The F28 code on a Heatline boiler signals an ignition lockout. After three unsuccessful attempts to establish a stable flame during the startup sequence, the boiler gives up and shuts itself down. You'll see F28 on the display and the boiler won't fire again until the fault is resolved and the unit is reset. Heatline is part of the Vaillant Group, so the F28 code behaves identically to the equivalent Vaillant ignition fault — causes, diagnostics, and repairs are consistent across both brands.

Causes, fixes & costs for Heatline F28 → · All Heatline codes

Protherm F.28

The F.28 code appears on Protherm boilers — including the Gepard and Panther ranges — when the burner fails to light on its first attempt. The boiler tries to ignite, detects no flame, and shuts itself down as a safety precaution. After several unsuccessful attempts the unit enters a full lockout, cutting off heating and hot water entirely. You may hear repeated clicking as the boiler tries and fails to spark. Because Protherm is part of the Vaillant Group and shares much of its engineering platform, F.28 carries the same ignition-failure meaning across Protherm, Vaillant ecoTEC, and Glow-Worm boilers.

Causes, fixes & costs for Protherm F.28 → · All Protherm codes

Saunier Duval F28

The F28 code means your Saunier Duval boiler has tried to light the burner three times in a row and failed every time. Rather than keep trying indefinitely — which could allow unburned gas to build up in the combustion chamber — the boiler locks itself out and displays F28 as a safety measure. The fault can stem from anything on the gas supply side, the ignition components, or the control electronics, and it affects all Saunier Duval models in the same way.

Causes, fixes & costs for Saunier Duval F28 → · All Saunier Duval codes

Vaillant F.28

F.28 is an ignition lockout fault. Your Vaillant boiler ran through its normal start-up sequence but failed to detect a stable flame during the ignition phase, so it has shut down as a safety precaution. The boiler will not attempt to fire again until the underlying cause is dealt with and the boiler is reset. This is the appliance's built-in protection working correctly — the lockout stops repeated unsafe attempts to light the burner. F.28 covers a wide range of possible causes, from something as trivial as a prepayment meter running out of credit or a frozen condensate pipe, right through to a defective gas valve or, very occasionally, a failed circuit board. It is closely related to F.29: F.28 means no flame was ever established at ignition, whereas F.29 means a flame was detected but then lost during operation.

Causes, fixes & costs for Vaillant F.28 → · All Vaillant codes

Frequently asked questions

Is the F28 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant the F28 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F28 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a F28 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.