This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Glow-worm F.27 fault codewhat it means and how to fix it
The boiler is sensing a flame when there should not be one. Glow-worm list moisture on the electronics, a failed flame monitor or a leaking gas valve. Not a reset job.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Glow-worm F.27
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Anything you can smell or hear?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
A smell of gas, however faint
Treat the leaking-valve cause as true.
Turn the gas off at the meter if you can reach it safely, leave the property and call 0800 111 999.
Water dripping inside the casing, or on the floor under the boiler
Moisture on the electronics is Glow-worm's first cause. Switch the boiler off at the wall so the board is not sitting live and wet.
Book a Gas Safe registered engineer; the leak and the board both need attention.
Nothing to smell, see or hear
Most likely the flame monitor circuit or a damp electrode lead.
Leave the boiler off and book an engineer. Do not reset it.
What F.27 means
Glow-worm print F.27 as "Safety shutdown: Fault in flame detection", and the Compact manual gives it the plainer name "flame simulation": the ionisation circuit is reporting a flame at a moment the board knows the gas valve is closed. That can only be one of two things. Either the sensing is lying — Glow-worm's first two causes are "moisture on the electronics" and "electronics (flame monitor) defective" — or the flame is real, which is their third cause, "gas solenoid valve leaking".
The boiler cannot tell which, so it treats the worst case as true and shuts down, and this page does the same: there are no householder steps for F.27 on any Glow-worm range, and the Compact and Energy7 user guides hand every code except F.22, F.28 and F.83 to a competent person. The Ultracom2, Betacom 24a and Betacom2 installation manuals carry F27 as "flame detection fault — abnormal flame detection" and tell the engineer to check the flame detection electrode, the main board and the igniter unit, which is the same three-way split.
On the Ultracom cxi and the CI-era boilers the number was F16, "flame detection fault". Resetting a boiler that thinks it has a flame with the valve shut is exactly the wrong instinct; if the valve really is passing, a reset asks the board to open it further.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Damp on the board or the electrode lead | Glow-worm's first cause. A leaking pump, a weeping heat exchanger joint or condensation dripping onto the PCB tracks the ionisation input to earth and the board reads a flame. The leak that caused it is the real fault. | Common |
| Flame monitor circuit on the PCB failed | The ionisation amplifier drifts and reports a signal with no flame. Glow-worm's wording is "electronics (flame monitor) defective", and it means a board replacement after the engineer has proved the electrode and lead are clean. | Occasional |
| Gas valve letting gas through when closed | The dangerous one and the reason for the severity here: a valve that does not seal leaves a small flame burning or gas accumulating. Glow-worm print it as "gas solenoid valve leaking" and the Compact manual as "not leak-tight". | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£120–£450
Time on site
1h 30m
Leak fixed and board dried £120–£250. PCB £250–£450. Gas valve £250–£400.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Glow-worm ignition and flame-sensing electrode
£18–£45Authorised Glow-worm parts distributor
Price checked 28 September 2026
Glow-worm gas valve assembly
£90–£200Authorised Glow-worm parts distributor
Price checked 28 September 2026
Glow-worm main PCB
£150–£320Authorised Glow-worm parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Glow-worm fault codes
- F.28Get it checkedThree ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Related: ignition and gas supply
- F1Get it checkedOn Ultracom, Flexicom and Ultrapower boilers F1 is an ignition lockout after five failed attempts, and Glow-worm's guides let you reset it. On Energy and Ultimate boilers F.01 is the return sensor instead.Related: ignition and gas supply
- F.29Get it checkedThe burner lit and then the flame went out while running. Glow-worm blame the gas supply, flue gas recirculating, earthing, the spark transformer and, on newer manuals, a blocked condensate pipe.Related: ignition and gas supply
- F.61Get it checkedThe board cannot drive the gas valve properly — a short in its wiring, coils shorted to earth, or the electronics. The valve stays shut until an engineer has traced it.Related: ignition and gas supply
- F.68Get it checkedThe flame is burning but its signal is unstable. Glow-worm list air in the gas, low gas pressure, a wrong combustion setting or injector, a poor electrode connection or a blocked condensate pipe.Related: ignition and gas supply
- F.62Stop — call an engineerThe gas valve did not shut off when told to — the flame signal lingered after the close command. Glow-worm list a delayed or leaking valve or the electronics. Switch off; call an engineer.Related: ignition and gas supply
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.27
Why is this danger when F.28 is caution?
F.28 is a boiler that could not light and closed the gas. F.27 is a boiler that may have gas or flame where it has not asked for any. One is an inconvenience; the other is the safety circuit finding something it cannot explain.
Can I switch it off at the wall?
Yes, and if there is any water near the electronics you should. Switching off does not close the gas valve any further, so the smell check still stands.
What will it cost?
A damp board dried and its leak fixed, £120–£250. A new PCB £250–£450. A gas valve £250–£400. The engineer will not know which until the electrode and lead have been checked, so ask for the diagnosis before the part.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.