Glow-worm F84 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F84 fault code mean?
The F84 code appears when the boiler's control board detects a persistent, irreconcilable difference between the readings from the heating flow NTC thermistor and the heating return NTC thermistor. Under normal operation, these two sensors work in tandem — the flow thermistor monitors water leaving the heat exchanger, while the return thermistor reads the cooler water coming back from the radiators. When the gap between their readings is permanently outside acceptable limits, the boiler concludes something is wrong with the sensing arrangement itself and locks out to prevent unsafe operation. This typically points to a faulty, swapped, or disconnected thermistor, damaged wiring between the sensors and the PCB, or — less commonly — a circulation problem that is preventing normal heat distribution around the system.
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
- Faulty heating flow NTC thermistor Common
The flow thermistor measures resistance to infer water temperature; a healthy sensor reads approximately 12 kOhms at 20°C. If the component has drifted out of spec or failed entirely, it will report a temperature that is permanently inconsistent with what the return sensor is seeing, triggering F84.
- Faulty heating return NTC thermistor Common
The return thermistor is subject to the same failure modes as the flow sensor. Corrosion, thermal fatigue, or physical damage can cause it to report incorrect values, creating an apparent permanent temperature difference even when the boiler is circulating water normally.
- Damaged or loose wiring between sensors and PCB Common
The thin wiring loom connecting each thermistor to the PCB can suffer from chafing, heat damage, or simply a loose plug-in connector. A poor connection may look fine visually but cause intermittent or permanently incorrect resistance readings at the board, mimicking a sensor failure.
- Flow and return thermistors fitted to the wrong pipes Sometimes
If a previous repair or installation involved disturbing the sensors, there is a possibility they were clipped back onto the wrong pipes — flow sensor on the return pipe and vice versa. The board then sees a temperature pattern that is the inverse of what it expects, and F84 is the result.
- Circulation problem reducing normal temperature differential Sometimes
A seized or incorrectly set pump, a significant airlock, or a blockage within the system can prevent water from circulating freely. This causes abnormal temperature readings across the two sensors that the PCB interprets as a sensor fault, even though the sensors themselves may be fine.
- Failed PCB Rare
In rare cases the fault lies not with the sensors or wiring but with the PCB's ability to interpret the sensor signals correctly. This is a last-resort diagnosis reached only after sensors and wiring have been checked and confirmed good.
How to fix it
- Check that your gas supply is working DIY safe
Before assuming a component fault, confirm other gas appliances in your home (hob, gas fire) are functioning normally. If your gas supply has been interrupted, contact your gas supplier rather than an engineer.
- Reset the boiler once or twice DIY safe
On most Glow-Worm models, turn the central heating control knob to the reset position (marked with a power symbol), hold for a moment, then release and set to your desired temperature. Wait a few minutes to see if the boiler fires successfully. If F84 returns immediately or after a short run, do not continue resetting — repeated lockouts indicate a genuine fault that needs investigation.
- Visually check the thermistor clips on the heating pipes DIY safe
With the boiler off, take a look at the two NTC thermistors — small cylindrical sensors clipped directly onto the heating flow and return pipework, usually near the boiler's heat exchanger. Check that both are firmly clipped against the pipe and have not fallen away or become dislodged. Do not attempt to unplug, swap, or test them yourself; simply note whether they appear securely fitted and report this to your engineer.
- Arrange for a Gas Safe engineer to test both NTC thermistors with a multimeter Gas Safe engineer
A qualified engineer will check the resistance of each thermistor at a known pipe temperature and compare the reading against the manufacturer's specification curve. This quickly confirms whether one or both sensors has failed and needs replacing.
- Have the engineer inspect all wiring between the thermistors and PCB Gas Safe engineer
The engineer should trace the full length of the wiring harness for each sensor, checking for signs of chafing, heat damage, or corrosion at connector plugs. A damaged loom or corroded terminal can produce the same fault code as a dead thermistor and is often cheaper to repair.
- Ask the engineer to confirm sensors are fitted to the correct pipes Gas Safe engineer
The engineer should verify that the flow thermistor is clipped to the flow pipe and the return thermistor to the return pipe. If a previous repair left them transposed, swapping them back resolves F84 immediately without any parts being needed.
- Have the engineer assess system circulation Gas Safe engineer
If sensors and wiring both check out, the engineer should test pump operation and check for airlocks or blockages that could be causing abnormal temperature differentials across the heat exchanger. A pump running at the wrong speed setting or partially seized can generate misleading sensor data.
- PCB diagnosis if all other checks are clear Gas Safe engineer
As a last resort, the engineer will assess whether the PCB itself is misreading the sensor signals. PCB replacement is expensive, so a competent engineer will exhaust all other possibilities before recommending this route. On an older boiler, a PCB quote is also a good point at which to consider whether replacement of the whole appliance is more economical.
- Call a Gas Safe registered engineer if the fault persists after your visual checks Gas Safe engineer
F84 cannot be resolved by a homeowner beyond the basic checks above. All thermistor testing, wiring inspection, and component replacement must be carried out by a Gas Safe registered engineer. You can find one at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- Heating flow NTC thermistor (Glow-Worm compatible) · from £18
- Heating return NTC thermistor (Glow-Worm compatible) · from £18
- Thermistor wiring harness · from £35
- Central heating pump (replacement) · from £65
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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Glow-Worm F84 myself?
The only things you can safely do yourself are confirming your gas supply is on, giving the boiler one or two resets, and visually checking that the thermistor clips are sitting firmly against the heating pipes. All actual diagnosis and repair — testing sensor resistance, inspecting wiring, replacing components — must be carried out by a Gas Safe registered engineer. Thermistors are inexpensive parts, but misdiagnosis or an incorrectly fitted sensor can cause further damage or unsafe operation.
How much does it cost to fix F84 on a Glow-Worm boiler?
For most households, the repair will cost between £150 and £320 including parts and labour. A single NTC thermistor typically costs under £20 as a part, so if the fault is a straightforward sensor failure the bill is largely for the engineer's time. If the wiring harness needs attention the cost is similar. A PCB replacement, which is only needed in a minority of F84 cases, can add significantly more — expect £250–£450 for the board alone — and at that point it is worth comparing the repair quote against the cost of a new boiler, particularly if your appliance is over ten years old.
Why does my Glow-Worm boiler show F84 even though it was working fine yesterday?
NTC thermistors and their wiring can fail gradually or quite suddenly. Temperature cycling over years of use can cause the sensor's resistance element to drift out of specification, or a connector to gradually loosen until one cold start finally tips it into fault territory. It can also happen if the boiler was recently serviced and a sensor was not clipped back properly. The fact that it appeared suddenly does not make it any more or less serious — it still needs an engineer to investigate.
Is F84 the same as F72 on Glow-Worm boilers?
They are closely related but distinct codes. F72 flags an abnormally high temperature reading specifically from the return sensor, suggesting that sensor is reading far higher than it should. F84 flags a persistent temperature difference between the flow and return sensors taken together, suggesting the two sensors are out of step with each other — whether due to a faulty sensor, swapped sensors, wiring issues, or a circulation problem. An engineer will diagnose which is actually present and whether the underlying cause is the same component.