Glow-worm F.25 fault codewhat it means and how to fix it
On current ranges F.25 is the flue gas safety cut-out — a break in its plug or harness. On an Ultracom, Flexicom or cxi, F25 is a circulation fault. Different generations, different fault.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.25
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.
Which display are you looking at?
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.
F.25 with a stop
Flue gas safety cut-out: a broken plug or wire, or the device has tripped on a hot flue.
Book a Gas Safe registered engineer. Do not reset repeatedly.
F25 on an Ultracom hxi, a Flexicom or a cxi
Glow-worm's "circulation fault" or "poor circulation" — pump, valve or air.
Check the pressure reads at least 1 bar and that radiator valves are open, reset once, then book an engineer if it returns.
What F.25 means
F.25 is another code whose meaning depends on the age of the board. On the Energy, Ultimate, Easicom and Compact it is "Safety shutdown: flue gas temperature too high", but read the causes before assuming the flue is hot: "break in plug connection for optional flue gas safety cut-out (SCO), break in cable harness". The flue gas safety cut-out is an optional device, and on a boiler that has one, a broken plug or wire is indistinguishable to the board from the device having tripped. The Compact manual gives the same two causes and two measures — check the plug, check the harness.
Where the cut-out has actually operated, the flue gases were hotter than Glow-worm allow, which points at the heat exchanger or combustion and is emphatically an engineer's finding. The two-digit Ultracom2 and Betacom tables have no 25.
The older Ultracom hxi and Flexicom list F25 as "circulation fault" and the cxi as "poor circulation" — a pump, a valve or air, nothing to do with the flue — so an owner of one of those boilers is reading the wrong page unless they also read F.23 and F.24. No Glow-worm user guide gives the householder anything to do for F25 on any range.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flue gas safety cut-out plug or harness broken — current ranges | Both of Glow-worm's causes. The optional cut-out sits in the flue connection and its plug is easy to disturb; a break reads exactly as a trip. | Common |
| Circulation fault — Ultracom, Flexicom and cxi | On the F1-to-F77 ranges F25 is water not moving: a pump, a stuck valve or air in the boiler. The checks are those on the F.23 and F.24 pages, not the flue. | Common |
| Cut-out has genuinely operated | Flue gas hotter than the device allows. A heat exchanger that is scaled or partly blocked on the water side gives up less heat to the water and more to the flue; combustion set wrongly does the same. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
1 hour
A plug or harness repaired £80–£150. A cut-out replaced £120–£200. A heat exchanger problem behind a genuine trip is a separate and larger estimate.
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.
Central heating circulating pump
CH-PUMP-15-60£90–£180Plumbing merchant
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.32Get it checkedF.32 is a fan fault — the fan speed is outside what the board will accept, or the fan cannot be seen at all. Older ranges show F2 or F3. An engineer's code throughout.Related: fan and flue
- F.77Get it checkedF.77 on current ranges is the flue non-return flap or condensate pump not answering. On a Betacom2 or Ultracom2 it is the gas valve motor; on an Ultracom or Flexicom the optional condensate pump.Related: fan and flue
- F.33Get it checkedF.33 is an air supply fault on the two ranges that carry it: a Compact's flue losing too much pressure, or a Betacom3's air pressure switch not proving. Not in the Energy or Ultimate tables.Related: fan and flue
- 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: same brand
- F.22Get it checkedGlow-worm's low-water-pressure shutdown, and the one code every Glow-worm user guide lets you clear yourself by topping the system up. The threshold differs by range.Related: same brand
- F.75Get it checkedGlow-worm's own documents give this code three different cause lists — pressure switch only, or sensor, pump or lack of water — and the fullest one is the Compact's eight-way diagnostic.Related: same brand
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
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.