The short answer
F4 on a Glow-worm points to an ignition or flame detection fault: the boiler did not establish or could not prove a flame. Glow-worm lists F4 as an ignition fault on its older CI Plus range, and newer Glow-worm models report ignition and flame problems under other codes such as F28 and F29. The usual causes are gas supply, a blocked condensate pipe, a worn ignition electrode, a poor earth or a failing gas valve. Check your gas and condensate pipe, reset once, and call an engineer if it returns. London repair costs run from £90 to £400.
What the law requires
What does F4 mean on a Glow-worm?
Glow-worm uses different fault codes across its ranges, and the same number can mean different things on different models. That is the first thing to know before acting on any code you find online.
On Glow-worm’s own fault information, F4 is listed as an ignition fault on the CI Plus range, with the same description as F1. In practice that means the boiler tried to light and did not prove a flame. On newer Glow-worm models, which report ignition failure and flame loss as F28 and F29, an F4 display is uncommon and the meaning may differ.
So check the model name on the front panel or the data plate inside the door. If it is a CI Plus or a similar older model, this guide applies directly. If it is a newer model, the manufacturer’s manual for that model is the source of truth, and an engineer can confirm quickly.
Why would a Glow-worm fail to detect a flame?
Gas supply.
Prepayment credit, a half-closed isolation valve, a faulty meter regulator or low pressure. If other gas appliances work normally, the supply is probably fine.
A blocked or frozen condensate pipe.
Backed-up condensate can stop ignition or snuff the flame. F4 that appears on cold mornings and clears later is the signature.
A worn or dirty ignition electrode.
The spark gap widens or the tip erodes with age. This is the most common cause on older boilers.
A poor earth connection.
Flame sensing depends on a clean earth. A corroded bond causes the board to miss a flame that is actually there.
A failing gas valve or control board.
Less common and more expensive. The valve does not open correctly, or the board no longer senses the flame properly.
A flue or air supply problem.
A flue or air supply problem.
What you can safely check yourself
None of this involves opening the case.
- Check the gas. Light a hob ring. Check meter credit and the isolation valve.
- Check the condensate pipe. If it is frozen, pour warm (not boiling) water along it.
- Look at the flue terminal for debris or obstruction.
- Reset once. Press the reset button. On some Glow-worm models the code stays on the screen after a reset until an engineer clears it, so do not be alarmed if it persists.
- Stop there. If it returns, book a repair. Repeated resets can wear the electrode and put stress on the gas valve.
Half the house cold? Describe which radiators and where they are cold, and we will usually name the fix on the phone.
What an engineer does
The engineer reads the model and the code against the right fault table first, because the code means different things on different Glow-worm ranges. Then gas pressure is checked at full rate, the electrode is inspected and its gap measured, the condensate route and flue are checked, and the earth is tested. The gas valve and board are the last things investigated, with a fixed quote before any part is fitted.
How much does a Glow-worm F4 repair cost in London?
Fixed price quoted after diagnosis.
- Diagnosis, reset and minor adjustment: £90 to £140.
- Electrode and lead replacement: £110 to £220 fitted.
- Condensate trap clean or reroute: £110 to £220.
- Flue repair: £120 to £400 depending on access.
- Gas valve replacement: £220 to £420 fitted.
- Control board replacement: £300 to £520 fitted.
On a Glow-worm older than twelve years, a gas valve or board failure is a point where a replacement boiler is a fair comparison. Our repair or replace guide shows how engineers make that call.
Is F4 dangerous?
No. The boiler shut off because it could not confirm a flame, which is the safe response. Do not tamper with the boiler, do not open the case, and do not keep resetting. If you smell gas, leave the property and call the National Gas Emergency line on 0800 111 999.
Glow-worm F4, quick answers
What does F4 mean on a Glow-worm?
On older models such as the CI Plus it is an ignition fault. Newer models use other codes, so confirm your model.
Is F4 the same as F28?
Not necessarily. F28 is the ignition failure code on newer models. Check your model and ask an engineer if unsure.
Why does F4 happen on cold mornings?
A frozen condensate pipe is the first suspect.
Can I fix it myself?
You can check the gas and the condensate pipe and reset once. Anything else is Gas Safe work.
Is my Glow-worm worth repairing?
For an electrode or condensate fix, yes. For a valve or board on a boiler past twelve years, run the half-cost rule.