Vaillant F.28 Fault Code: Ignition Failure Explained
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.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Vaillant F.28 fault code mean?
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.
Common causes
- No or low gas supply Common
Air in the gas supply, low gas pressure, or simply no gas reaching the boiler. A prepayment meter out of credit, a closed gas isolation valve, or works on the gas network can all cause this. It is one of the most common reasons for F.28.
- Frozen or blocked condensate pipe Common
In freezing weather the condensate pipe — especially where it runs outside — can freeze and back up, preventing the boiler from firing. This is a very common winter cause and is one of the few things a homeowner can safely remedy.
- Faulty ignition electrode or wiring Sometimes
A worn, dirty or misaligned ignition/flame-sensing electrode, or damaged wiring to it, can prevent the boiler from creating or detecting a spark and flame.
- Flue or air intake fault Sometimes
A blocked, restricted or incorrectly fitted flue can cause flue-gas recirculation and stop a stable flame forming, triggering the lockout.
- Defective gas valve Sometimes
If the gas valve assembly is not opening correctly or is failing, the burner cannot receive the right amount of gas to establish a flame. This must be diagnosed and replaced by an engineer.
- Faulty PCB (circuit board) Rare
A failed control board can misread or fail to manage the ignition sequence. This is a diagnosis of last resort, confirmed only after every other component has been tested and ruled out.
How to fix it
- Check your gas supply is on and you have credit DIY safe
Try another gas appliance such as a hob or gas fire. If nothing works, the problem is likely your gas supply, meter or prepayment credit rather than the boiler itself. Check the gas isolation valve under the boiler is in the 'on' position (handle in line with the pipe).
- Check for a frozen condensate pipe in cold weather DIY safe
Locate the condensate pipe (usually a plastic pipe running outside to a drain). If it is frozen, you can thaw it by pouring warm — not boiling — water along its length, or holding a wrapped hot water bottle against it. Once thawed, reset the boiler.
- Reset the boiler once DIY safe
Press and hold the reset button as described in your Vaillant user guide. Allow the boiler to attempt ignition. If F.28 returns, do not keep resetting — repeated resets will not fix the underlying fault and can mask a genuine problem.
- Inspect and test the ignition electrode and wiring Gas Safe engineer
A Gas Safe engineer will check the ignition/flame-sensing electrode for wear, fouling and correct positioning, and inspect the associated wiring. This requires removing boiler casing and must not be attempted by a homeowner.
- Test gas pressures and the gas valve Gas Safe engineer
An engineer will measure the gas inlet and working pressures and check the gas valve operation. Any gas pressure or valve work is gas work and is legally restricted to Gas Safe registered engineers.
- Check the flue and air intake Gas Safe engineer
The engineer will verify the flue is unobstructed, correctly installed and not causing flue-gas recirculation, as this can prevent a stable flame.
- Call a Gas Safe registered engineer DIY safe
If the gas supply is fine, the condensate pipe is clear and a single reset has not cleared the fault, book a Gas Safe registered engineer. They will diagnose ignition, gas valve, flue or — as a last resort — PCB issues. It is both unsafe and illegal to carry out gas repairs yourself.
Parts you may need
- Ignition / flame-sensing electrode · from £35
- Ignition lead / wiring harness · from £25
- Gas valve assembly · from £180
- PCB (control board) · from £260
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£250, depending on the underlying cause.
Frequently asked questions
Can I fix a Vaillant F.28 fault myself?
Only the safe basics: check your gas supply and prepayment credit, thaw a frozen condensate pipe in cold weather, and try a single reset. Anything beyond that — ignition electrodes, gas pressure, the gas valve, flue or PCB — must be handled by a Gas Safe registered engineer. Gas work by anyone unqualified is unsafe and illegal.
Why does my Vaillant boiler keep showing F.28 after resetting?
A reset only clears the lockout; it does not fix the cause. If F.28 returns, there is an unresolved problem such as low gas pressure, a faulty ignition electrode, a flue issue or a failing gas valve. Stop resetting after two or three attempts and call a Gas Safe engineer to diagnose it properly.
How much does it cost to fix a Vaillant F.28 fault?
Most people pay between £100 and £250 for typical repairs like replacing an ignition electrode or repairing wiring, including labour. Simple fixes such as thawing a condensate pipe may cost little or nothing. A gas valve replacement can run £250–£400, and in the rare case a PCB is genuinely at fault, expect £400–£600 — though a reputable engineer will only suggest that after ruling everything else out.
What is the difference between F.28 and F.29 on a Vaillant boiler?
Both are ignition-related. F.28 means the boiler never established a flame during ignition, while F.29 means a flame was detected at start-up but then went out during operation. They often share causes such as gas supply problems, but the stage at which the flame is lost differs.