Quick answer: F28 means your Vaillant boiler tried to light but didn't detect a flame, so it locked out for safety. Check your gas supply, any prepayment meter credit and, in freezing weather, the condensate pipe. Reset once; if it comes back, you need a Gas Safe registered engineer.
Smell gas? Stop. Don't touch electrical switches or use naked flames. Open doors and windows, turn the gas off at the meter if you can do it safely, leave the property and call the National Gas Emergency Service on 0800 111 999.
What F28 means
Vaillant describes F28 as the boiler not detecting a flame during start-up. In plain English, it tried to light and failed, so it locked itself out for safety.
A closely related code, F29, means the flame went out while the boiler was already running. The checks below apply to both.
Vaillant lists a frozen condensate pipe as one possible cause, so check that first if it's been below freezing.
Safe things to check first
These checks don't involve opening the boiler. Work through them in order.
- Is gas reaching your home? Try your gas hob, if you have one. If that won't light either, the problem is your gas supply, not the boiler.
- Prepayment meter? Check it has credit, and that the emergency credit hasn't run out.
- Is the gas turned on at the meter? The lever should run in line with the pipe. If it has been turned off and you didn't do it, call your gas supplier before turning it back on.
- Is it below freezing outside? A frozen condensate pipe is one of the most common causes of this fault in winter. How to spot and thaw a frozen condensate pipe.
- Reset once. Use the reset or restart button exactly as your boiler's user manual describes. If the fault comes straight back, stop. Resetting over and over won't fix it.
When to call an engineer
Call a Gas Safe registered engineer if your gas supply is fine, the condensate pipe isn't frozen and the fault returns after one reset. Ignition faults often come down to a part inside the boiler, such as the ignition electrode, the gas valve, the fan or the circuit board, and finding out which one needs an engineer.
Never take the casing off your boiler or try to repair gas parts yourself. That work must be done by a Gas Safe registered engineer. With boiler cover, your provider sends one for you. Compare boiler cover.
Have boiler cover?
Call your provider before paying an engineer yourself. A repair like this is exactly what cover is for.
No cover yet? A new plan won't pay for a fault you already have, because problems that exist when you join aren't covered and there's a waiting period at the start. It can protect you next time.
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Last updated 30 October 2026