Glow Worm F9 Fault Code: Causes & How To Fix It

Last Updated on January 23, 2025

If your Glow Worm boiler is displaying the F9 fault code, it indicates that your system is experiencing an issue with water pressure. This might sound like a minor problem, but it can disrupt the heating and hot water supply. 

In this guide, we will break down what the F9 fault code means, why it occurs, and how you can fix it.

What Does the Glow Worm F9 Fault Code Mean?

The F9 fault code on your Glow Worm boiler is a warning that the boiler has detected an issue with the water pressure sensor. 

This sensor is responsible for measuring the pressure within your boiler system and sending that information to the printed circuit board (PCB). The PCB controls the electrical operations of the boiler.

If the sensor detects that the pressure is too high or too low, or if the PCB cannot receive data from the sensor, the boiler will shut down and display the F9 error code.

Common Causes of the Glow Worm F9 Fault Code

1. Low or High Water Pressure

The most common cause of the F9 fault code is incorrect water pressure. This could be caused by:

• A drop in system pressure due to leaks, airlocks, or a faulty pressure relief valve.
• Excessively high pressure, which can also trigger the error.

If the boiler pressure is too low, the sensor will register it and stop the system to prevent further damage. Similarly, if the boiler pressure is too high, it can stress the system and trigger the same error.

2. Faulty Pressure Sensor

If the pressure sensor itself is malfunctioning, it may not provide accurate readings to the PCB. This can lead the boiler to believe that the pressure is either too high or too low, even if the actual pressure is fine. In this case, the sensor may need to be cleaned, recalibrated, or replaced.

3. Leaks in the System

A leak in the system, whether in the radiators, pipework, or the boiler itself can cause gradual pressure loss over time. Even small leaks can cause a significant drop in pressure, which can trigger the F9 error code. 

If you notice any water around the boiler or damp patches near your radiators, it’s essential to investigate for leaks.

4. Airlocks in the Heating System

An airlock can prevent water from circulating properly through the system, leading to pressure fluctuations. These airlocks can form in radiators or pipework, disrupting the pressure balance and potentially triggering the F9 fault.

5. Faulty PCB

The printed circuit board (PCB) is the central control unit for your boiler. If the PCB fails or becomes damaged, it may fail to receive the correct readings from the pressure sensor. This can cause the F9 error to appear. 

In this case, the PCB may need to be inspected or replaced.

6. Expansion Vessel Issues

The expansion vessel helps regulate the pressure in the system by absorbing the expansion of water when it’s heated. If the expansion vessel becomes faulty or loses its charge, it can cause pressure fluctuations that trigger the F9 error. 

A Gas Safe registered engineer will need to assess and recharge or replace the vessel if necessary.

7. Defective Pressure Relief Valve

The pressure relief valve regulates the pressure within the boiler. If the valve is defective or stuck, it can cause the system to register incorrect pressure readings, leading to the F9 fault code. A faulty valve may need to be replaced.

How to Fix the Glow Worm F9 Fault Code

1. Repressurise the Boiler

If the issue is caused by low water pressure, you can repressurise the system yourself:

• Locate the filling loop (usually underneath the boiler).
• Open the valve to allow water into the system.
• Monitor the pressure gauge and stop when it reaches the recommended level (typically 1-1.5 bar).
• Close the valve and check for any leaks.

2. Check for Leaks

Inspect your system for any visible leaks, especially around the radiators, pipework, and boiler. Even small leaks can cause significant pressure drops over time. If you find any leaks, they will need to be repaired to restore the pressure.

3. Bleed the Radiators

If airlocks are suspected, bleeding your radiators can help release any trapped air:

• Turn off the heating system and allow the radiators to cool.
• Use a radiator key to open the valve at the top of each radiator.
• Let the air escape until water starts to flow steadily, then close the valve.

4. Inspect or Replace the Pressure Sensor

If the pressure sensor is malfunctioning, it will need to be inspected or replaced. This is typically not a DIY fix, so it’s best to call in a qualified engineer to handle the sensor replacement.

5. Check the Expansion Vessel

If the expansion vessel is not functioning properly, it may need to be recharged or replaced. A professional engineer will be able to assess the vessel and restore its functionality.

6. Reset the Boiler

Once the issue is resolved, you can reset the boiler to clear the F9 fault code. Follow the manufacturer’s instructions on how to reset your particular Glow Worm model.

Considering a New Boiler?

If your Glow Worm boiler frequently displays the F9 fault code, it might be a sign that your current system is reaching the end of its lifespan. Over time, older boilers can become less efficient, prone to overheating, and costly to repair. 

If you’re facing constant issues with your existing model, it could be time to consider upgrading to a newer, more reliable unit. Modern Glow Worm boilers are designed to provide better energy efficiency, reducing your heating costs in the long run. 

To learn more about the latest models and their performance, take a look at our Glow Worm boiler prices and reviews to help you make an informed decision about your next boiler purchase.

Get a Free Quote for a New Boiler

If you’re facing frequent errors with your boiler or it’s nearing the end of its lifespan, it might be time to consider a new one. 

We offer free, no-obligation quotes for new boilers! Simply fill out our online form, answer a few questions, and receive your quote within minutes. Upgrade to a more efficient boiler today and enjoy reliable heating and hot water all year round.

Conclusion

The Glow Worm F9 fault code is a warning that your boiler is experiencing water pressure issues, which could stem from a faulty pressure sensor, leaks, airlocks, or other system faults. 

While some causes can be easily fixed by repressurising or bleeding the system, others may require professional help. 

If you’re unable to resolve the issue or if your boiler continues to display error codes, it’s always a good idea to contact a Gas Safe registered engineer for a thorough inspection.

By following the steps outlined above, you can quickly identify and fix the root cause of the F9 fault. And if you find that your boiler needs replacing, don’t hesitate to request a free quote for a new unit tailored to your needs.