Glow-worm F23 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F23 fault code mean?
The F23 code means your Glow-Worm boiler has detected an excessive temperature difference between the flow and return pipes and has shut itself down as a safety measure. Your boiler has two NTC thermistors — one monitoring the water temperature as it leaves the boiler (flow) and one monitoring it as it returns after circulating through your radiators. Some temperature drop across the system is perfectly normal, but when the gap between the two readings becomes too large, the boiler interprets this as a circulation problem and locks out to prevent overheating or damage. The boiler will remain off until the root cause is addressed and the fault is cleared.
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
- Weak or failing circulation pump Common
If the pump is not pushing water around the system effectively, heated water sits near the boiler and the return temperature drops far below the flow temperature. A noisy pump, one that runs intermittently, or one that has seized are all common triggers for F23. This is the single most frequent cause.
- Sludge or limescale restricting flow Common
Magnetite sludge and limescale deposits can build up in the heat exchanger, pipework, or pump over time, creating a partial or full blockage. Restricted flow means water spends longer in the system, losing far more heat before returning, which widens the temperature differential beyond the boiler's tolerance.
- Airlock in the system Common
A pocket of trapped air in the pipework or heat exchanger can disrupt water circulation in much the same way as a physical blockage, preventing sufficient flow and causing the return temperature to read unusually low.
- Zone valve or radiator valves closed or stuck Sometimes
If thermostatic radiator valves are turned fully down or a zone valve has failed in the closed position, water cannot circulate freely. The boiler heats water that has nowhere to go, creating a large temperature spread between flow and return.
- Faulty NTC thermistor(s) Sometimes
One or both of the NTC thermistors may have drifted out of calibration or failed outright, causing the boiler to read a false temperature differential even when circulation is perfectly normal. Less common than circulation issues but worth investigating once flow problems have been ruled out.
- NTC thermistors fitted to the wrong pipes Rare
In rare cases following a previous repair, the flow and return thermistors may have been swapped. This produces a persistent and misleading temperature differential reading that points to F23.
How to fix it
- Check that all radiator valves and zone valves are open DIY safe
Walk around the property and make sure thermostatic radiator valves are not turned to zero or the frost setting, and that any manually operated zone valves are in the open position. Fully closed valves prevent circulation and can cause F23 directly.
- Bleed your radiators to remove trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve slightly until water (rather than air) escapes, then close it again. Work back towards the boiler. Once done, check the system pressure on the boiler gauge — it should read between 1 and 1.5 bar. If it has dropped, top up via the filling loop.
- Top up system pressure if needed DIY safe
If pressure has fallen below 1 bar after bleeding, attach the filling loop and slowly open both valves until the gauge reads approximately 1.2–1.5 bar, then close the valves and disconnect the loop. Low pressure on its own can contribute to poor circulation.
- Reset the boiler once DIY safe
Press and hold the reset button as described in your boiler's user guide. Allow the boiler to complete its start-up sequence. If the F23 code returns promptly, do not reset again — repeated resets without fixing the underlying problem can cause further damage.
- Listen for unusual pump noise DIY safe
Stand next to the boiler and listen while it attempts to fire. A grinding, rattling, or unusually quiet pump (or no pump noise at all) points strongly towards a pump fault. Note what you hear and report it to the engineer — it helps them diagnose faster.
- Have the circulation pump tested and replaced if faulty Gas Safe engineer
A Gas Safe registered engineer will use a multimeter to check that the pump, its wiring, and the PCB connections are all receiving the correct voltage. If the pump has seized or is underperforming, it will need to be replaced — typically with a Grundfos or equivalent unit suited to your Glow-Worm model.
- Have the system flushed to remove sludge Gas Safe engineer
An engineer will assess the severity of any sludge contamination. A chemical flush (circulating cleaning solution through the system) is sufficient for mild build-up. Heavily contaminated systems with cold radiators or visible corrosion may require a full powerflush. Either way, the system should be re-dosed with corrosion inhibitor afterwards, and fitting a magnetic filter at the same time significantly reduces future sludge accumulation.
- Have the NTC thermistors tested and replaced if necessary Gas Safe engineer
If circulation checks out but F23 persists, an engineer will measure the resistance of both thermistors across a range of temperatures and compare the readings against the manufacturer's specification. A thermistor that has drifted out of tolerance or failed should be replaced. The engineer will also confirm the flow and return sensors are fitted to the correct pipes.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If your own checks (bleeding, pressure top-up, single reset) have not resolved the F23 code, book a Gas Safe registered engineer. Provide them with a description of any noises, how long the fault has been present, and the age of the system — this helps them arrive prepared with the most likely replacement parts.
Parts you may need
- Circulation pump (e.g. Grundfos UPS2 or equivalent) · from £85
- NTC thermistor (flow or return) · from £25
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £65
- Zone valve motor head · from £30
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–£400, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Glow-Worm boiler to clear the F23 code?
You should only reset once or twice at most. If the fault returns each time, resetting is simply masking a problem that will not fix itself. Repeated resets without addressing the root cause can put additional strain on the pump and heat exchanger. Book an engineer rather than continuing to reset.
Is F23 always a pump problem?
Not always, but the pump is the most common culprit. Sludge build-up and airlocks are also frequent causes and can mimic a pump fault. If the pump sounds normal and your radiators heat evenly, the engineer will turn their attention to the thermistors. A proper diagnosis with a multimeter is the only reliable way to be certain.
How much does it cost to fix a Glow-Worm F23 fault code?
For most homeowners, the repair falls between £150 and £400. A circulation pump replacement typically costs around £150–£350 all in, while thermistor replacement is usually £80–£160 for parts and labour. If sludge is the cause, a chemical flush adds roughly £100–£200; a full powerflush on a heavily contaminated system can cost £400–£700. Fitting a magnetic filter at the same time (around £80–£150 extra) is strongly recommended as it helps prevent a repeat. In rare cases where the PCB is involved, costs can be higher — your engineer will advise.
How can I prevent F23 from happening again?
Annual boiler servicing catches pump and thermistor issues before they cause a lockout. Fitting a magnetic filter (if you do not already have one) captures iron oxide sludge before it reaches the pump or heat exchanger. Bleeding radiators once a year and keeping system pressure between 1 and 1.5 bar also helps maintain good circulation. Most Gas Safe engineers recommend a full powerflush every 5–10 years on older systems, especially in hard-water areas.