How to Fix the Glowworm F25 Fault Code | Step-by-Step Guide
If your boiler is displaying the Glowworm F25 fault code, it indicates a circulation fault. This means your boiler cannot circulate hot water effectively through your central heating system. As a safety measure, the boiler shuts down to prevent overheating and potential damage.
While some causes of the Glowworm F25 fault code can be resolved with simple checks, others require the expertise of a Gas Safe registered engineer. In this guide, we’ll explain what the fault means, its common causes, and the steps you can take to troubleshoot it safely.
Table of contents
What Does the Glowworm F25 Fault Code Mean?
The Glowworm F25 fault code signals that the boiler has detected poor or insufficient water circulation within the heating system.
Normally, the circulation pump moves heated water through your radiators and back to the boiler. If this process is interrupted, the water temperature inside the boiler rises too quickly, triggering the F25 safety lockout.
Common Causes of the Glowworm F25 Fault Code
Several issues can cause the Glowworm F25 fault code to appear.
1. Low Boiler Water Pressure
Low pressure is one of the most common reasons for circulation problems.
If the pressure falls below 1.0 bar, the circulation pump may struggle to move water effectively.
Signs include:
- Pressure gauge below the recommended level
- Cold radiators
- Boiler repeatedly locking out
2. Airlocks in the Heating System
Air trapped inside radiators or pipework restricts water circulation.
Common symptoms include:
- Radiators cold at the top
- Gurgling or bubbling noises
- Uneven heating throughout the property
3. Seized or Blocked Circulation Pump
Over time, sludge (magnetite), limescale, or debris can cause the pump to become partially or completely blocked.
A faulty pump may:
- Stop circulating water
- Produce humming or grinding noises
- Cause repeated overheating faults
4. Faulty Temperature Sensors (Thermistors)
Glowworm boilers use flow and return thermistors to monitor water temperature.
If these sensors provide incorrect readings, the boiler may mistakenly detect overheating and trigger the Glowworm F25 fault code.
5. Sludge or Blockages in the Heating System
Older heating systems often develop sludge deposits that restrict water flow.
Signs include:
- Cold spots on radiators
- Slow heating
- Frequent circulation faults
Step-by-Step Guide to Fix the Glowworm F25 Fault Code
Before calling an engineer, there are several safe checks you can perform.
Time needed: 5 minutes
Before calling an engineer, there are several safe checks you can perform.
- Check the Boiler Pressure
Check the pressure gauge on the front of your boiler to ensure the system pressure is between 1.0 and 1.5 bar when the boiler is cold. If the pressure is below this range, use the filling loop to carefully top up the system until it reaches the recommended level. Once the correct pressure is achieved, make sure to close the filling loop securely to prevent over-pressurising the system.
- Bleed Your Radiators
If you suspect trapped air in your heating system, switch off the heating and bleed each radiator using a radiator key until water flows steadily without air. Once you’ve finished, check the boiler pressure and top it up if it has dropped below the recommended level.
- Reset the Boiler
After restoring the correct boiler pressure and bleeding any trapped air from the system, press and hold the Reset button for 5–10 seconds to restart the boiler. If the Glowworm F25 fault code disappears, continue to monitor the boiler over the next few days to ensure it operates normally and the fault does not return.
- Check for Heating System Blockages
If your radiators stay cold or heat unevenly, sludge or debris may be restricting water circulation in your heating system. In this case, a professional heating engineer may recommend cleaning the system or carrying out a power flush to restore proper flow and improve heating performance.
- Contact a Gas Safe Registered Engineer
If the Glowworm F25 fault code persists after checking the boiler pressure, bleeding the radiators, and resetting the system, it’s time to contact a Gas Safe registered engineer. They can inspect the circulation pump, test the flow and return thermistors, check for blocked heat exchangers, diagnose PCB faults, and clean sludge from the heating system if necessary. For your safety, never remove the boiler casing or attempt repairs involving gas or electrical components yourself.
When Should You Call an Engineer?
You should contact a Gas Safe engineer if:
- The boiler repeatedly displays the Glowworm F25 fault code
- Boiler pressure keeps dropping
- The pump appears noisy or has stopped working
- Radiators remain cold after bleeding
- You suspect a faulty sensor or internal component
Preventing the Glowworm F25 Fault Code
Regular maintenance can significantly reduce the risk of circulation faults.
Book an Annual Boiler Service
A yearly service helps identify worn components before they fail.
Monitor Boiler Pressure
Check the pressure gauge every month and keep it between 1.0 and 1.5 bar.
Bleed Radiators Regularly
Removing trapped air improves circulation and heating performance.
Install a Magnetic Filter
Magnetic filters capture sludge and debris before they damage the pump or heat exchanger.
Consider a Power Flush
If your heating system is older or suffers from sludge buildup, a professional power flush can restore circulation and improve efficiency.
Conclusion
The Glowworm F25 fault code indicates that your boiler is struggling to circulate water safely through your heating system. In many cases, checking the pressure, bleeding radiators, and resetting the boiler can resolve the issue.
However, if the fault continues to appear, it’s likely that the circulation pump, thermistors, or heating system requires professional attention. Prompt repairs can prevent further damage and keep your heating system operating efficiently.
Frequently Asked Questions
The Glow-worm F32 fault indicates a fan-related issue. It means your boiler’s fan is not running at the correct speed, is disconnected, or has failed entirely. Because the fan vents harmful combustion gases, the boiler safely locks out to prevent carbon monoxide buildup.
The Glow-worm F25 fault indicates a circulation or flue gas safety issue, meaning the boiler is struggling to disperse heat correctly. It often points to trapped air, a blocked heat exchanger, a faulty circulation pump, or low water pressure.
An F75 fault on a boiler (most common on Vaillant and Glow-worm models) indicates a pressure problem or a fault with the water pressure sensor/pump. It means the boiler cannot detect a pressure rise when the pump starts.
