How to Fix Glow Worm 15hxi Faults?

Understanding Glow Worm 15hxi Fault Codes

Your Glow Worm 15hxi uses fault codes to communicate problems. These alphanumeric codes appear on the display panel and indicate specific issues within the system. Recognising these codes helps you determine whether a simple reset will solve the problem or if you need professional assistance.

Some faults can be resolved with basic checks, whilst others require a Gas Safe-registered engineer for a professional boiler repair job.

Common Glow Worm 15hxi Faults and Solutions

F1 and F4: Ignition Faults

What it means: Your boiler has failed to light (F1) or has gone out after lighting (F4).

Possible causes:

• No gas supply or insufficient gas pressure
• Frozen condensate pipe (particularly common in winter)
• Faulty ignition electrode or lead
• Defective electronic igniter
• Blocked air inlet duct
• Incorrect gas valve adjustment

What you can do:

1. Check that your gas supply is turned on.
2. Inspect the external condensate pipe (the white plastic pipe outside) for ice. If frozen, carefully thaw it using warm (not boiling) water or a hot water bottle.
3. Reset the boiler by pressing the reset button for several seconds.
4. Check other gas appliances in your home; if they're not working, contact your gas supplier.

If these steps don't resolve the issue, call a Gas Safe engineer. Problems with the ignition electrode, gas valve, or internal components require professional diagnosis and repair.

Take a look at our blog “How Often Do Boiler Electrodes Need Replacing?” for a clearer understanding of maintenance timelines.

F5: Overheat Fault

What it means: Your boiler has overheated, triggering the safety thermostat.

Possible causes:

• Blocked or sludge-filled heat exchanger
• Pump failure is preventing proper water circulation
• Air trapped in the system
• Faulty overheat thermostat connection
• All radiator valves are closed, restricting flow

What you can do:

1. Ensure at least one radiator valve is fully open
2. Check that the pump is running when heating is on
3. Bleed your radiators to remove trapped air
4. Reset the boiler after addressing circulation issues

Repeated F5 faults often indicate system sludge or a failing pump. A professional power flush and pump inspection can prevent further problems and improve efficiency.

F6 and F10: Thermistor Faults

What it means: The temperature sensors on your flow (F6) or return (F10) pipes are faulty or disconnected.

Possible causes:

• Broken or defective thermistor cable
• The thermistor is not correctly attached to the pipe
• Faulty temperature sensor

What you can do:
Check that the thermistor is securely clipped to the heating pipe. Beyond this visual check, thermistor faults require professional diagnosis and replacement.

F9 and F22: Low Water Pressure

What it means: System pressure has dropped below the minimum operating level.

Possible causes:

• System leak
• Recent bleeding of radiators
• Faulty pressure sensor
• Pressure relief valve discharged

What you can do:

1. Check the pressure gauge, it should read between 1 and 1.5 bar
2. Locate your external filling loop (usually under the boiler)
3. Slowly open both valves to repressurise the system
4. Close the valves once pressure reaches 1.2 bar
5. Reset the boiler

F14 and F24: High Temperature Faults

What it means: Flow temperature exceeds 95°C (F14) or return temperature exceeds 90°C (F24).

Possible causes:

• Pump failure
• System circulation problems
• Faulty flow or return thermistor

What you can do:
These faults indicate serious circulation issues that require immediate professional attention. Turn off the boiler and contact a qualified engineer.

F11, F12, F13: Connection Faults

What it means: Communication problems between the main PCB and the user interface.

Possible causes:

• Loose or damaged wiring
• Faulty connections
• PCB or display panel failure

What you can do:
These electrical faults require professional diagnosis. The Glow Worm 15hxi range is particularly prone to PCB failures, especially after power cuts or when the boiler has been switched off.

No Display or Power

What it means: The user interface is completely blank.

Possible causes:

• Faulty main circuit board (PCB)
• Blown fuse
• Power supply issues
• Loose electrical connections

What you can do:
Check your consumer unit (fuse box) to ensure the boiler's circuit hasn't tripped. If power is reaching the boiler but the display remains blank, you likely need a new PCB, a common failure point in the HXi range.

Glow Worm 15hxi Not Firing Up

If your boiler won't ignite and you're hearing a pulsing noise with fluctuating temperature readings, this suggests problems with flame detection or the air pressure switch.

Check the following:

• Ignition electrode and flame sensor for dirt or damage
• Fan operation (you should hear it running)
• Air pressure switch tubing for blockages
• Condensate pipe for blockages or freezing
• The gas supply valve is fully open

Reset the boiler and observe the ignition sequence. If it still won't fire, the PCB, gas valve, or flame detection components may need professional testing and replacement.

The Importance of Regular Boiler Servicing

Many Glow Worm 15hxi faults can be prevented with annual servicing. A qualified engineer will:

• Clean the burner and heat exchanger
• Test safety devices and controls
• Check for internal corrosion or wear
• Verify the correct gas pressure and combustion
• Inspect electrical connections
• Test the condensate trap and discharge pipe

Regular maintenance and annual boiler service extend your boiler's lifespan, maintains efficiency, and helps diagnose potential problems before they cause breakdowns. It's also essential for maintaining your warranty and ensuring your household's safety.

When to Call Serviceteam

Whilst some basic checks can be performed safely, most Glow Worm 15hxi repairs require a Gas Safe registered engineer.

Contact Serviceteam immediately if:

• You smell gas (turn off the gas supply and call the National Gas Emergency Service on 0800 111 999)
• Basic troubleshooting hasn't resolved the fault
• The fault involves electrical components or the PCB
• You're unsure about any procedure
• The problem keeps recurring
• You notice unusual noises, smells, or leaks

Serviceteam's experienced engineers are fully qualified to diagnose and repair all Glow Worm boiler faults.

Why Choose Professional Boiler Repair?

Attempting DIY repairs on gas appliances is illegal unless you're Gas Safe registered. Beyond the legal requirements, professional repair ensures:

• Safety: Gas leaks and carbon monoxide pose serious health risks
• Warranty protection: DIY repairs typically void manufacturer warranties
• Correct diagnosis: Similar symptoms can have different causes
• Quality parts: Genuine replacements maintain performance and safety
• Guaranteed work: Professional repairs come with warranties

The Glow Worm 15hxi has known vulnerabilities, particularly with PCB failures. An experienced engineer familiar with this model can quickly identify common issues and implement lasting solutions.

Protect Your Heating System

Don't let a small problem become a major breakdown. Regular servicing and prompt repairs keep your boiler running efficiently and safely. Serviceteam offers comprehensive boiler repair services across London, with Gas Safe-registered engineers available for emergency callouts and routine maintenance.

Contact Serviceteam today to book your boiler service or repair.

Frequently Asked Questions

What do the fault symbols mean on a Glow-worm boiler?

Glow-worm boilers display fault symbols to indicate specific issues. For example, F1 points to an ignition or gas supply problem, F2 relates to a flame fault, F3 signals an issue with the fan or air pressure switch, and F4 indicates a fault with the heat exchanger. These codes help identify the cause of a breakdown, allowing the right repair to be carried out.

Why is my Glow-worm boiler not heating up?

A Glow-worm boiler that isn't heating up and shows no fault code may be caused by a blocked plate heat exchanger, which stops heat from transferring properly, or a faulty diverter valve that fails to direct hot water where it's needed.

What is the best pressure for a Glow-worm boiler?

The ideal pressure for a Glow-worm boiler is between 1 and 1.5 bar. Keep an eye on the gauge and avoid letting it rise above 2.5 bar, as excessive pressure can damage the boiler and the heating system.