Glossary · Faults, servicing and gas safety
What does the F28 fault code mean?
F28 is the fault code a Vaillant or Glow-worm boiler shows when it tries to light and no flame signal is detected. The causes include no gas reaching the boiler, a frozen condensate pipe, a faulty electrode or circuit board or a flue problem. Some checks are safe for householders. The rest is for a Gas Safe engineer.
Key points
- Vaillant's official wording for F.28 is that the flame signal was not detected during the ignition phase. Glow-worm words it as the boiler failing to ignite.
- Causes listed include low gas pressure or an empty prepayment meter, a frozen condensate pipe, a faulty electrode or circuit board and a flue issue.
- Vaillant says you can thaw a frozen condensate pipe with warm water. It warns against using boiling water from a kettle.
- Beyond the gas supply and condensate checks, Vaillant and Glow-worm say F.28 is a case for an engineer, and not to open the boiler casing.
- If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.
What causes an F28 fault
A gas boiler lights its burner and then checks for a flame. On a Vaillant boiler, F.28 means the flame signal was not detected during the ignition phase.[1]Vaillant, Boiler fault codes Glow-worm lists the same code as the boiler having failed to ignite.[3]Glow-worm, Boiler fault codes Vaillant says the boiler has tried to light several times without success.[2]Vaillant, Understanding an F28 fault code on a Vaillant boiler
Both makers list the same possible causes.[1]Vaillant, Boiler fault codes[3]Glow-worm, Boiler fault codes
| Possible cause | What it means |
|---|---|
| Not enough gas reaching the boiler | For example low gas pressure, or not enough credit on a prepayment meter |
| Blocked condensate pipe | Condensate has backed up in the combustion chamber |
| Faulty ignition electrode | A part inside the boiler |
| Faulty circuit board (PCB) | A part inside the boiler |
| A flue system issue | The flue may be affecting the boiler |
Vaillant explains the frozen-pipe cause. A condensing boiler produces waste water, and the condensate pipe often ends outside, where it can freeze in cold weather. If it freezes, the boiler will most likely fail to start.[2]Vaillant, Understanding an F28 fault code on a Vaillant boiler Vaillant adds that pipes are more likely to freeze if the heating has not been used during a spell of freezing weather, such as when you are away on holiday. It advises keeping the boiler active on a timer.[2]Vaillant, Understanding an F28 fault code on a Vaillant boiler
What you can check yourself
These are the checks Vaillant and Glow-worm describe.[1]Vaillant, Boiler fault codes[3]Glow-worm, Boiler fault codes
- Check whether other gas appliances in your home are working. If not, there may be a problem with the gas supply.
- If you have a prepayment gas meter, check you have enough credit and top up if needed.
- In cold weather, check for ice in any outdoor condensate pipe.
Vaillant’s guide to thawing a frozen pipe says the pipe is usually a white plastic pipe leaving the wall near the boiler, running to a drain or waste pipe. A bubbling or gurgling sound from the boiler can be a sign. To thaw it, pour warm water along the affected part of the pipe from a watering can or jug until the ice falls away or the pipe drains. Do not use boiling water from a kettle, which can damage or crack the pipe. Then restart the boiler as your manual describes.[2]Vaillant, Understanding an F28 fault code on a Vaillant boiler Insulating the pipe reduces the chance of it freezing again.[2]Vaillant, Understanding an F28 fault code on a Vaillant boiler
When to call an engineer
If the gas is on, other appliances work, and the pipe is not frozen, Vaillant says the boiler has tried to light without success and it is time to call a professional.[2]Vaillant, Understanding an F28 fault code on a Vaillant boiler Outside the checks above, both makers say F.28 is a case for their engineer and that you should not try to solve it yourself, because opening the boiler casing can be dangerous.[1]Vaillant, Boiler fault codes[3]Glow-worm, Boiler fault codes
Work on a gas appliance, including its ignition parts, gas valve and flue, must be done by a Gas Safe registered engineer.[4]Health and Safety Executive, Domestic gas: frequently asked questions A boiler repair visit can test the electrode, the circuit board and the flue. If you smell gas at any point, leave the property and call the National Gas Emergency Service on 0800 111 999.[1]Vaillant, Boiler fault codes Vaillant and Glow-worm both say they reserve the right to charge a call-out fee for non-product faults.[1]Vaillant, Boiler fault codes[3]Glow-worm, Boiler fault codes
Example
Gurpreet in Bradford wakes to a cold house in a hard frost. The Vaillant boiler shows F.28. The hob works, so the gas is on, and she is on a credit meter. She finds the white pipe outside, which is frozen. She pours warm water from a watering can along it, not boiling water, until the ice clears, then restarts the boiler as her manual says. It fires up. She asks her engineer to insulate the pipe.
If the boiler had not restarted, she would have booked an engineer rather than touch the boiler itself.
What people get wrong
The common mistake is assuming F.28 means a broken boiler part. Vaillant lists a frozen condensate pipe and an empty prepayment meter among the causes, and both are quick to check. The reverse mistake is to keep resetting a boiler that has a genuine ignition fault.
The second mistake is reaching for a kettle. Vaillant says boiling water can damage the pipe and leave you without a working boiler for longer.[2]Vaillant, Understanding an F28 fault code on a Vaillant boiler
The bottom line
F28 means the boiler did not detect a flame when it tried to light. Check the gas supply, any prepayment credit and, in frosty weather, the outdoor condensate pipe, using warm water only. If that does not clear it, book a Gas Safe registered engineer and keep the casing shut.
Common questions
What does F28 mean on a Vaillant boiler?
The flame signal was not detected during the ignition phase. The boiler tried to light several times without success. A frozen condensate pipe is one cause Vaillant lists, and it says you can fix that yourself.
Can I fix an F28 fault myself?
Only some of it. Vaillant says to check other gas appliances work, top up prepayment meter credit and thaw a frozen condensate pipe. Outside those, it is a case for an engineer, and you should not open the casing.
How do I thaw a frozen condensate pipe?
Vaillant says to pour warm water along the affected area of the pipe, using a watering can or jug, until the ice clears. Do not use boiling water from a kettle, which can crack the pipe. Then restart the boiler as your manual describes.
Does F28 mean the same thing on every boiler?
No. Vaillant and Glow-worm both use F.28 for a failed ignition. Baxi uses E133 and Worcester Bosch uses EA. Check your own manufacturer's list.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Vaillant. Boiler fault codes. Checked 11 November 2026. ↩
- Vaillant. Understanding an F28 fault code on a Vaillant boiler. Checked 11 November 2026. ↩
- Glow-worm. Boiler fault codes. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩