A Glow-worm F75 fault code usually points to a problem with boiler pressure detection or water circulation. In many cases the issue is linked to the pressure sensor, circulation pump or system pressure.
If your heating or hot water has stopped, this guide explains what the fault means, the most common causes and the checks you can safely carry out before calling an engineer. If you're seeing a different error, our guide to Glow-worm boiler fault codes explains what each code means and the next steps.
- Content Guide
- » What Does the Glow-worm F75 Fault Code Mean?
- » What Causes a Glow-worm F75 Fault?
- - Faulty or Blocked Pressure Sensor
- - Circulation Pump Problems
- - Low Boiler Pressure
- » How to Fix a Glow-worm F75 Fault
- - Reset the Boiler
- - Check the Boiler Pressure
- - When DIY Stops
- » When to Call a Gas Safe Engineer
- » How to Help Prevent Another F75 Fault
- » Need Help With a Glow-worm F75 Fault?
What Does the Glow-worm F75 Fault Code Mean?
The Glow-worm F75 fault code appears when the boiler cannot detect the expected change in water pressure during start-up.
The pressure sensor monitors water pressure and sends information to the printed circuit board (PCB), which controls how the boiler operates safely.
If the PCB cannot detect the expected pressure increase, it locks the boiler and displays the F75 error.
Common symptoms include:
- F75 displayed on the screen
- No heating
- No hot water
- Gurgling, banging or vibrating noises
What Causes a Glow-worm F75 Fault?
The fault is usually caused by one of three issues.
Faulty or blocked pressure sensor
Over time, limescale, sludge and debris can affect the pressure sensor, preventing accurate readings.
Electrical failure can also stop the sensor from communicating with the PCB.
Circulation pump problems
The pump moves heated water around your heating system.
If it sticks, develops worn bearings, contains air or becomes blocked with sludge, the boiler may not detect sufficient pressure movement and the F75 code appears. Common signs include:
- Vibrating pump
- Banging noises
- Pump failing to start
- Airlocks within the system
Low boiler pressure
Low system pressure is less commonly the only cause but can contribute to an F75 fault.
Pressure may fall because of:
- Small leaks
- Faulty filling loop
- Water loss from the heating system
If pressure is low, restoring it may temporarily clear the fault, although the underlying issue may still need repairing.
How to Fix a Glow-worm F75 Fault
Some basic checks are safe for homeowners.
Reset the boiler
Hold the reset button for around five seconds following the manufacturer's instructions.
If the fault clears but quickly returns, further investigation is needed.
Check the boiler pressure
If pressure has dropped below the recommended level, repressurise the boiler using the filling loop until it reaches around 1.0–1.5 bar.
Only do this if you're confident following the manufacturer's guidance.
If pressure repeatedly falls, stop topping it up and arrange a repair.
When DIY Stops
Problems involving pressure sensors, circulation pumps or the PCB require diagnosis by a Gas Safe registered engineer.
Attempting internal boiler repairs yourself is unsafe. If the fault won't clear or keeps returning, arrange a professional boiler repair in London with one of our Gas Safe engineers.
When to Call a Gas Safe Engineer
Our engineers diagnose Glow-worm boiler faults every day and carry replacement parts for many common repairs. We aim to identify the fault quickly and provide an accurate quotation before any work begins.
How to Help Prevent Another F75 Fault
Regular maintenance helps reduce the chances of the F75 fault returning. We recommend:
- Annual boiler servicing
- Keeping the correct system pressure
- Installing a magnetic filter where appropriate
- Treating heating systems with inhibitor
- Addressing leaks before they become larger problems
Need Help With a Glow-worm F75 Fault?
If your Glow-worm boiler is displaying an F75 fault code and simple checks haven't solved the problem, book a Gas Safe engineer.
We'll diagnose the fault, explain what's failed and carry out the repair where possible on the first visit.
