Glow-worm fault codes are an F followed by a number, and each one tells you which part of the boiler has stopped it running. The two you’ll see most are F22, which means the water pressure is too low, and F28, which means the boiler tried to light and couldn’t. Only F22, F28 and F29 have checks you can safely make yourself. Every other code needs a Gas Safe registered engineer.
We’re Clever Energy Boilers: Gas Safe registered (no. 554138), accredited Worcester Bosch installers and based in Bingley, fitting boilers across Yorkshire, the North West and the Midlands. We’ve completed more than 25,000 installations in the last 15 years, so Glow-worm codes are a familiar sight. Below is every current code, a lookup for the old codes on earlier models, and the codes Glow-worm shares with Vaillant. An older Glow-worm that keeps showing faults is usually better replaced with a new boiler, for lower running costs and an end to the repair bills.
What do Glow-worm boiler fault codes mean?
Each code points to the area of the boiler that has stopped it. One code can have more than one cause, though, which is why an engineer tests the parts before replacing anything.
Glow-worm has been part of Vaillant Group since 2001 and still makes its boilers in Belper, Derbyshire. That shared ownership is the reason Glow-worm uses the same F code system as Vaillant, and why several numbers mean the same thing on both brands.
Which Glow-worm boiler do you have?
The current Glow-worm ranges are the Energy, Ultimate and Easicom. Older boilers still running in homes around Bradford, Leeds and Keighley include the Flexicom, Ultracom, Betacom, the cxi, sxi and hxi models, and the Xtrafast.
The list below comes from Glow-worm’s trade fault-finding lists for the Betacom A and Energy ranges. Older ranges used some different numbers, so if your code isn’t listed, check the manual for your model.
What are the Glow-worm boiler fault codes?
This is the full list, with who should deal with each code:
| Glow-worm code | What it means | Who can deal with it |
|---|---|---|
| F00 / F10 | Flow sensor fault | Gas Safe registered engineer |
| F01 / F11 | Return sensor fault | Gas Safe registered engineer |
| F20 | Overheat | Gas Safe registered engineer |
| F22 | Low water pressure | You: top up to 1 to 1.5 bar, then reset |
| F23 / F24 | Circulation fault: temperature difference too big, or temperature rising too fast | Gas Safe registered engineer |
| F27 | Flame detection fault | Gas Safe registered engineer |
| F28 | Ignition failure | You: check gas credit and the condensate pipe, then reset once |
| F29 | Flame loss | You: check gas credit and the condensate pipe, then reset once |
| F32 | Fan or air pressure fault | Gas Safe registered engineer |
| F49 | eBUS voltage fault | Gas Safe registered engineer |
| F61 to F65, F67 | Main board (PCB) fault | Gas Safe registered engineer |
| F68 | Unstable flame | Gas Safe registered engineer |
| F70 | Invalid product code or incompatible user interface | Gas Safe registered engineer |
| F71 | Flow sensor fault | Gas Safe registered engineer |
| F72 / F84 | Difference between the flow and return sensors | Gas Safe registered engineer |
| F73 / F74 | Pressure sensor fault | Gas Safe registered engineer |
| F75 | Pressure sensor fault, or no pressure rise when the pump starts | Gas Safe registered engineer |
| F76 | Thermal fuse | Gas Safe registered engineer |
| F83 | No water, or too little temperature rise | Gas Safe registered engineer |
| F85 | Sensor fault | Gas Safe registered engineer |
| Err | User interface fault | Gas Safe registered engineer |
F22: Glow-worm low pressure fault
The water pressure in your heating system has dropped too low for the boiler to run. Top it back up to 1 to 1.5 bar with the filling loop, then reset. Our guide on how to repressurise a boiler takes you through it, and if you’re not sure what the gauge should read, see what your boiler pressure should be.
F28 and F29: Glow-worm ignition and flame faults
F28 means the boiler tried to light and couldn’t. F29 means it lit, then lost the flame. Glow-worm’s advice for both is the same: make sure a prepayment meter has credit, thaw the condensate pipe if it’s frozen, then reset. If that doesn’t clear it, it’s a job for an engineer. Our Glow-worm F28 fault code guide goes into the causes.
F75: Glow-worm pressure sensor fault
The boiler didn’t see the pressure rise it expected when the pump started, or the pressure sensor itself has failed. It isn’t one to fix yourself. Our Glow-worm F75 fault code guide explains what an engineer will look at.
F61 to F65 and F67: Glow-worm main board faults
These point to the main circuit board. A new board typically costs £250 to £380 or more, and on an older boiler that’s often where replacing the whole boiler starts to make more sense. Our guide to boiler PCB faults covers what goes wrong with them.
What do the old Glow-worm fault codes mean?
On an older Glow-worm, the number on the display may not match the list above. Glow-worm says its older ranges used F9 for low pressure and F1 for ignition. This is how they line up with today’s codes:
| Old Glow-worm code | Meaning on older ranges | Current equivalent |
|---|---|---|
| F9 | Low water pressure | F22 |
| F1 | Ignition fault | F28 (ignition failure) or F29 (flame loss) |
The safe checks don’t change. Top up the pressure for an old F9. For an old F1, check the gas supply and the condensate pipe before one reset.
Are Glow-worm and Vaillant fault codes the same?
Six of them are. We’ve checked F20, F22, F28, F29, F75 and F84 against Vaillant’s own fault code list and the meanings match, so our Vaillant guides apply to your Glow-worm as well:
| Code | Meaning on both brands | Our Vaillant guide |
|---|---|---|
| F20 | Overheat | Vaillant F20 fault code |
| F22 | Low water pressure | Vaillant F22 fault code |
| F28 | Ignition failure | Vaillant F28 fault code |
| F29 | Flame loss | Vaillant F29 error code |
| F75 | Pressure sensor fault or no pressure rise at pump start | Vaillant F75 fault code |
| F84 | Flow and return sensor difference | Vaillant F84 fault code |
For any code outside those six, stick with the Glow-worm table. Our Vaillant boiler error codes guide has the rest of the Vaillant list, and if you’re weighing up the two brands, our Glow-worm vs Vaillant comparison sets out the differences.
Is a Glow-worm fault code dangerous?
The code itself isn’t. It means the boiler has spotted a problem and stopped, which is exactly what it’s meant to do.
A smell of gas is different. If you notice one, open the windows, don’t touch any switches, turn the gas off at the meter and call the National Gas Emergency Service on 0800 111 999. Don’t wait to see whether the code clears.
What can I safely check at home when my Glow-worm shows a fault code?
Glow-worm’s own homeowner advice is short. Repressurise for F22. For F28 or F29, top up your prepayment gas or thaw a frozen condensate pipe. Then reset. For any other code, Glow-worm says to contact an installer and not attempt a fix. None of these checks involve opening the boiler:
- Is the pressure low? With the boiler cold, the gauge should read 1 to 1.5 bar. If it’s lower, switch off, let it cool and top up with the filling loop, then close both valves firmly.
- Is the gas getting through? On a prepayment meter, check there’s credit. If the hob won’t light either, it’s the supply, not the boiler.
- Has the condensate pipe frozen? In a hard frost, pour warm water over the plastic pipe outside, never boiling. Pipes on exposed walls freeze first, so if you’re up on the Pennine hillsides around Queensbury or Thornton, check it before anything else. Our guide on how to defrost your condensate pipe shows where to look.
- Reset once. Then watch whether the boiler fires and stays on.
When should I stop resetting a Glow-worm fault and get a Gas Safe engineer?
If the code comes back after one reset, stop pressing the button. Get a Gas Safe registered engineer out if:
- The code returns straight after a reset
- It’s anything other than F22, F28 or F29
- Pressure drops again within a few days of a top-up
- Water is dripping from the boiler or the pipes underneath it
Sensor, fan, pump, gas valve and board work is for a Gas Safe registered engineer only. Never take the casing off yourself.
Why do Glow-worm fault codes keep coming back?
A code that clears and returns usually means a part is on its way out, or a slow leak is draining the system. An F22 that’s back within days means water is escaping somewhere, and our guide to a boiler losing pressure explains where to look. A reset buys you a few days at most. On an older Glow-worm, one call-out tends to lead to another.
Is a Glow-worm fault code a sign you need a new boiler?
On a young boiler, often not. On a legacy Glow-worm, it often is. These are typical UK repair costs for the main parts behind Glow-worm codes:
| Part | Typical UK repair cost |
|---|---|
| Diverter valve | £170 to £240 |
| Pump | £190 to £260 |
| Fan | £200 to £280 |
| Gas valve | £200 to £280 |
| PCB (main board) | £250 to £380 or more |
| Heat exchanger | £500 to £900 |
One repair on a younger boiler is usually worth doing. Boilers over 8 years old become noticeably less efficient, though, and one repair tends to lead to the next. On a Flexicom, Ultracom or Betacom, repeat F28 or F75 codes or a main board fault are the cue to replace rather than repair. A heat exchanger fault at up to £900 takes you a long way towards the price of a new boiler.
Our guide on how to know when your boiler needs replacing lists the signs, and our new boiler cost guide shows what to budget. If you’d rather spread the cost, there are boiler finance options too.
Replace your old Glow-worm with Clever Energy
We fit Worcester Bosch, Ideal, Vaillant and Baxi boilers in Bingley, Bradford, Leeds, Keighley, Huddersfield, Halifax and across Yorkshire, the North West and the Midlands, from Manchester and Liverpool down to Birmingham and Leicester. Every installation is carried out by Gas Safe registered engineers, and a long, market-leading warranty is available through Clever Energy. Have a look at our boiler installation service, get a fixed price online in a few minutes, or request a callback. Once it’s in, a Clever Care plan keeps your new boiler protected. If you’d rather talk it through, call 0330 055 2210 between 8am and 8pm on weekdays or 10am to 4pm at weekends.
Glow-worm boiler fault code FAQs
Is Glow-worm the same as Vaillant?
Not quite, but they’re closely linked. Glow-worm has been part of Vaillant Group since 2001, both use F codes, and F20, F22, F28, F29, F75 and F84 mean the same thing on both.
How do I reset a Glow-worm boiler?
Once, with the reset button, after checking the pressure, the gas supply and the condensate pipe. Your user manual shows where the button sits on your model. If the code returns, stop and get a Gas Safe registered engineer.
Why does my Glow-worm keep losing pressure (F22)?
Water is escaping from the system or the boiler. Topping up gets you going again, but a leak needs an engineer to find it.
What’s the difference between F28 and F29 on a Glow-worm?
Timing. F28 means the boiler couldn’t light at all, while F29 means it lit and then lost the flame. Both share the same safe checks: gas credit, the condensate pipe and one reset.
Are Glow-worm boilers still made?
Yes. Glow-worm still makes boilers in Belper, Derbyshire, and its current ranges are the Energy, Ultimate and Easicom. The Flexicom, Ultracom and Betacom are legacy ranges.
More Glow-worm and Vaillant error codes
Our detailed Glow-worm guides cover the Glow-worm F75 fault code and the Glow-worm F28 fault code. Because the numbers overlap, our Vaillant guides to F20, F22, F28, F29, F75 and F84 help too, and the full list is on our Vaillant boiler error codes guide.
Got a different brand? See our Potterton boiler fault codes, Alpha boiler fault codes and Worcester boiler error codes guides, or read our Glow-worm boilers review for more on the brand.