Glow-worm F5 Fault Guide for Homeowners: Meaning, Symptoms, Safety, Causes, Prevention

The Glow-worm F5 fault code indicates a problem with the flow or return temperature sensor. F5 fault code is displayed when your boiler is overheating. Symptoms include the boiler not working to prevent it from damaging itself. Operating with this fault is unsafe and may damage internal components. Causes range from limescale build-up, faulty or failed components, and airway blockages. Prevention is through regular professional maintenance. Use a Gas Safe registered engineer for diagnosing and addressing the F5 fault for safety reasons.

Meaning of the Glow-worm F5 Fault

The Glow-worm F5 fault code means the boiler has shut down because internal temperatures exceed safe operating levels. The boiler will not restart – this is known as locking out. This halts all heating and hot water supply, which means your boiler won’t work until it’s repaired.

What Are the Risks Associated with the F5 Fault?

The main risk of the F5 fault is damage to the boiler’s internal components. This occurs if the boiler continues to operate while too hot. There are also safety concerns, as overheating can lead to the risk of fires.

Can I Resolve the F5 Fault on My Own?

No, you cannot resolve the F5 fault on your own, you will need professional help. Check your owner’s manual in case there are some safe steps for your model that you can take. Clearing limescale build-up, replacing faulty components, and removing blockages are all jobs for a Gas Safe engineer.

How Does a Boiler Engineer Fix an Overheat Fault?

A professional will start by finding the cause of overheating. This may involve:

• Checking for limescale and clearing it from the heat exchanger.
• Testing the pressure release valve (PRV) and thermistors
• Inspecting the pump for failures
• Making sure there are no air blockages.

Symptoms of the Glow-worm F5 Fault

There are 4 main symptoms of the F5 fault code: boiler lockout, no hot water or heating, display showing error code F5, and unusual noises.

• Boiler Lockout: The boiler shuts down to prevent damage from overheating.
• No Hot Water or Heating: The boiler is shut down and ceases to provide hot water and heating.
• Error Codes: The display panel on the boiler will show the F5 fault code, signalling an overheat issue.
• Unusual Noises: Knocking, hissing, or gurgling sounds. These may occur due to boiling water within the system, indicative of overheating.

Recognising the F5 Fault Across Different Glow-worm Boilers

Combi Boilers: For Glow-worm Combi boilers, the F5 fault will be clearly displayed on the digital screen.

System Boilers: Like Combi boilers, System models will display the F5 fault code on their digital display.

Older Non-Digital Boilers: You cannot easily recognise the F5 code on older models without a digital display. In this case, your boiler engineer will diagnose the problem during a visit.

The following section explores the actions to take if your Glow-worm boiler displays an F5 fault.

What to Do When Your Boiler Shows the F5 Fault

When your boiler shows the F5 fault, check your manual for any recommended actions and if you cannot resolve it from that, call a Gas Safe registered engineer. Do not attempt to reset the boiler.

When dealing with gas appliances, always put safety first.

• Gas Safety: Working on gas appliances, including boilers, should only be done by professionals who are Gas Safe registered. Attempting repairs or diagnostics yourself can pose serious risks, including gas leaks, carbon monoxide exposure, and fires.
• Professional Assistance: Always enlist the help of a Gas Safe registered engineer to diagnose and repair faults with your boiler. They have the training, experience, and tools to safely and effectively address the issue.

Should I Turn Off My Boiler?

Yes, turning off the boiler until a professional can assess it is generally a safe action. However, consult your boiler’s manual to ensure this is recommended for an F5 fault.

Do I Need to Shut Off the Gas Supply?

Shutting off the gas supply to your entire home is usually not necessary unless you smell gas or suspect a leak. In those cases, turn off the gas, ventilate the area, and contact emergency services or a Gas Safe engineer immediately. 

Causes of the Glow-worm F5 Fault

The F5 fault in Glow-worm boilers is caused by 3 main issues, limescale build-up, faulty components (pressure release valve, thermistor, or water pump) or blocked airways preventing the boiler from cooling. This list explains these issues in more detail.

• Limescale Build-up: Limescale accumulation, especially in the heat exchanger, can restrict the transfer of heat, resulting in the boiler operating above normal temperatures and overheating.
• Faulty Pressure Release Valve (PRV): The PRV is designed to relieve excess pressure within the boiler. If it fails and cannot open, pressure can build up and lead to overheating.
• Failed Thermistor: Thermistors monitor the temperature of water within the boiler. A failure here can lead to incorrect temperature readings, causing the boiler to overheat.
• Pump Failure: The pump circulates water through the boiler and heating system. If it fails, water may not circulate properly, causing localised overheating.
• Airway blockages: in the flue and condensate pipe can prevent the hot gases produced by the boiler from being expelled. These build up and prevent cooler air getting in, so the boiler overheats.

How Does Limescale Lead to My Boiler Overheating?

Limescale acts as an insulator on heat exchanger surfaces, preventing efficient heat transfer. This forces the boiler to work harder and hotter, increasing the risk of overheating.

What Should I Do If My Boiler’s Pressure Release Valve Is Faulty?

If you suspect the PRV is faulty, do not attempt to repair it yourself. A Gas Safe registered engineer should assess and replace it if necessary.

Can I Test the Thermistor Myself?

No, you cannot test the thermistor yourself. It’s best to leave this to a professional.

What Signs Indicate Pump Failure?

Signs of pump failure include poor heating performance, unusual noises from the pump, and localised overheating. If you notice these signs, a professional inspection is needed.

How Can I Prevent the F5 Fault?

Regular maintenance and checks by a Gas Safe engineer help to keep your boiler operating efficiently and catch potential issues early.

How often should my boiler be serviced?

It’s recommended to have your boiler serviced annually by a Gas Safe registered engineer.

How can I prevent limescale buildup in my boiler?

Discuss with your heating professional during annual servicing. Solutions include a water softener for your home’s water supply, or a chemical flush by a heating engineer. 

Is pressure flushing my central heating a good idea?

We do not recommend pressure flushes because of the strain this can place on your plumbing.

Safety Disclaimer

This guide aims to provide homeowners with information about the F5 fault in Glow-worm boilers. There are considerable dangers and risks associated with attempting to diagnose or repair gas appliances without proper expertise and certification.

Risks of DIY Repairs on Gas Appliances

• Gas Leaks: Incorrect handling can lead to gas leaks, posing serious risks of fire or explosion.
• Carbon Monoxide Poisoning: Improper repairs may result in incomplete combustion, leading to carbon monoxide buildup, which is a colourless, odourless gas that can be deadly.
• Legal and Insurance Implications: In the UK, gas appliance repairs and modifications must be carried out by certified professionals. Unauthorised work can void warranties, break the law, and potentially invalidate insurance claims in the event of an accident.

Importance of Using a Gas Safe Registered Engineer

To ensure the safety and proper functioning of your boiler, all servicing, repairs, or modifications should be performed by a Gas Safe registered engineer. They have undergone extensive training and certification to work safely and effectively on gas appliances, adhering to strict safety standards and regulations.

• Certification: Always verify the engineer’s Gas Safe registration to ensure they are legally permitted to work on gas appliances.
• Experience: Certified engineers possess the necessary experience to diagnose and repair faults safely, reducing the risk of further damage or safety hazards.