How to Fix the Glowworm F75 Fault Code | Step-by-Step Guide
If your boiler is displaying the Glowworm F75 fault code, don’t panic. While this error can prevent your heating and hot water from working, it doesn’t necessarily mean your boiler needs replacing.
The Glowworm F75 fault code is one of the more common faults found on Glow-worm boilers. In many cases, it’s linked to a problem with the boiler’s pressure sensor, water circulation, or pump. Some basic checks can be carried out safely by homeowners, while other repairs require a Gas Safe registered engineer.
In this guide, we’ll explain what the Glowworm F75 fault code means, its most common causes, and the steps you can take to resolve the issue safely.
Table of contents
What Does the Glowworm F75 Fault Code Mean?
The Glowworm F75 fault code indicates that the boiler has failed to detect a change in water pressure when the circulation pump starts.
Your boiler relies on a pressure sensor to monitor the movement and pressure of water through the heating system. This information is sent to the Printed Circuit Board (PCB), which controls the boiler’s operation.
If the PCB cannot confirm that water is circulating correctly, it will place the boiler into lockout mode as a safety precaution and display the F75 fault code.
Common Causes of the Glowworm F75 Fault Code
Several issues can trigger an F75 fault. Below are the most common causes.
1. Faulty Pressure Sensor
The most common reason for the Glowworm F75 fault code is a failed pressure sensor.
The sensor monitors system pressure and communicates with the PCB. If it develops an electrical fault or stops responding correctly, the boiler cannot verify water circulation.
Common signs include:
- Boiler locks out with F75
- Pressure reading remains unchanged when heating starts
- Intermittent heating performance
2. Blocked Pressure Sensor
Over time, heating systems naturally develop sludge made up of rust, debris, and magnetite.
Small particles can block the tiny opening inside the pressure sensor, preventing accurate readings and triggering the fault.
3. Low Boiler Pressure
Low system pressure can also contribute to the Glowworm F75 fault code.
Pressure may gradually reduce because of:
- Small leaks
- Recently bled radiators
- Expansion vessel issues
- Natural pressure loss over time
Most Glow-worm boilers should operate between 1.0 and 2.0 bar, with around 1.2–1.5 bar being ideal when the system is cold.
4. Faulty Boiler Pump
The circulation pump moves heated water around your central heating system.
If the pump isn’t operating correctly, the boiler won’t detect the expected pressure increase.
Common causes include:
- Worn bearings
- Airlocks
- Internal blockages
- Sludge buildup
- Incorrect pump speed settings
- Mechanical failure
Typical symptoms include:
- Vibrating noises
- Clanging or grinding sounds
- Boiler locking out shortly after startup
How to Fix the Glowworm F75 Fault Code
Time needed: 5 minutes
Some checks can safely be carried out before contacting a heating engineer.
- Check the Boiler Pressure
Check your boiler’s pressure gauge to ensure the system pressure is within the recommended range of 1.0–2.0 bar, with 1.2–1.5 bar being ideal when the boiler is cold. If the pressure is too low, use the filling loop to top up the system until it reaches the correct level, then close both filling loop valves securely to prevent over-pressurising the boiler.
- Reset the Boiler
After restoring the pressure:
Locate the reset button.
Press and hold it for several seconds.
Allow the boiler to restart.
If the fault returns immediately, further investigation is required. - Listen to the Pump
When the boiler starts, listen for any unusual grinding, vibrating, or clanging noises and check if the pump is running normally. If the pump is unusually noisy or doesn’t appear to operate at all, it may be faulty and should be inspected or replaced by a Gas Safe registered engineer.
- Check for Signs of Sludge
If your radiators have cold spots, poor circulation, or dirty heating system water, sludge may have built up inside the system. This can restrict water flow and affect components such as the pressure sensor. A heating engineer may recommend a system flush, installing a magnetic filter, or cleaning or replacing the pressure sensor to restore efficient operation.
- Contact a Gas Safe Registered Engineer
If the Glowworm F75 fault code continues to appear after basic troubleshooting, it’s time to contact a Gas Safe registered engineer. They can diagnose the fault by testing the pressure sensor, inspecting the circulation pump, clearing any blockages, checking the PCB, and verifying the expansion vessel pressure. Because the F75 fault involves internal boiler components, you should never remove the boiler casing or attempt repairs yourself, as this can be dangerous and must only be carried out by a qualified professional.
Can You Fix a Glowworm F75 Fault Yourself?
Some basic troubleshooting is safe, including:
- Checking boiler pressure
- Topping up the system
- Resetting the boiler once
- Looking for obvious leaks
However, repairs involving:
- Pressure sensors
- Pumps
- PCB
- Electrical components
should always be left to a qualified professional.
How to Prevent the Glowworm F75 Fault Code
Regular maintenance helps reduce the likelihood of pressure-related faults.
Schedule an Annual Boiler Service
A yearly service helps identify worn components before they fail.
Check Boiler Pressure Monthly
Keep pressure within the manufacturer’s recommended range.
Install a Magnetic Filter
Magnetic filters remove sludge and debris before they reach sensitive boiler components.
Consider a Power Flush
If your heating system contains excessive sludge, a professional power flush can improve circulation and help prevent future faults.
Address Small Leaks Promptly
Persistent pressure loss often indicates a leak that should be repaired before it causes additional damage.
Conclusion
The Glowworm F75 fault code usually indicates that the boiler cannot detect the expected pressure change during startup. While low pressure is sometimes the cause, the fault is more commonly linked to a faulty pressure sensor, blocked sensor, circulation pump issue, or heating system sludge.
Simple checks like topping up the pressure and resetting the boiler may restore normal operation. However, if the fault continues, it’s important to arrange an inspection by a Gas Safe registered engineer to ensure the boiler is repaired safely and efficiently.
Frequently Asked Questions
We always recommend that you speak with a Gas Safe engineer the moment you see the Vaillant F75 fault code pop up on your boiler’s display. However, there are ways to help diagnose the cause of the fault: Press the reset button on your boiler and wait a few minutes.
An F75 fault on a boiler (most commonly found on Vaillant and Glow-worm models) means the system failed to detect a pressure change when the circulation pump started. The boiler shuts down as a safety measure because it cannot confirm water is circulating properly.
To reset a Glow-worm boiler, locate the reset button (often marked with a flame, a cross, or a circular arrow) on the front panel. Press and hold it for about 5 to 10 seconds until the display flickers or resets. Wait 1 to 2 minutes for the boiler to reboot.
