Glossary · Faults, servicing and gas safety
What do Glow-worm boiler fault codes mean?
Glow-worm fault codes are the F-numbers, such as F.22 or F.28, that a Glow-worm boiler shows when it detects a problem. Each points to an area such as water pressure, ignition or a sensor. Glow-worm publishes the meanings and says most need an engineer, and that you should not open the boiler casing.
Key points
- F.22 is low water pressure. Glow-worm says that under 1 bar you can top up to between 1 and 1.5 bar using the filling loop, if you are competent to do so.
- F.28 means the boiler has failed to ignite. Glow-worm lists no gas, a frozen condensate pipe and an electrode or circuit board fault among the causes.
- Glow-worm says not to open the boiler casing, because it can be dangerous. Most codes on its list say to call an engineer.
- If you smell gas, Glow-worm says to leave the property and call the National Gas Emergency Service on 0800 111 999.
- Glow-worm says resetting can clear a fault or get the boiler working again for a while. A fault that returns needs an engineer.
How Glow-worm fault codes work
When a Glow-worm boiler detects a problem, it shows a code beginning with F, followed by a number. Glow-worm publishes a list of what each code means, its possible causes and what to do next. Its page says that a fault code is not always something to worry about. It could be something you can do yourself, or it may need a Glow-worm installer or engineer.[1]Glow-worm, Boiler fault codes
On resetting, the page says that resetting a boiler can resolve a fault or get it working again for a while. It points you to your user manual for the correct reset method.[1]Glow-worm, Boiler fault codes It also strongly advises you not to gain access inside your boiler, because this can be dangerous. If you smell gas, vacate the property immediately and call the National Gas Emergency Service on 0800 111 999.[1]Glow-worm, Boiler fault codes
By law, work on a gas appliance in a home must be done by a Gas Safe registered engineer.[2]Health and Safety Executive, Domestic gas: frequently asked questions
Common codes and what Glow-worm says
The table is taken from Glow-worm’s list. It covers some of the codes and is not complete. If your code is not shown, Glow-worm’s page gives a phone number for further help.[1]Glow-worm, Boiler fault codes
| Code | Glow-worm’s description | Who deals with it |
|---|---|---|
| F.0 | Flow temperature sensor interrupted | Glow-worm engineer |
| F.1 | Return temperature sensor interrupted | Glow-worm engineer |
| F.20 | Safety thermal cut out has operated and shut the boiler off | Installer in the first instance |
| F.22 | Low water pressure in the system | You, if under 1 bar and competent. Engineer if over 1 bar |
| F.23 | Temperature difference between flow and return pipe is too great | Installer in the first instance |
| F.27 | A flame signal detected while the burner is off | An expert. Do not attempt it yourself |
| F.28 | The boiler has failed to ignite | Checks below, then an engineer |
| F.29 | The flame has gone out and the boiler has failed to reignite | Checks below, then an engineer |
| F.32 | Fan fault | Glow-worm engineer |
| F.75 | Pressure change too low when the heating pump is started up | You, if under 1 bar and competent. Engineer if over 1 bar |
| F.88 | Electrical connection to the gas valve interrupted | Glow-worm engineer |
What you can check yourself
Glow-worm describes three checks, and these are the only ones to attempt.[1]Glow-worm, Boiler fault codes
Low pressure (F.22 and F.75). The normal range is 1 to 1.5 bar. If your gauge shows under 1 bar, Glow-worm says this can be solved without an engineer if you are competent to do so. Find your filling loop in the manual and slowly raise the pressure to between 1 and 1.5 bar. Glow-worm says its guidance is used at your own risk. If the pressure is already above 1 bar, it says to contact a Glow-worm engineer.[1]Glow-worm, Boiler fault codes
Failed ignition (F.28 and F.29). Check whether other gas appliances in your home are working. If you have a prepayment gas meter, check there is credit. In cold weather, check for ice in any outdoor condensate pipe. Beyond these, Glow-worm says it is a case for an engineer.[1]Glow-worm, Boiler fault codes
Reset. Try a reset as your manual describes.[1]Glow-worm, Boiler fault codes If the same code returns, do not keep resetting.
When to call an engineer
For most codes, the answer from Glow-worm is to contact its customer service, or your installer, for a visit. Glow-worm says it will attend any fault code but reserves the right to charge a call-out fee for non-product faults.[1]Glow-worm, Boiler fault codes A fault with the sensors, fan, gas valve or circuit board involves the inside of the boiler. Do not open the casing. Boiler repair by a Gas Safe registered engineer is the safe route.
Example
Chloe in Pudsey has a Glow-worm combi that shows F.22 after she bleeds a radiator. The pressure gauge reads 0.6 bar. She follows the steps in her user manual to open the filling loop slowly until the gauge reads between 1 and 1.5 bar, then closes it. The code clears. Two weeks later it returns. She stops topping up and books an engineer, because a code that keeps returning needs diagnosing. Our entry on low water pressure covers the wider issue.
What people get wrong
It is easy to read a code as a diagnosis of a broken part. F.28 says the boiler failed to ignite. It does not say the ignition electrode is faulty. Glow-worm lists no gas, a blocked condensate pipe, the electrode, the circuit board and the flue among the possible causes.
The other mistake is treating a fix that worked once as a permanent answer. Repeated codes point to an underlying cause.
The bottom line
Glow-worm fault codes give you a starting point. Use Glow-worm’s own list for your boiler, do only the pressure, gas supply, frozen pipe and reset checks it describes, and keep the casing closed. For anything else, or if a code keeps returning, book a Gas Safe registered engineer.
Common questions
What does F.22 mean on a Glow-worm boiler?
Low water pressure in the system. Glow-worm says the normal range is 1 to 1.5 bar. Under 1 bar, you can repressurise using the filling loop if you are competent. Above 1 bar, it says to contact a Glow-worm engineer.
What does F.28 mean on a Glow-worm boiler?
The boiler has failed to ignite. Check that other gas appliances work, that a prepayment meter has credit and for ice in an outdoor condensate pipe. Beyond that, Glow-worm says it is a case for an engineer.
Can I fix a Glow-worm fault code myself?
Only the checks Glow-worm describes. It says not to open the casing because it can be dangerous. Gas work must be done by a Gas Safe registered engineer.
Will Glow-worm charge for a call-out?
Glow-worm says it will attend any fault code but reserves the right to charge a call-out fee for non-product faults. Check the terms of your guarantee or plan.
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.
- Glow-worm. Boiler fault codes. Checked 11 November 2026. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩