Glow-worm F.76 fault codewhat it means and how to fix it
The thermal fuse on the primary heat exchanger has operated or failed. It exists to catch a dry fire the sensors missed, and Glow-worm's test for it is not one an owner performs.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Glow-worm F.76
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.
What F.76 means
F.76 is a code most Glow-worm manuals do not print, and the ones that do describe a last-line safety device. The Energy7 manual calls it "the safety cut-out in the primary heat exchanger is defective — safety cut-out feedback does not match the gas valve assembly feedback"; the Compact manual, "Fault: Thermal cut-out — defective thermal fuse"; and the Betacom 24a manual, which uses the two-digit F76, "thermal fuse fault — thermal fuse disconnected, defective thermal fuse", adding in its commissioning section that the fuse "protects the appliance against dry fire". It is a one-shot device fixed to the heat exchanger body.
If the metal reaches a temperature the flow sensor should have stopped long before, the fuse opens, the gas valve loses its permission, and the boiler shows F.76 for good — a blown fuse cannot be reset. The Compact manual's procedure for the engineer explains why this page is danger rather than caution: "check the heat exchanger for leakages. If the heat exchanger has no leaks, bypass the thermal fuse. If you can then start the product, replace the thermal fuse."
Bypassing a safety fuse to test a boiler is a competent person's diagnostic step under controlled conditions and nothing else; the Betacom 24a manual's remedy is blunter still, "replace the heat exchanger", because on that boiler the fuse is part of it. A fuse that has genuinely operated means the heat exchanger has already been much too hot once, and the reason — no water, a dead pump, a sensor off its pipe — has to be found before the boiler runs again. The Ultimate3, Easicom3, Betacom3 and Betacom4 tables do not list F.76, and the Ultracom, Flexicom and Ultrapower lists have no 76 at all.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Heat exchanger overheated and the fuse opened | The fuse did its job. Something let the exchanger run dry or stagnant — low pressure, a stuck pump, a sensor reading the wrong pipe — and the sensors did not catch it first. That cause has to be found as well as the fuse. | Common |
| Fuse or its plug disconnected | The Betacom 24a manual's "thermal fuse disconnected" and its instruction to check the fuse's connections. A plug left off after a heat exchanger clean produces F.76 with nothing overheated. | Occasional |
| Heat exchanger leaking | The first thing the Compact manual tells the engineer to check. A weeping exchanger can wet the fuse or its wiring, and a fuse replaced on a leaking exchanger will go again. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£150–£800
Time on site
2 hours
Thermal fuse and cause £150–£350. Heat exchanger £450–£800.
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 flow or return temperature sensor (clip-on NTC)
£12–£35Authorised 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.20Get it checkedThe safety thermal cut-out has tripped and stopped the boiler. Glow-worm's homeowner page lets you check pressure, radiator valves and airlocks first; on Ultracom2 boilers F20 trips at 97 °C.Related: overheat and circulation
- F.83Get it checkedThe burner lit but neither sensor registered any warming — Glow-worm call it dry fire. Low pressure or a sensor off its pipe; the Energy7 guide allows two resets before a 45-minute lockout.Related: overheat and circulation
- F.24Get it checkedThe temperature at the sensor climbed faster than water flowing properly would allow. Glow-worm list the pump, air, low pressure and a stuck non-return valve, and send it to an engineer.Related: overheat and circulation
- F.23Get it checkedThe flow and return temperatures are too far apart, which means water is barely moving through the boiler. Glow-worm point at the pump, air, or the two sensors swapped over.Related: overheat and circulation
- F.86Get it checkedThe external safety contact — usually an underfloor heating limit thermostat wired to the boiler — is open, or its link is missing. The boiler will not fire until the contact closes.Related: overheat and circulation
- 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
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.76
Can it be reset?
No. A thermal fuse that has operated is open circuit for good, and the board will show F.76 until it is replaced. Pressing reset repeatedly changes nothing.
Why danger, when the boiler is stopped?
Because the diagnostic Glow-worm print involves bypassing a safety device, and because a fuse that has blown means the heat exchanger has already been overheated once. Neither is territory for anyone but a Gas Safe registered engineer.
What will it cost?
A fuse and the fault behind it, £150–£350. If the heat exchanger has to be replaced, as the Betacom 24a manual requires, £450–£800 — at which point on an older boiler the conversation may turn to replacement.
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.