Vaillant F.28 and F.29 fault codes — ignition failure or flame loss
⚠️ This needs a qualified professional
This fault involves gas or mains electrical work. Don't attempt a DIY repair — UK law requires gas work to be carried out by a Gas Safe registered engineer. Below is what to check and describe to them, not step-by-step repair instructions.
Vaillant’s own official guidance confirms F.28 means the boiler attempted to ignite but no flame signal was detected, while F.29 means it lit briefly and then the flame went out. Both lock the boiler out for safety after repeated failed attempts.
What’s safe to check yourself
- Check another gas appliance in the house (a hob burner, a gas fire) still lights — if nothing gas-powered works anywhere, it’s a supply or meter issue, not the boiler.
- In cold weather, check the condensate pipe (white plastic pipe, usually exiting to an outside drain) for visible ice — this is one of the most frequent causes of F.28/F.29 in winter. If safely reachable, pour warm (not boiling) water over the frozen section.
- Try resetting the boiler once per your manual’s exact procedure. Modern Vaillant boilers lock out after repeated resets — if the code returns immediately, stop resetting and book an engineer instead.
⚠️ This needs a Gas Safe engineer
Beyond the condensate and gas-supply checks above, the likely causes — a faulty gas valve, a failed ignition electrode, or a wiring fault — all involve gas or safety-critical components and must be diagnosed by a Gas Safe registered engineer. If you ever smell gas, don’t reset the boiler; turn off the supply if safe to do so and call the National Gas Emergency Service on 0800 111 999.