The boiler lit correctly but the flame went out while it was running, so it has locked out.
Most common on: ecoTEC plus, ecoTEC pro
F.29 means the boiler successfully lit but then lost the flame during operation — the flame went out after the burner had established. The control board detects the loss and shuts the gas valve for safety.
It is closely related to F.28: F.28 is a failure to light in the first place, while F.29 is a flame that goes out once running. Both usually stem from the gas supply, the ignition/flame-sensing electrode or the gas valve, and a recurring F.29 needs a Gas Safe engineer.
Check other gas appliances light steadily, the boiler gas cock is open and any prepayment meter has credit.
In freezing weather, thaw the external condensate pipe if it is blocked with ice, then reset.
Press reset and see whether the boiler stays lit. If F.29 returns, do not keep resetting.
A flame that keeps dropping out needs a Gas Safe engineer to test gas pressure, the electrode and the gas valve.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
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The boiler lit correctly but the flame went out while it was running, so it has locked out. F.29 means the boiler successfully lit but then lost the flame during operation — the flame went out after the burner had established. The control board detects the loss and shuts the gas valve for safety.
You can try a single reset and the safe checks below, but Vaillant F.29 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring F.29 means the underlying cause has not been fixed — most often interrupted or fluctuating gas supply, or low gas pressure. Resetting repeatedly will not solve it; get the boiler diagnosed properly.