Glow-worm F20 Fault Guide for Homeowners: Meaning, Symptoms, Actions, Costs, Causes, Prevention
The Glow-worm F20 fault code indicates that your boiler has overheated due to an issue with temperature regulation. The main symptoms include the boiler shutting down, displaying the F20 error code, and radiators or hot water becoming excessively hot before the system locks out. To resolve this issue, you need to attempt a boiler reset, but if the fault persists, professional assistance is required. Repair costs vary, ranging from £80 for a reset to £300 or more if components such as the thermistor, heat exchanger, or pump require replacement. The most common causes of this fault include faulty temperature sensors, restricted water flow, limescale buildup, or a failing pump. Preventative measures include annual servicing, flushing the system, and checking radiator balancing. Always use a Gas Safe-registered engineer for any repairs, as overheating issues can pose safety risks. The next section will explain the meaning of the F20 fault in more detail.
Meaning of the Glow-worm F20 Fault
The Glow-worm F20 fault code appears when the boiler detects an unsafe rise in temperature, triggering an automatic shutdown. This means that the internal components are overheating due to a lack of proper heat dissipation. The thermistor (temperature sensor) or heat exchanger may be faulty, or the water circulation system may be restricted. Overheating can cause long-term damage if left unresolved, so the system locks out to prevent further risks.
What does the F20 fault code indicate on a Glow-worm boiler?
The F20 fault code means that your boiler has overheated, triggering a safety shutdown to prevent damage.
Why does my Glow-worm boiler display the F20 fault?
Your boiler displays the F20 fault when it detects an abnormal rise in temperature, often due to a blocked heat exchanger, failing pump, or faulty thermistor.
Is the F20 fault dangerous?
The F20 fault is not immediately dangerous, but it indicates excessive heat buildup, which could lead to internal damage if ignored.
Can I fix the Glow-worm F20 fault myself?
You can attempt a simple boiler reset, but if the fault returns, a Gas Safe engineer must diagnose the cause.
Will the F20 fault reset itself?
The F20 fault does not reset itself. The boiler requires a manual reset, but overheating issues often need professional repairs.
What happens if I ignore the F20 fault?
Ignoring the F20 fault can cause serious damage to the boiler’s internal components, leading to costly repairs.
The next section will describe the symptoms of the F20 fault in more detail.
Symptoms of the Glow-worm F20 Fault
The Glow-worm F20 fault causes the boiler to overheat and lock out, preventing further operation. You may notice radiators becoming excessively hot before the system shuts down. The digital display will show the F20 error code, and the boiler may make unusual clicking or hissing noises due to overheating. If the circulation is restricted, some radiators may remain cold while others overheat.
How To Recognise The Glow-worm F20 Fault on Different Types of Boilers
Glow-worm Combi Boilers
Combi boilers will display F20 on the digital screen, preventing the boiler from producing heating or hot water. The boiler may make gurgling or boiling noises before shutting down.
Glow-worm System Boilers
System boilers show the F20 fault code on the control panel, often accompanied by fluctuating pressure readings. Water temperature may become inconsistent before the boiler locks out.
Older Non-Digital Boilers
Older Glow-worm boilers without digital displays may overheat and shut down unexpectedly. You may hear bubbling noises, and the reset button may need pressing to restart.
The next section suggests actions to take in the event of the F20 fault.
What to Do When Your Glow-worm Boiler Shows the F20 Fault
When your Glow-worm boiler displays the F20 fault code, the first step is to attempt a manual reset by holding the reset button for 3–5 seconds. If the boiler reignites successfully, monitor it to see if the fault returns. Check if the radiators are unevenly heating, as this may indicate a circulation issue.
If the fault persists, turn off the boiler and contact a Gas Safe engineer. Do not attempt to open the boiler casing or adjust internal components, as overheating issues require professional diagnosis.
Safety Warning
Only perform actions described in your boiler’s user manual. Do not attempt to dismantle the boiler or adjust temperature sensors. A Gas Safe-registered engineer must handle overheating-related repairs.
How do I reset my Glow-worm boiler after an F20 fault?
Press and hold the reset button for 3–5 seconds. If the boiler restarts, monitor it for further overheating.
Should I turn off my boiler if it shows the F20 fault?
Yes, overheating issues require professional inspection. Turn off the boiler and call an engineer if the fault persists.
Where is the reset button on my Glow-worm boiler?
The reset button is located on the boiler’s front panel, usually near the display screen.
Can I fix the F20 fault myself?
You can reset the boiler and check for radiator issues. Any further repairs require a Gas Safe engineer.
The next section will describe the cost to repair Glow-worm F20 fault.
Cost to Repair Glow-worm F20 Fault
The cost to repair the Glow-worm F20 fault depends on the cause of the overheating. The minimum repair cost for a basic reset or minor adjustment starts at £80–£100. If the issue involves thermistor replacement or system flushing, you can expect to pay between £100–£200.
If faulty components require replacement, costs increase. A new thermistor or pump typically costs between £150–£250, including parts and labour. A full heat exchanger replacement ranges from £250–£500, depending on the model and installation complexity.
In rare cases, if multiple parts fail, a full boiler replacement may be necessary. A new Glow-worm boiler installation costs between £1,500–£3,000. Getting multiple quotes from Gas Safe engineers ensures you receive a fair price. A boiler cover plan can help reduce unexpected repair costs.
The 5 Major Causes of the Glow-worm F20 Fault
The Glow-worm F20 fault is caused by overheating, preventing safe operation. The most common causes include faulty thermistors, blocked heat exchangers, restricted circulation, failing pumps, and excessive limescale buildup.
1. Faulty Thermistor
The thermistor measures water temperature. A malfunctioning thermistor may send incorrect readings, causing the boiler to overheat unnecessarily.
2. Blocked Heat Exchanger
Limescale and sludge buildup in the heat exchanger restrict water flow, trapping heat inside the system and leading to overheating.
3. Restricted Circulation
Blocked pipes or poor radiator balancing can prevent water from circulating properly, leading to temperature imbalances and overheating.
4. Failing Pump
A weak or failing pump cannot push hot water through the system efficiently, causing the boiler to retain excess heat and shut down.
5. Excessive Limescale Buildup
Hard water areas cause limescale buildup, which clogs internal components and prevents proper heat dissipation.
The next section will explain how to prevent this fault from occurring.
Preventing Future Glow-worm F20 Faults
Annual boiler servicing ensures that thermistors, pumps, and circulation systems function correctly. A boiler service contract includes regular inspections and emergency callouts, reducing the risk of faults. Flushing the system, checking for radiator balancing, and descaling components in hard water areas help prevent overheating issues.
Disclaimer
Only a Gas Safe-registered engineer should repair internal boiler faults. Do not attempt to dismantle the boiler or adjust temperature components. Always follow the manufacturer’s manual for safe operation.