Glow-worm F81 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F81 fault code mean?
The F.83 code — which on some Glow-Worm displays can be misread as F81 due to the small digits — indicates a 'dry fire' condition. When the burner ignites, the boiler expects to see a measurable rise in temperature at the flow or return sensor within a set period. If that temperature change is too small or absent entirely, the boiler concludes that either there is insufficient water circulating through the system, or the temperature sensors are not reading correctly. As a safety measure, the boiler locks out to prevent overheating or damage to internal components. If your display is definitely showing F81 and not F.83, check your model-specific manual or call Glow-Worm technical support directly, as F81 does not appear in standard published references for this brand.
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
- Low system pressure Common
If the water pressure in the heating system has dropped below the minimum threshold — typically below 1 bar — there may not be enough water present for the boiler to register a proper temperature rise after ignition. This is the most common trigger for F.83 and is straightforward to address by topping up via the filling loop.
- Air trapped in the system or boiler Common
Air locks in the heat exchanger or pipework can prevent water from circulating properly. Without adequate flow, the temperature sensors may not detect the expected rise even when the burner is firing normally.
- Faulty or incorrectly positioned NTC thermistors Sometimes
The flow and return temperature sensors (NTC thermistors) must be firmly seated against the pipework and making good thermal contact. If either sensor has worked loose, its wiring has become corroded or disconnected, or the sensor itself has drifted out of calibration, it will report an inaccurate temperature and trigger the F.83 lockout.
- Scaled or restricted heat exchanger Sometimes
A build-up of limescale inside the heat exchanger — more common in hard-water areas — reduces the transfer of heat into the water. The burner fires but the water temperature does not rise as quickly as the boiler expects, mimicking a dry-fire condition.
- PCB or internal communication fault Rare
In a small number of cases the printed circuit board fails to correctly process the signals from the temperature sensors, producing a false F.83 even when the sensors and water circuit are functioning normally.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, the low water level is very likely causing the fault.
- Top up system pressure using the filling loop DIY safe
Locate the filling loop — usually a braided silver or grey hose with one or two valves beneath the boiler. Slowly open the valve(s) until the gauge reaches approximately 1.2–1.5 bar, then close the valve(s) securely. Do not overfill beyond 2 bar.
- Bleed the radiators to remove trapped air DIY safe
Using a radiator bleed key, open each radiator bleed valve slightly until any hissing air escapes and a steady trickle of water appears, then close the valve. Work upstairs first, then downstairs. After bleeding, re-check and top up the system pressure if it has dropped again.
- Reset the boiler DIY safe
Once pressure is correct and radiators have been bled, press the reset button (consult your boiler manual for its exact location — typically a flame symbol button held for 3 seconds). Allow the boiler to complete its ignition sequence. If the fault clears, monitor the boiler over the next 24 hours. Do not reset more than two or three times in total — repeated resets without a fix can mask a deeper problem.
- Check that the flow and return sensor leads are connected Gas Safe engineer
A Gas Safe engineer should inspect the NTC thermistors on the flow and return pipes, confirming they are clipped firmly against the pipe and that the wiring back to the PCB shows no signs of damage, corrosion, or loose terminals. Faulty sensors are replaced at this stage.
- Test and if necessary descale or replace the heat exchanger Gas Safe engineer
An engineer will assess whether limescale restriction is reducing heat transfer. A chemical flush or descale may restore performance; in more severe cases the heat exchanger will need replacing.
- Inspect and test the PCB Gas Safe engineer
If all sensors and water-circuit components test correctly, the engineer will assess whether the PCB is misreading sensor signals. PCB replacement is a last resort but may be necessary if all other causes are ruled out.
- Call a Gas Safe registered engineer Gas Safe engineer
If the fault returns after resetting, or if you are unable to identify the cause through the homeowner checks above, contact a Gas Safe registered engineer. They have the tools and authority to safely diagnose and repair gas-side and electrical components. You can find registered engineers at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- NTC flow thermistor · from £25
- NTC return thermistor · from £25
- Thermistor wiring harness · from £35
- PCB (printed circuit board) · from £220
- Heat exchanger · from £180
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 £90–£300, depending on the underlying cause.
Frequently asked questions
Is F81 a real Glow-Worm fault code?
F81 does not appear in any published Glow-Worm fault code documentation. If your boiler is displaying what looks like F81, look again carefully — on small LCD displays the code F.83 is very easily misread as F81. F.83 is a well-documented Glow-Worm fault relating to the boiler detecting insufficient temperature rise after ignition. If after a second look the code is definitely not F.83, check your boiler's model handbook or call Glow-Worm technical support, as some older or region-specific models can carry codes not listed in general guides.
Can I fix a Glow-Worm F.83 fault myself?
Possibly, yes — if low pressure is the cause. Check the pressure gauge: if it reads below 1 bar, top up via the filling loop to around 1.2–1.5 bar, bleed any air from the radiators, and reset the boiler. This resolves a significant proportion of F.83 faults without any engineer involvement. However, if the fault reappears after resetting, or if pressure was already correct, the underlying cause is likely a faulty sensor, wiring issue, or internal component — all of which require a Gas Safe registered engineer.
How much does it cost to repair a Glow-Worm F.83 fault in the UK?
Most people with this fault pay somewhere between £90 and £300 all-in, depending on whether the fix is as simple as a pressure top-up and service call (lower end) or involves replacing one or both NTC thermistors with associated labour (mid-range). Heat exchanger replacement is less common but can cost £350–£600 including parts and labour. A full PCB replacement sits at the higher end too, typically £300–£500. These larger repairs are worth weighing against the age and condition of your boiler — if it is over 10–12 years old, a new boiler may be more economical in the long run.
Will my Glow-Worm guarantee cover an F.83 repair?
It may, provided you have kept up with the conditions of the guarantee. Glow-Worm requires the boiler to have been installed and commissioned by a Gas Safe registered engineer, registered within 30 days of installation, and serviced annually by a Gas Safe engineer. If any of these conditions have not been met, the guarantee may have lapsed to 12 months from installation. Check your paperwork and contact Glow-Worm directly if you think your boiler is still within guarantee — their technical helpline can confirm cover before you book an independent engineer.