Glow-worm F15 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F15 fault code mean?
The F15 fault code on a Glow-Worm boiler points to a problem with the return thermistor — the sensor responsible for measuring the temperature of water as it flows back into the boiler after circulating through your heating system. When this thermistor fails or sends readings outside the expected range, the boiler's control board can no longer accurately monitor return water temperature. This disrupts the boiler's ability to regulate its heating cycles correctly, so it shuts down and displays F15 to protect itself. The fault is typically caused by a failed thermistor, a wiring fault in the sensor circuit, or in less common cases, a faulty PCB. While F15 does not usually represent an immediate safety emergency, it will prevent your boiler from running normally until the underlying cause is resolved.
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
- Failed Return Thermistor Common
The NTC thermistor itself is the most frequent culprit. Over time the thermistor's internal resistance drifts or fails completely, causing it to report temperatures that are wildly inaccurate or not register at all. The boiler's PCB detects a reading outside its acceptable limits and triggers F15. Thermistors are consumable components and failure rates increase significantly in older boilers.
- Damaged or Corroded Thermistor Wiring Sometimes
The wiring loom connecting the return thermistor to the PCB can develop faults through vibration, heat cycling, or age. Corroded terminals, a chafed cable, or a loose plug connector can all mimic the symptoms of a dead thermistor by breaking or distorting the signal reaching the control board. An engineer will check continuity and resistance through the wiring before condemning the sensor itself.
- Faulty PCB Rare
If the thermistor and its wiring both test within specification, attention turns to the PCB. A damaged input circuit on the board can misinterpret or fail to read the thermistor signal altogether, producing an F15 even when the sensor is perfectly healthy. PCB faults are the least common explanation but must be considered when everything else checks out.
How to fix it
- Switch the boiler off at the programmer or room thermostat and allow it to cool for a few minutes DIY safe
Turning the boiler off gives any temporary sensor glitch a chance to clear. Do not touch any internal components — simply use your controls or the boiler's power switch.
- Reset the boiler following the manufacturer's reset procedure DIY safe
Locate the reset button (usually a flame symbol with a line through it) and hold it for the duration stated in your user manual, typically around three seconds. The boiler should attempt to restart. If it fires up and runs normally, monitor it over the next day or two. If F15 returns quickly, do not keep resetting — repeated resets without resolving the cause can mask a worsening fault.
- Check your boiler's system pressure is within the normal range (1–1.5 bar when cold) DIY safe
Low pressure is unrelated to F15 but is worth confirming so you can give the engineer accurate information. If pressure is below 1 bar, top up via the filling loop following your boiler's instructions. If pressure is correct, leave it alone.
- Note any other symptoms and contact a Gas Safe registered engineer DIY safe
Make a note of how frequently F15 appears, whether it came on suddenly or has been intermittent, and whether heating or hot water has been affected. This information will help the engineer diagnose the fault more efficiently.
- Engineer: test the return thermistor resistance and compare against the manufacturer's temperature–resistance curve Gas Safe engineer
A Gas Safe engineer will use a multimeter to measure the thermistor's resistance at a known water temperature and compare it against Glow-Worm's published values. A reading that falls well outside the specified range confirms the sensor has failed and needs replacing.
- Engineer: inspect the thermistor wiring loom and connector for damage or corrosion Gas Safe engineer
If the thermistor resistance tests correctly, the engineer checks continuity through the wiring back to the PCB, looks for chafed insulation, and checks that the connector is seated firmly. A faulty wire or corroded terminal is repaired or the loom section replaced.
- Engineer: replace the return thermistor if confirmed faulty Gas Safe engineer
Replacement involves draining down the relevant section of the boiler, removing the old sensor, fitting a compatible new thermistor, refilling, and testing. The boiler is then run through a full heating cycle to confirm the fault has cleared and temperatures are reading correctly.
- Engineer: test and if necessary replace the PCB if thermistor and wiring are both sound Gas Safe engineer
If sensor and wiring checks are both satisfactory, the PCB input circuit is the remaining suspect. PCB replacement is a more involved and costly repair; the engineer will confirm this diagnosis before proceeding.
Parts you may need
- Return NTC Thermistor (Glow-Worm compatible) · from £18
- Thermistor Wiring Loom / Connector · from £25
- PCB (Printed Circuit Board) · 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 £120–£300, depending on the underlying cause.
Frequently asked questions
Is the Glow-Worm F15 fault dangerous?
F15 is not generally considered a safety-critical fault in the same way as a gas leak or carbon monoxide warning. The return thermistor monitors water temperature for efficiency and protection purposes, and when it fails the boiler shuts itself down rather than running dangerously. That said, you should still get it looked at promptly — leaving a fault unresolved can put extra stress on other components over time.
Can I fix the F15 fault myself?
The only things a homeowner can safely try are switching the boiler off, performing a single reset, and checking system pressure. The actual cause of F15 — whether a failed thermistor, wiring fault, or PCB — involves accessing internal boiler components and requires a Gas Safe registered engineer. Attempting to replace internal parts yourself is unsafe and will invalidate your warranty or any existing service plan.
How much does it cost to fix a Glow-Worm F15 fault?
Most homeowners pay between £120 and £300 to resolve an F15 fault. A straightforward thermistor replacement at the lower end of that range typically costs around £120–£180 including labour and parts. If the wiring loom needs attention the cost is similar. A PCB replacement sits at the top of the range or can exceed it — PCBs for Glow-Worm boilers typically cost £150–£200 for the part alone, with labour on top. If your boiler is over ten years old and needs a new PCB, it is worth getting a quote for a new boiler at the same time for comparison.
Will an annual boiler service help prevent the F15 fault?
Yes. During a service a Gas Safe engineer checks sensor readings, inspects wiring connections, and cleans components that can affect thermistor performance. Catching a marginal thermistor reading or a loose connector early — before it deteriorates into a full F15 shutdown — is one of the practical benefits of keeping up with annual servicing.