A Glow-worm F5 fault usually means your boiler has overheated. This often happens because water is not circulating properly through the heating system. If left unresolved, the boiler may stop providing heating and hot water.
Some causes are straightforward, such as low system pressure. Others require professional diagnosis. This guide explains the most common reasons for the fault, the checks you can carry out safely, and when it's time to call a Gas Safe engineer.
- Content Guide
- » What Does the Glow-worm F5 Fault Mean?
- » Common Causes of a Glow-worm F5 Fault
- - Low system pressure
- - Faulty thermistor
- - Blocked heat exchanger or pipework
- - Faulty circulation pump
- - Pressure relief valve problems
- - PCB fault
- » How to Troubleshoot a Glow-worm F5 Fault
- - Check the boiler pressure
- - Listen to the pump
- - Check for blocked pipework
- - Reset the boiler
- » When to Call a Gas Safe Engineer
- » Typical Glow-worm F5 Repair Costs
- » How Serviceteam Can Help
What Does the Glow-worm F5 Fault Mean?
The Glow-worm F5 fault code indicates the boiler has detected overheating.
In many cases, poor water circulation is responsible. Low pressure, a faulty pump, blocked pipework, a failed thermistor or an internal component fault can all trigger the error. If you're seeing a different error, our guide to Glow-worm boiler fault codes explains what each code means and the checks you can carry out first.
Common Causes of a Glow-worm F5 Fault
Low system pressure
If pressure falls below the recommended level, water cannot circulate correctly and the boiler may overheat.
Faulty thermistor
The thermistor monitors water temperature. Incorrect readings can cause the boiler to display an F5 fault.
Blocked heat exchanger or pipework
Sludge, debris and limescale restrict water flow. This increases operating temperatures and may trigger the fault.
Faulty circulation pump
If the pump is not moving water around the heating system efficiently, overheating can occur.
Pressure relief valve problems
A faulty pressure relief valve can affect system pressure and contribute to circulation issues.
PCB fault
The printed circuit board controls the boiler's operation. If it develops a fault, incorrect error codes or overheating issues may follow.
How to Troubleshoot a Glow-worm F5 Fault
Check the boiler pressure
Locate the pressure gauge. Most Glow-worm boilers should operate at around 1 to 1.5 bar when cold. If pressure is low, top it up using the filling loop following the manufacturer's instructions.
Listen to the pump
Turn the boiler off first. If the pump is making grinding, buzzing or banging noises, it may not be circulating water correctly.
Check for blocked pipework
Look for obvious signs of sludge or restricted flow around visible pipework. Internal blockages or a blocked heat exchanger normally require professional cleaning.
Reset the boiler
Some temporary faults clear after a reset. Switch the boiler off, wait around one minute, then follow the manufacturer's reset procedure. If the code returns, further investigation is needed.
Never remove boiler covers or attempt repairs yourself.
When to Call a Gas Safe Engineer
If the F5 fault keeps returning after these checks, arrange a professional inspection. A Gas Safe engineer can diagnose faults involving:
- Faulty pumps
- Thermistors
- Heat exchangers
- Printed circuit boards
- Internal circulation problems
These repairs require specialist tools and should not be carried out by homeowners. If the fault doesn't clear or keeps returning, book our boiler repair service in London and one of our Gas Safe engineers will diagnose the problem.
Typical Glow-worm F5 Repair Costs
Simple issues such as repressurising the system are relatively inexpensive.
Replacing components such as pumps, PCBs or heat exchangers is more costly. The final repair cost depends on the failed part, labour required and the age of the boiler.
How Serviceteam Can Help
Our Gas Safe engineers diagnose and repair Glow-worm boiler faults across London.
We provide clear advice, transparent pricing and repairs for a wide range of Glow-worm boiler problems. If replacement is the better option, we'll explain why before any work begins.
If your boiler keeps showing the F5 fault after the basic checks above, don't leave it running. Book a visit with one of our Gas Safe engineers for fast fault diagnosis and professional boiler repairs across London.
