F29 Boiler Fault Code: What It Means on Every Brand

F29 reads the same on all five brands that share Vaillant Group electronics — Vaillant, Glow-worm, Saunier Duval, Heatline and Protherm: the burner lit but the flame then went out, and the boiler locked out. Where F28 is 'would not light', F29 is 'lit then lost it', which points more toward gas-supply pressure dips, flue problems, a struggling flame-sensing electrode, or wind affecting the flue terminal. Gas supply and condensate pipe are the homeowner checks; the rest is engineer work.

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 F29 means	Severity	DIY?
GWGlow-worm	The F29 code on a Glow-Worm boiler means the burner managed to light but the flame then cut out unexpectedly while the boiler was running. Full guide →	Lockout — engineer likely	Some checks
HHeatline	The F29 code appears on Heatline boilers when the appliance cannot establish or maintain a stable flame during an ignition attempt. Full guide →	Lockout — engineer likely	No
PProthermshown as F.29	The F.29 code appears on Protherm boilers when the appliance detects that a flame has been lost — or could not be established — during the burner sequence. Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F29 code indicates that the boiler's burner successfully ignited but the flame then went out unexpectedly during operation. Full guide →	Lockout — engineer likely	Some checks
VVaillantshown as F.29	F.29 is a flame-failure lockout. Full guide →	Lockout — engineer likely	No

F29 brand by brand

Glow-worm F29

The F29 code on a Glow-Worm boiler means the burner managed to light but the flame then cut out unexpectedly while the boiler was running. The boiler's flame-sensing circuit detected the loss and triggered an automatic safety lockout to prevent unburned gas from building up. It is worth distinguishing F29 from the closely related F28 code: F28 means the boiler failed to establish a flame in the first place, whereas F29 means ignition succeeded but the flame could not be maintained. Both codes point broadly to gas supply or ignition issues, but F29 specifically indicates the problem occurs after the initial spark. The boiler will stop producing heat and hot water until the underlying cause is resolved and the unit is reset.

Causes, fixes & costs for Glow-worm F29 → · All Glow-worm codes

Heatline F29

The F29 code appears on Heatline boilers when the appliance cannot establish or maintain a stable flame during an ignition attempt. The boiler's control system detects that combustion has either failed to start or has cut out shortly after lighting, and immediately shuts the boiler down as a safety lockout. Heatline is part of the Vaillant Group, so the F29 code shares the same core logic as the equivalent Vaillant flame-failure code. The lockout itself is not dangerous — it is the boiler doing exactly what it should when something prevents reliable combustion. However, the underlying cause does need to be found and resolved, because repeated flame failures can in some cases indicate a gas or combustion issue that a Gas Safe engineer should assess.

Causes, fixes & costs for Heatline F29 → · All Heatline codes

Protherm F.29

The F.29 code appears on Protherm boilers when the appliance detects that a flame has been lost — or could not be established — during the burner sequence. In plain terms, your boiler attempted to ignite but either failed to light at all, or the flame went out shortly after ignition and the built-in safety system shut everything down to prevent gas from accumulating. The result is a complete lockout: no heating and no hot water until the underlying cause is resolved. Because Protherm boilers share the same core platform as Vaillant and Glow-worm models, the F.29 definition is consistent across all three brands.

Causes, fixes & costs for Protherm F.29 → · All Protherm codes

Saunier Duval F29

The F29 code indicates that the boiler's burner successfully ignited but the flame then went out unexpectedly during operation. As soon as the control board detects this loss of flame signal, it triggers an immediate safety lockout — cutting all heating and hot water — to prevent unburned gas from accumulating. This is distinct from the F28 code, which relates to a failure to ignite in the first place. You'll typically notice the boiler attempting to start, briefly firing, then shutting itself down, often repeating this cycle before locking out completely.

Causes, fixes & costs for Saunier Duval F29 → · All Saunier Duval codes

Vaillant F.29

F.29 is a flame-failure lockout. It means your Vaillant boiler successfully lit but then lost the flame while running, or couldn't keep it stable. When the flame-detection system no longer senses a flame, the boiler shuts down and locks out as a safety measure. This prevents gas from being released without combustion and protects both the appliance and your home. F.29 is closely related to F.28 (failure to ignite at start-up), but F.29 specifically refers to the flame going out after it was established.

Causes, fixes & costs for Vaillant F.29 → · All Vaillant codes

Frequently asked questions

Is the F29 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant the F29 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 F29 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 F29 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.