What the F28 code means
Each time your Glow-worm needs to heat water, it runs through a short start-up routine: the fan clears the combustion chamber, the gas valve opens and an electrode sparks to light the gas. A sensor then checks that a flame has actually formed. If it does not see one, the boiler tries again. When those attempts fail, it closes the gas valve and shows F.28.
Glow-worm's installation manual for the Energy range calls F28 a failed ignition when starting up, and the Betacom 4 operating manual explains that it appears after three unsuccessful ignition attempts in a row. Either way, the boiler is refusing to keep releasing unlit gas, which is exactly the right thing to do. The code exists on Energy, Ultimate, Betacom and many older Glow-worm ranges, and it matches the ignition code on Vaillant boilers, which we cover on our Vaillant F28 page.
Common causes of F28
No gas reaching the boiler
This is the first thing to rule out. A prepayment meter that has run out of credit, an emergency credit limit reached, a gas isolation valve that has been knocked closed, or work on the gas supply in your street can all stop gas getting through. Glow-worm's manual also lists low gas pressure and a defective meter or tripped gas pressure regulator.
Air in the gas pipe
After a new meter is fitted or the gas has been off, air can sit in the pipework. The boiler may need a few attempts before gas reaches the burner, and it can lock out in the meantime.
A frozen condensate pipe
In a hard frost, the plastic condensate pipe that runs outside can freeze. Water then backs up inside the boiler and it will not light. Installer sources list F28 as one of the codes Glow-worm boilers commonly show when this happens.
Ignition or flame detection faults
The manual also lists faults in the ignition system (the transformer, lead, plug or electrode), an interrupted flame detection signal, poor earthing, a gas valve fault or faulty electronics. These are all engineer jobs.
| Cause | What you might notice | Who fixes it |
|---|---|---|
| Prepayment credit run out | Gas hob will not light either | You, by topping up |
| Gas isolation valve closed | Handle under the boiler sits across the pipe | You, by turning it back in line |
| Frozen condensate pipe | Freezing weather, gurgling from the boiler | You, by thawing it with warm water |
| Air in the gas line | New meter or recent gas work | Often clears after a reset, otherwise an engineer |
| Electrode, lead or gas valve | Gas supply fine, F28 keeps returning | Gas Safe engineer |
Safe checks you can do yourself
- Try another gas appliance, such as the hob. If that will not light either, the problem is with the gas supply rather than the boiler.
- Check the gas meter. On a prepayment meter, make sure there is credit and that you have not used up emergency credit.
- Find the gas isolation valve on the pipe under the boiler. The handle should run in line with the pipe. If it is across the pipe, turn it back in line.
- In freezing weather, find where the condensate pipe leaves the house. If it looks frozen, thaw it by pouring warm (not boiling) water along it or holding a hot water bottle against it. Our frozen condensate pipe guide shows how.
- Reset the boiler following your manual. On the Energy7, press the reset button for about a second and the boiler will make a fresh attempt to light.
- Stop after two or three resets. Glow-worm's own manuals advise contacting a competent person if the fault continues.
If you smell gas, do not reset the boiler or use electrical switches. Turn off the gas at the meter if it is safe to do so, open doors and windows and call the National Gas Emergency Service on 0800 111 999.
When to call a Gas Safe engineer
If your gas supply is fine, the condensate pipe is clear and F28 still keeps coming back, the problem is inside the boiler. Book a Gas Safe registered engineer, and let them know which checks you have already done. They will check the gas pressure at the boiler, then inspect the ignition electrode and lead, the flame detection circuit, the gas valve and the condensate trap.
If F28 only happens now and then, note when it occurs, for example first thing in the morning or during very cold weather. That pattern can help an engineer narrow things down quickly.
Typical repair costs in the UK
Many F28 faults cost nothing to fix because the cause is the gas supply or a frozen pipe. If a part is needed, these are typical ranges.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Replacing the ignition electrode | Typically £90 to £180 |
| Replacing the ignition lead or spark generator | Typically £100 to £200 |
| Replacing the gas valve | Typically £200 to £400 |
| Replacing the main PCB | Typically £250 to £500 |
How to stop F28 coming back
If you use a prepayment meter, keep an eye on the balance, especially in winter when the boiler uses more gas. Ask your installer whether the outside section of the condensate pipe could be insulated, rerouted or upgraded to a wider pipe to make freezing less likely. A yearly service lets an engineer clean and check the electrode, burner and condensate trap, which are common sources of ignition trouble. Our boiler service cost guide explains what a service typically covers. If your boiler lights but then cuts out, see our Glow-worm F29 page instead.