Glow-worm Compact Boiler Guide

The Glow-worm Compact range was introduced as a smaller, wall-mounted boiler series for homes with limited space. These units were efficient and easy to service for their time, but most have now been running for more than three decades. As with all older appliances, mechanical fatigue, scaling, and corrosion are now widespread, and replacement parts are becoming increasingly difficult to find. The range includes the Compact 40, 50, 60, 70, and 80, each with slightly different power outputs and internal layouts, but the same set of recurring faults.

Pilot light keeps going out (Compact 40 and 50)

Smaller Compact models, such as the 40 and 50, are well known for pilot light issues. Over time, thermocouples weaken, preventing them from sensing the flame correctly, while the pilot jet collects dust and debris that restrict gas flow. In some cases, draughts entering the open-flue system disturb the flame. Cleaning or replacing the pilot assembly may fix the problem temporarily, but repeated failure is often a sign of a worn gas valve or deteriorated combustion components.

Ignition and flame failure (Compact 60 and 70)

Mid-sized Compact models like the 60 and 70 often experience intermittent ignition or short-lived flames. These issues are usually caused by corroded electrodes, poor electrical grounding, or weakened gas valves. Over time, electrical insulation around the ignition electrode can degrade, producing a weak spark that fails to ignite the gas. Occasionally, low gas pressure or a sticking valve coil can also cause the boiler to light and immediately go out.

Overheating and frequent shutdowns (Compact 70 and 80)

Larger models in the Compact range are more prone to overheating because of the higher output temperatures they operate at. Scale buildup inside the heat exchanger restricts water flow, trapping heat and triggering the overheat thermostat. A sluggish pump or faulty system thermostat can make the issue worse. When overheating becomes a regular occurrence, it often indicates that internal corrosion and scale have reached an advanced stage.

Rumbling or kettling noises (All models, particularly 60–80)

A loud rumbling or boiling sound during operation, often described as kettling, is caused by limescale accumulating inside the heat exchanger. The noise comes from small pockets of water boiling at different temperatures due to uneven heat transfer. This problem is especially noticeable in the 60, 70, and 80 models. A chemical flush can reduce the noise temporarily, but once the metal begins to pit or corrode, the problem quickly returns.

Leaks and corrosion (Compact 70 and 80)

The larger Compact units run hotter for longer periods, putting stress on soldered joints, seals, and the heat exchanger. Over time, these parts corrode and develop small leaks. Even minor drips can cause pressure loss and eventually damage the electrical components below. Once a main heat exchanger begins leaking, repair is rarely feasible, as replacements are no longer produced.

Circulation problems and cold radiators (Compact 50 and 60)

When radiators heat slowly or unevenly, the issue usually lies with sludge buildup or a worn circulation pump. The Compact 50 and 60 models are particularly sensitive to poor circulation due to smaller internal waterways. Iron oxide from corroded radiators restricts flow, which can also lead to short cycling and overheating. A power flush may help temporarily, but if the pump impeller or motor is worn, performance will continue to decline.

Thermostat and control faults (Compact 80)

The mechanical thermostats and control knobs used in the larger Compact 80 often become unreliable after years of use. They can cause erratic operation, short cycling, or a failure to maintain steady heat output. Because replacement parts are now obsolete, modern electronic thermostats cannot be installed directly without extensive rewiring. This makes ongoing repair less viable for these models.

Gas valve sticking or failure (All models)

All Compact boilers use a gas valve that regulates burner pressure. With age, these valves become stiff or stick partially open, leading to delayed ignition or weak flame output. The coil inside the valve can also fail after years of heat exposure. Since genuine replacements are discontinued, engineers are often limited to refurbished parts that may not meet original safety standards.

Flue and ventilation problems (All models)

Every Compact boiler relies on an open-flue design that draws air from the surrounding room. Corrosion in the flue terminal or blockages caused by debris can restrict exhaust gases, leading to incomplete combustion and a risk of carbon monoxide. Even well-maintained flues are vulnerable after decades of exposure. Modern sealed systems are significantly safer and more efficient.

General age and part availability (All models)

After more than 30 years, internal wiring, seals, and pipe fittings naturally degrade. Replacement parts such as gas valves, pumps, and ignition kits are now discontinued, making even minor repairs difficult to complete. Once multiple issues occur within a short period, engineers usually recommend full replacement rather than further repair.

While some Compact models can still be kept running through repair, most are now well beyond their serviceable life. Replacing them with a new A-rated boiler provides long-term efficiency, lower running costs, and better safety for the home.