Glow-worm F76 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F76 fault code mean?
The F76 code appears when the thermal fuse built into the primary heat exchanger has tripped. This fuse sits at the top of the heat exchanger and acts as an emergency cut-out — when the heat exchanger reaches a dangerously high temperature, the fuse opens and the boiler locks out to protect itself from damage. The code can also appear if the wiring connecting that fuse to the PCB is broken or has a poor connection, meaning the boiler may not actually be overheating but is receiving a false signal indicating it is. Either way, the boiler will not restart until the underlying problem is diagnosed and resolved by a qualified engineer.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Poor circulation due to a failing or seized pump Common
If the central heating pump is not moving water around the system efficiently, heat builds up rapidly inside the primary heat exchanger and the thermal fuse trips. A pump running at reduced speed, making unusual noises, or seized entirely is one of the most frequent triggers for F76.
- Trapped air in the heat exchanger or system Common
Air locks within the heat exchanger pipework or elsewhere in the heating circuit prevent water from flowing freely. Without proper water flow to carry heat away, temperatures inside the exchanger climb until the thermal fuse responds.
- Low system pressure causing dry running Common
If system pressure has dropped significantly, the boiler can run with insufficient water in the heat exchanger. Operating 'dry' causes rapid overheating. This is sometimes compounded by a faulty pressure sensor that fails to alert the boiler's controls to the low-water condition.
- Physical blockage or sludge in the system Sometimes
A build-up of magnetite sludge or scale in the heat exchanger or pipework restricts water flow in a similar way to trapped air, leading to heat stress on the exchanger. A system that has never been power-flushed or chemically cleaned is particularly susceptible.
- Faulty or open-circuit thermal fuse wiring Sometimes
The cable or connector linking the thermal fuse to the PCB can develop a break or a loose connection. The PCB then reads the fuse as having tripped even when the heat exchanger temperature is normal. This is an electrical fault rather than a genuine overheating event, but it produces an identical lockout.
- Defective temperature sensor Sometimes
A temperature sensor sending inaccurate readings to the PCB can cause the boiler's control system to misinterpret normal operating temperatures as dangerously high, triggering the F76 lockout as a precaution.
- Primary heat exchanger failure Rare
In some cases — particularly on older boilers or those that have experienced repeated overheating — the heat exchanger itself becomes damaged or cracked. This is the most serious diagnosis and may make replacement of the entire boiler the most cost-effective option.
How to fix it
- Do not attempt repeated resets DIY safe
The F76 is a safety lockout. Attempting to reset the boiler multiple times without identifying the cause risks further damage to the heat exchanger. Try one reset using the boiler's reset button and observe whether it locks out again immediately — if it does, leave it and move to the next steps.
- Check system pressure on the boiler gauge DIY safe
Look at the pressure gauge on the front of the boiler. For most Glow-Worm combi and system boilers, normal cold pressure is between 1.0 and 1.5 bar. If the needle is below 0.5 bar, the system is too low and may have been running dry. Top up carefully via the filling loop until the gauge reads around 1.2 bar, then attempt one reset. If pressure drops again quickly, there is likely a leak or a faulty pressure relief valve — call an engineer.
- Check that your gas supply is live DIY safe
Confirm that other gas appliances in the property (cooker, gas fire) are working normally. If they are not, contact your gas supplier, as a wider supply interruption can sometimes cause erratic boiler behaviour before a lockout.
- Do not attempt to access internal boiler components Gas Safe engineer
The thermal fuse, pump, heat exchanger, wiring harness, and PCB are all internal components that must only be inspected and worked on by a Gas Safe registered engineer. Removing the boiler casing and touching internal parts is both dangerous and illegal for an unqualified person.
- Call a Gas Safe registered engineer for diagnosis Gas Safe engineer
An engineer will use specialist diagnostic equipment to determine whether the fault is a genuine overheating event (pump failure, air lock, sludge, or heat exchanger damage) or an electrical fault with the thermal fuse wiring or PCB. They will carry out any necessary repairs — which may include a power flush, pump replacement, wiring repair, or in serious cases, heat exchanger or full boiler replacement. Do not delay: repeated overheating events cause cumulative damage to the heat exchanger.
Parts you may need
- Circulating pump · from £85
- Thermal fuse / thermal cut-out · from £25
- Temperature sensor (NTC) · from £20
- Wiring harness · from £60
- Primary heat exchanger · from £350
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£450, depending on the underlying cause.
Frequently asked questions
Can I reset my Glow-Worm boiler to clear the F76 code myself?
You can try a single reset, but it is unlikely to clear the fault permanently because F76 is caused by a physical problem — either genuine overheating or a wiring fault — that does not resolve itself. If the boiler locks out again immediately after resetting, do not keep trying; arrange a visit from a Gas Safe engineer instead.
Does F76 always mean I need a new boiler?
Not necessarily. Many F76 faults are resolved by replacing the circulating pump, clearing an air lock, performing a power flush to remove sludge, or repairing a faulty wiring connection to the thermal fuse. These repairs typically cost between £150 and £450. A new heat exchanger is a more expensive outcome (typically £500–£900 including labour) and a full boiler replacement more expensive still, but those scenarios arise mainly on older boilers with significant internal damage. An engineer's diagnosis will clarify which applies to your boiler.
What is the thermal fuse in a Glow-Worm boiler and why does it trip?
The thermal fuse is a one-time safety device fitted to the top of the primary heat exchanger. When the exchanger overheats beyond a set threshold, the fuse permanently opens the circuit and cuts power to trigger the lockout. Unlike a resettable thermostat, a thermal fuse that has genuinely tripped due to overheating cannot be reset — it must be replaced, and the root cause of the overheating (poor circulation, low water, sludge) must be identified and fixed at the same time to prevent it from happening again.
Could regular boiler servicing have prevented the F76 fault?
In many cases, yes. An annual service by a Gas Safe engineer gives the opportunity to check pump performance, test system pressure, inspect for early signs of sludge build-up, and verify that sensors and wiring are in good condition. Catching a weakening pump or a pressure issue early is far less costly than dealing with a damaged heat exchanger later. If your boiler has not been serviced recently, it is worth booking one as part of the repair visit.