Glow-worm F25 Fault: Meaning, Symptoms, Actions, Costs, Causes, Prevention
The Glow-worm F25 fault means your boiler has detected low water pressure, preventing it from operating safely. The main symptoms include a locked-out boiler, an error code on the display, and possibly cold radiators or no hot water. When you see this fault, you need to check the pressure gauge and repressurise the system if it is below 1 bar. The cost to fix the F25 fault depends on the cause, ranging from a simple pressure top-up to replacing a faulty pressure sensor or fixing a leak £100–£300. The most common causes include system leaks, a faulty pressure sensor, or trapped air reducing circulation. You can prevent this fault by regularly checking the pressure, bleeding radiators, and fixing leaks promptly. Always use a Gas Safe engineer to diagnose and repair your boiler safely. The next section explains the meaning of the Glow-worm F25 fault in more detail.
Meaning of the Glow-worm F25 Fault
The Glow-worm F25 fault code signals that the boiler has detected a low water pressure issue, which prevents safe operation. Glow-worm boilers require a minimum water pressure level to circulate heated water efficiently. When the pressure drops below the required threshold, the boiler locks out to prevent damage or overheating. The F25 error typically results from a loss of pressure in the system, caused by leaks, trapped air, or a malfunctioning pressure sensor. The built-in safety mechanism ensures that the boiler does not run under unsafe conditions, protecting internal components from overheating or dry operation.
Why does my Glow-worm boiler display the F25 error code?
Your Glow-worm boiler displays the F25 error code when the pressure in the heating system drops too low. This typically happens due to leaks, trapped air, or a faulty pressure sensor that misreads the pressure level.
How does low pressure affect a Glow-worm boiler?
Low pressure affects a Glow-worm boiler by reducing the circulation of hot water through the system. Without sufficient pressure, the boiler struggles to push water through radiators and pipes, leading to inefficiency and potential overheating of internal components.
Does the F25 fault indicate a serious problem?
The F25 fault itself is not inherently serious but requires prompt attention to restore proper boiler function. If left unresolved, continuous low pressure can cause long-term damage to components such as the pump or heat exchanger.
Can the F25 fault cause my boiler to stop working?
Yes, the F25 fault triggers a safety lockout, which stops the boiler from operating until the underlying issue is fixed. The lockout prevents potential damage caused by dry operation or overheating.
The next section will describe the symptoms of the F25 fault in more detail.
Symptoms of the Glow-worm F25 Fault
The Glow-worm F25 fault causes noticeable symptoms that indicate a pressure issue within the boiler. The most common sign is a fault code displayed on the boiler’s control panel, preventing normal operation. The boiler may lock out, meaning it stops heating water and supplying warmth to radiators. You may notice that the pressure gauge shows a reading below 1 bar, which confirms a low-pressure issue. In some cases, radiators may stay cold, or hot water may become intermittent or completely unavailable. If the fault is caused by a leak, you may also see visible water dripping from pipe joints, radiators, or the boiler itself.
How To Recognise The Glow-worm F25 Fault on Different Types of Boilers
Glow-worm Combi Boilers
On Glow-worm combi boilers, the F25 fault appears as a digital error code on the display panel. The boiler stops producing hot water and heating, and the pressure gauge will likely show a reading below 1 bar. Some models may display a flashing warning light or an exclamation mark alongside the fault code.
Glow-worm System Boilers
Glow-worm system boilers also show the F25 error on their digital display, triggering a lockout. The main difference is that system boilers rely on a separate hot water cylinder, so stored hot water may still be available even when the fault occurs. However, central heating will stop working, and the boiler will not be able to heat the water tank.
Older Non-Digital Glow-worm Boilers
Older Glow-worm boilers without a digital display do not show an F25 fault code. Instead, you may notice that the boiler is unresponsive, with no heating or hot water. The pressure gauge may indicate low pressure, and some models may have a flashing fault light.
The next section suggests actions to take in the event of the Glow-worm F25 Fault.
What to Do When Your Glow-worm Boiler Shows the F25 Fault
When your Glow-worm boiler displays the F25 fault, you need to check the system pressure and take basic troubleshooting steps. First, look at the pressure gauge on the boiler panel. If the reading is below 1 bar, the pressure is too low, and the system may need to be repressurised. You can safely top up the pressure using the filling loop, following the instructions in the manufacturer’s manual. After increasing the pressure to the recommended level (usually between 1 and 1.5 bar), reset the boiler to see if it resumes normal operation.
If the pressure drops again after repressurising, check for visible leaks around the boiler, radiators, and pipework. If you notice a water leak or cannot restore the pressure, contact a Gas Safe engineer for further inspection. Never attempt to open the boiler casing or adjust internal components, as this requires professional expertise. Always follow the Glow-worm user manual and avoid any actions that go beyond standard homeowner maintenance.
How do I safely turn off my Glow-worm boiler with the F25 fault?
You can safely turn off your Glow-worm boiler by pressing the power button or switching it off at the main control panel. If your boiler has a dedicated power switch near the unit, turn it off there. Avoid disconnecting any internal components or removing the boiler casing.
Should I shut off the gas supply if my boiler has the F25 fault?
The F25 fault is a water pressure issue, not a gas-related problem. You do not need to shut off the gas unless you smell gas or suspect a leak. If you notice a gas smell, turn off the gas at the mains and contact the National Gas Emergency Service at 0800 111 999 immediately.
Where do I find the filling loop to repressurise my Glow-worm boiler?
The filling loop is usually located underneath the boiler, attached to the pipework. It consists of a flexible hose with two valves. To repressurise the system, slowly open both valves and allow water to enter the system until the pressure gauge reaches 1 to 1.5 bar. Always close both valves after topping up the pressure.
How do I reset my Glow-worm boiler after fixing the F25 fault?
To reset your Glow-worm boiler, locate the reset button, which is usually marked with a circular arrow or ‘RESET.’ Hold the button for a few seconds until the boiler restarts. If the fault reappears, contact a Gas Safe engineer for further diagnosis.
Should I call a professional if the F25 fault keeps returning?
Yes, if the F25 fault keeps coming back, a professional inspection is necessary. Recurring pressure loss may indicate a leak, a faulty expansion vessel, or a malfunctioning pressure sensor. A Gas Safe engineer can diagnose and fix the underlying problem safely.
The next section will describe the costs to repair this fault.
Cost to Repair Glow-worm F25 Fault
The cost to repair the Glow-worm F25 fault depends on the cause of the issue and the repairs needed. The minimum cost is £0 if the problem is a simple pressure drop that you can fix by repressurising the system yourself. A minor repair, such as replacing a faulty pressure sensor or fixing a small leak, typically costs between £100 and £300. If the issue involves a major system leak, damaged expansion vessel, or multiple failing components, repair costs can reach £500 or more.
A Gas Safe engineer usually charges between £80 and £150 for a standard call-out, depending on your location. If parts need replacement, the total cost increases based on the component price and labour time. Emergency repairs or weekend call-outs can add £50 to £100 to the final cost.
If your boiler is old and requires frequent repairs, replacing it may be a more cost-effective option in the long run. A new Glow-worm boiler costs between £1,500 and £3,000, including installation.
The next section will describe the major causes of the fault in more detail.
The 5 Major Causes of The Glow-worm F25 Fault
The Glow-worm F25 fault occurs due to low water pressure, which prevents the boiler from operating safely. The most common causes include system leaks, a faulty pressure sensor, trapped air in the system, an expansion vessel failure, or a blocked pressure relief valve. Each of these issues disrupts normal boiler function and may require repairs to restore proper pressure levels.
1. System Leaks
Leaks in the heating system are the most common cause of the F25 fault. Water can escape from radiators, pipe joints, or inside the boiler itself, leading to a gradual pressure drop. Signs of a leak include damp patches, dripping water, or visible corrosion on pipes. Even small leaks cause continuous pressure loss, requiring frequent repressurisation. A Gas Safe engineer can locate and repair leaks to restore system integrity.
2. Faulty Pressure Sensor
The boiler’s pressure sensor monitors system pressure and triggers safety shut-offs if the reading is too low. A malfunctioning sensor may send incorrect signals to the boiler, causing it to lock out with an F25 fault even if the pressure is within the normal range. Pressure sensors degrade over time due to dirt, limescale, or electrical faults. Replacing a faulty sensor typically resolves the issue.
3. Trapped Air in the System
Air pockets in the central heating system reduce water circulation and lower the effective pressure. Trapped air is common after recent maintenance, radiator bleeding, or if the system has not been used for a long period. Symptoms include cold spots on radiators, gurgling noises, or fluctuating pressure readings. Bleeding the radiators and topping up the pressure with the filling loop usually fixes the problem.
4. Expansion Vessel Failure
The expansion vessel regulates pressure changes as water heats and cools. If the vessel’s internal diaphragm fails, the system loses pressure rapidly, triggering the F25 fault. A faulty expansion vessel may cause repeated pressure drops even after repressurising the system. Replacing the expansion vessel is necessary if it no longer maintains stable pressure.
5. Blocked Pressure Relief Valve
The pressure relief valve (PRV) prevents excessive pressure buildup by releasing water when needed. If the valve becomes blocked or stuck open, it may cause a continuous loss of water and pressure. A faulty PRV may also leak, leading to visible water pooling near the boiler. Cleaning or replacing the valve restores proper pressure control.
The next section will explain how to prevent the Glow-worm F25 fault from occurring.
Preventing Future Glow-worm F25 Fault
You can prevent the Glow-worm F25 fault by maintaining stable boiler pressure and keeping the system in good condition. Regularly check the pressure gauge to ensure it stays between 1 and 1.5 bar. If pressure drops frequently, inspect radiators and pipework for leaks and bleed radiators to remove trapped air.
Annual boiler servicing helps detect and fix minor issues before they cause a fault. A Gas Safe engineer can check the expansion vessel, pressure sensor, and safety valves during a routine service. Signing up for a boiler service contract ensures yearly maintenance, reducing the risk of unexpected breakdowns.
Disclaimer
Any work beyond standard user maintenance should be performed by a Gas Safe registered engineer. Opening the boiler casing, replacing components, or adjusting internal settings requires professional expertise to ensure safety and compliance with UK gas regulations.