How to Fix Glow-worm 15hxi Faults

The Glow-worm Ultracom 15hxi is a regular (heat-only) condensing boiler. Like any heating system, it can develop faults over time, and the displayed error code can help indicate whether a simple external check may restore operation or whether professional attention is needed.

This guide explains the most common Glow-worm 15hxi faults, their likely causes and the safe checks you can carry out before calling an engineer.

Understanding Glow-worm 15hxi Fault Codes

Glow-worm boilers display fault codes whenever they detect a problem. These codes help narrow down the cause of the issue and speed up diagnosis.

Some faults can be resolved with simple checks, such as restoring system pressure or resetting the boiler. Others involve gas, electrical or safety components and should only be repaired by a Gas Safe registered engineer.

Never attempt to dismantle or repair a gas boiler yourself.

If you're troubleshooting a different model or error code, see our complete guide to Glow-worm boiler fault codes.

F1 and F4 Ignition Faults

What it means: the boiler failed to ignite (F1) or the flame went out after ignition (F4).

Common causes:

• No gas supply
• Low gas pressure
• Frozen condensate pipe
• Faulty ignition electrode
• Faulty gas valve
• Blocked air intake

Safe checks:

• Confirm your gas supply is on.
• Check whether other gas appliances are working.
• Inspect the condensate pipe for freezing during cold weather.
• Reset the boiler following the manufacturer's instructions.

If the fault returns, the ignition system or gas valve should be tested by a qualified engineer.

F5 Overheat Fault

This code means the boiler has overheated. Possible causes include:

• Pump failure
• Blocked heat exchanger
• Air in the heating system
• Restricted water circulation

Check that at least one radiator valve is open and bleed radiators if required. If the fault continues, the circulation system should be inspected professionally.

F6 and F10 Thermistor Faults

These faults indicate a problem with the flow or return temperature sensors. Possible causes include:

• Damaged wiring
• Loose thermistor
• Failed temperature sensor

Do not remove the casing or inspect the thermistors yourself. Sensor, wiring and resistance checks should be carried out by a qualified engineer.

F9 and F22 Low Water Pressure

Low pressure is one of the most common Glow-worm 15hxi faults.

Check that system pressure is between 1 and 1.5 bar. If it's low:

• Open the filling loop slowly.
• Refill to around 1.2 bar.
• Close both valves.
• Reset the boiler.

If pressure continues to fall, there may be a leak or another system fault that requires repair.

F14 and F24 High Temperature Faults

These codes indicate excessive water temperature inside the boiler. Common causes include:

• Pump failure
• Poor circulation
• Faulty thermistor

Turn the boiler off and arrange a professional inspection. Continued operation could damage internal components.

F11, F12 and F13 Connection Faults

These faults indicate communication problems between the PCB and user interface. They are commonly caused by:

• Loose wiring
• Damaged connections
• PCB failure

These faults require electrical testing and component diagnosis by a Gas Safe engineer.

No Display or No Power

A blank display usually indicates:

• Electrical supply problems
• Blown fuse
• Faulty PCB
• Loose electrical connections

Check your consumer unit first. If the boiler has power but remains blank, professional diagnosis is recommended.

Glow-worm 15hxi Not Firing Up

If the boiler will not ignite, safe external checks are limited to:

• Gas supply
• Condensate pipe
• Whether other gas appliances are working
• A single reset using the manufacturer's instructions
• No further internal checks as a homeowner

If it still fails to fire after the safe external checks, the fan, air-pressure system, ignition/flame-proving components, gas valve or PCB may need engineer diagnosis.

Preventing Future Faults

Annual servicing helps identify worn parts before they fail. A boiler service normally includes:

• Cleaning the burner
• Inspecting the heat exchanger
• Testing safety controls
• Checking gas pressure
• Inspecting electrical connections
• Checking the condensate system

Regular servicing improves reliability and helps maintain efficiency.

When to Call a Gas Safe Engineer

Arrange professional assistance if:

• You smell gas.
• The boiler repeatedly locks out.
• The fault returns after resetting.
• The boiler leaks.
• The fault involves electrical or gas components.
• You are unsure how to proceed safely.

If these checks don't resolve the issue, book a professional Glow-worm boiler repair in London with one of our Gas Safe engineers.

Why Professional Boiler Repair Matters

Gas boilers contain safety-critical components. Professional repairs provide:

• Safe diagnosis
• Correct replacement parts
• Reliable testing
• Warranty protection
• Compliance with Gas Safe regulations

The Glow-worm 15hxi is known to experience PCB and ignition-related faults as it ages, making accurate diagnosis particularly important.

Frequently Asked Questions - (FAQs)

Fault symbols indicate specific operating faults. The displayed code identifies which system requires investigation.

Common causes include low pressure, ignition faults, circulation problems or faulty internal components.

Normal cold operating pressure is around 1–1.5 bar.

Need Help With a Glow-worm 15hxi Fault?

If your boiler keeps displaying an error code, won't reset or has stopped producing heating or hot water, our Gas Safe engineers can diagnose and repair the fault.

Contact Serviceteam today to arrange a boiler repair.