Glow-worm Classic Wall-Hung Boiler Range (23C–80B Series) Guide

The Glow-worm Classic Wall-Hung boilers were built to a high standard, but after three decades of service, most have begun to show their age. Common problems include ignition failures, leaks, and pressure loss. These faults occur across the 23C, 24Ci, 30Ci Plus, 35Ci, and 80B Series, though the specific symptoms often vary by model. Below is a more detailed look at what typically goes wrong and what can realistically be done.

Boiler not firing up
This is one of the most frequent problems on older Classic models, especially the 23C and 24Ci. Homeowners often report hearing a repetitive clicking sound without the burner igniting. The fault usually lies with the ignition PCB or a weak gas valve coil that no longer opens properly. On some units, the spark electrode becomes carbonised, preventing ignition. Engineers can sometimes clean and reset these parts, but permanent fixes are rare because replacements are discontinued. A few refurbished PCBs are still available, though they carry no warranty and can fail again within months.

No hot water or fluctuating temperature
Many combination models in the Classic range, such as the 30Ci Plus and 35Ci, suffer from inconsistent hot water delivery. Limescale builds up inside the plate heat exchanger, restricting flow and reducing heat transfer. You might notice the water running hot for a few seconds, then turning lukewarm. In areas with hard water, this problem can appear every year. Engineers can flush the heat exchanger with descaling chemicals, but efficiency rarely returns to normal. Diverter valve faults also cause similar symptoms. When the valve fails to direct flow correctly between heating and hot water circuits, the boiler either fails to switch over or heats both at once, reducing performance.

Low pressure and frequent lockouts
Most Classic Wall-Hung boilers are sealed systems that rely on consistent water pressure. With age, leaks form around joints or inside the expansion vessel, causing the boiler to lose pressure repeatedly. The 23C and 24Ci are especially prone to this, often triggering a safety lockout. Repressurising may restart the system temporarily, but the root cause is usually a failed expansion vessel diaphragm or corrosion in the pipework. In many cases, even replacing the expansion vessel does not solve the problem because the surrounding components are also deteriorating.

Fan or flue faults (80B Series)
The larger 65–80B models use a fan-assisted flue system. Over time, dust and debris build up around the fan motor, causing noisy operation or complete failure. When the fan does not start, the boiler’s safety circuit prevents ignition. Because genuine replacement fans have been discontinued for many years, engineers are often forced to adapt parts from other units or attempt to repair the existing fan. Neither solution is reliable for long-term use. Flue blockages are also common in these older models, particularly where chimneys have deteriorated internally.

Overheating and kettling noises
A loud rumbling or boiling sound during operation is a clear sign of kettling. It happens when scale or sludge forms inside the heat exchanger, creating hotspots that boil the water unevenly. The 30Ci and 35Ci are particularly vulnerable to this issue. Kettling reduces efficiency and can cause the boiler to trip out on high temperature. Engineers may try a chemical flush or descaler, but results are temporary if the system water quality remains poor. In severe cases, internal scaling damages the heat exchanger beyond repair, leading to complete replacement.

Leaks and corrosion
Internal leaks are a serious concern in any Classic Wall-Hung boiler over twenty years old. The seals and gaskets around the pump housing, diverter valve, and manifold connections degrade over time. When moisture reaches electrical components, the boiler may trip or short out. The 23C and 24Ci are especially susceptible to corrosion around the pump and diverter valve area. Even if the leak seems minor, repeated exposure to moisture often causes further internal damage. Since new replacement parts are no longer produced, repairs are usually a temporary measure.

Ignition lockout and pilot flame failure
Some Classic models, especially early 23C and 24Ci units, suffer from intermittent ignition lockouts where the pilot light goes out or fails to stay lit. This can be caused by a weak flame rectification signal or a faulty thermocouple. If the pilot continues to extinguish itself after cleaning, it usually means the gas valve or flame sensor has worn out. Incomplete combustion can also leave soot on the electrode, making the issue more frequent over time.

Incomplete combustion or sooting
The open-flued 80B Series relies on proper airflow for safe operation. Any obstruction in the flue or lack of ventilation can cause incomplete combustion. Homeowners sometimes notice dark marks or soot deposits around the boiler casing, or the flame burning yellow instead of blue. These are warning signs that carbon monoxide may be forming. The boiler should be switched off immediately and inspected by a Gas Safe registered engineer.

Parts discontinued
This issue affects every model in the Classic Wall-Hung range. Glow-worm stopped manufacturing spares years ago, and most authorised distributors no longer stock original components. While a few reconditioned parts occasionally appear on the market, they are expensive and unreliable. When a key part fails, there is often no safe or practical way to restore the boiler to working condition.

Summary
The Glow-worm Classic Wall-Hung range has delivered decades of service, but the reality is that most remaining units are beyond economical repair. Frequent ignition faults, leaks, and scaling are inevitable at this age. Even when repairs succeed, they tend to be short-lived. Replacing the boiler with a modern A-rated system is the only realistic way to achieve reliable performance, safety, and lower running costs.