Glow-worm F8 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F8 fault code mean?
The F8 code on a Glow-Worm boiler signals a problem with the tank thermistor — an NTC (Negative Temperature Coefficient) sensor that monitors the temperature of stored domestic hot water in a cylinder or tank. This sensor is primarily relevant to system boilers and heat-only boilers paired with a hot water storage cylinder. When the boiler's control board receives a reading from this sensor that falls outside its expected range — or receives no reading at all — it treats the situation as a fault and locks the boiler out to prevent damage. The F8 code is confirmed on older Glow-Worm ranges including the Ultracom, Flexicom, Xtramax, and Energy series; always verify against your specific model's documentation as fault code definitions can vary between product generations.
General guidance only — not a substitute for professional advice. 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 or failed tank thermistor Common
The thermistor itself is the most frequent culprit. These sensors work by varying their electrical resistance in response to temperature; the PCB reads that resistance and converts it into a temperature value. Over time, age, heat cycling, and general wear can cause the sensor's resistance characteristics to drift or fail entirely, sending readings outside the boiler's accepted tolerances and triggering a lockout. A multimeter test by an engineer will confirm whether the sensor is out of spec.
- Loose, corroded, or damaged wiring Sometimes
The wiring loom connecting the tank thermistor to the control board can suffer from loose terminals, corrosion at the connector, or physical damage — particularly in older installations or where the cylinder is in a damp or confined space. Any interruption to that circuit can mimic a failed sensor and produce the same F8 code without the thermistor itself being at fault.
- PCB (printed circuit board) fault Rare
If the control board itself has developed a fault — whether through component failure, moisture ingress, or a power surge — it may misinterpret or fail to process signals from an otherwise healthy thermistor. A PCB fault is less common and usually only confirmed once the thermistor and wiring have been ruled out, but it can be the root cause, particularly on older boilers.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or reset symbol) for around three seconds, then release and wait up to a minute for the boiler to restart. If the F8 code clears and the boiler runs normally, monitor it over the next few hours. A one-off fault can occasionally be caused by a temporary sensor glitch. Do not reset more than two or three times — repeated lockouts indicate a genuine underlying problem that resetting will not fix.
- Check system pressure DIY safe
Glance at the pressure gauge on the boiler fascia. For most Glow-Worm system boilers, normal operating pressure when cold is between 1 and 1.5 bar. If it has dropped below 1 bar, top it up using the filling loop — a braided flexible hose or valve arrangement usually located beneath the boiler. Open the valve(s) slowly, watch the gauge rise to around 1.2–1.5 bar, then close the valve fully. A low-pressure condition alone is unlikely to cause F8 specifically, but it is worth ruling out before calling an engineer.
- Call a Gas Safe registered engineer to inspect the tank thermistor and wiring Gas Safe engineer
If the fault returns after resetting, the thermistor circuit needs professional diagnosis. An engineer will use a multimeter to test the thermistor's resistance at a known water temperature and compare the reading against the manufacturer's specification. They will also inspect the wiring loom and connector for signs of corrosion, loose pins, or chafing. This work involves accessing internal boiler components and the hot water cylinder sensor pocket — it must only be carried out by a Gas Safe registered engineer.
- Have the thermistor replaced if found to be faulty Gas Safe engineer
If testing confirms the sensor is out of specification, the engineer will fit a replacement tank thermistor. This involves draining relevant parts of the system where necessary, removing the old sensor from its pocket in the cylinder or pipework, fitting the new one, reconnecting the wiring correctly, and testing the boiler through a full heating and hot water cycle to confirm the fault is resolved.
- Have the PCB inspected or replaced if wiring and thermistor test satisfactory Gas Safe engineer
Should the thermistor and its wiring both test correctly, the fault points toward the control board. PCB diagnosis requires specialist equipment and expertise; replacement must be carried out by a Gas Safe engineer using an approved part for your specific Glow-Worm model. Once replaced, the boiler will need to be commissioned and tested before being returned to service.
Parts you may need
- Tank NTC thermistor (cylinder sensor) · from £25
- Thermistor wiring harness / connector · from £30
- Glow-Worm PCB (model-specific) · from £220
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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F8 fault myself?
The initial reset is something any homeowner can safely attempt. Beyond that, diagnosing and replacing a tank thermistor involves accessing internal components, testing electrical circuits, and potentially working on a pressurised hot water cylinder — all of which should be left to a Gas Safe registered engineer. Thermistors are not gas components, but the work still requires the right tools, knowledge, and access to approved parts.
My boiler is a combi — why am I seeing F8?
The F8 tank thermistor fault is specifically associated with system and heat-only Glow-Worm boilers that work with a separate hot water storage cylinder. Combi boilers heat water on demand and do not have a tank thermistor in the same way. If you have a combi and you're seeing F8, double-check your model number against its manual, as fault code meanings can differ across product ranges — what is F8 on one Glow-Worm series may mean something different on another.
How much does it cost to fix the F8 fault on a Glow-Worm boiler?
Most people pay somewhere between £120 and £320 all-in, which typically covers the engineer's call-out, diagnosis, a replacement thermistor, and labour. If the wiring loom also needs attention, costs sit toward the upper end of that range. In the less common situation where the PCB is the root cause, the total bill can rise considerably higher — PCB replacement alone can add £200–£400 on top of labour, so it is worth getting a clear quote before authorising that repair. On older boilers, compare the repair cost against a new boiler quote.
Will an annual boiler service help prevent the F8 fault?
Yes, in part. During a service, a Gas Safe engineer will visually inspect sensors and wiring connections, which can catch a deteriorating thermistor or a corroding connector before it causes a lockout. Glow-Worm also requires annual servicing by a registered engineer to keep the manufacturer's warranty valid (up to 10 years on some models). While no service can guarantee a thermistor won't eventually fail, keeping on top of annual maintenance is the best way to catch developing faults early.