Glow-worm Fuelsaver Mk 1 & Mk 2 Boiler Guide

The Glow-worm Fuelsaver Mk 1 and Mk 2 boilers were solidly built, but age, corrosion, and outdated components now cause recurring issues. Both generations share similar internals, though the Mk 2 introduced small efficiency and safety improvements. Most surviving units are now more than 30 years old, so mechanical and electrical reliability has declined sharply.

Pilot light keeps going out (Mk 1 and Mk 2 – 40, 50 models)

The smaller Fuelsaver 40 and 50 units often suffer from pilot lights that fail to stay lit. This typically results from a weakened thermocouple, a dirty pilot jet, or a gas valve that no longer maintains stable flow at low pressure. Carbon deposits around the jet restrict gas flow, while age-related wear on the thermocouple stops it from detecting heat correctly. Cleaning or replacing these components may restore function temporarily, but most parts are now discontinued, so recurring failure is common.

Ignition and flame failure (Mk 2 – 50, 60, 70 models)

The Mk 2 introduced a spark ignition system, which tends to degrade over time due to corroded electrodes or broken ignition leads. When the spark becomes weak or inconsistent, the boiler may light and immediately shut down. The issue can also stem from worn gas valves or low inlet pressure. These faults are typical in the mid-range Mk 2 50 and 60 models, where ageing control boards and electrodes struggle to maintain a steady flame.

Overheating and lockouts (Mk 1 and Mk 2 – 70, 80 models)

Larger Fuelsaver boilers like the 70 and 80 generate higher heat loads, which makes them vulnerable to overheating as scale builds in the heat exchanger. Restricted flow, a faulty thermostat, or a failing circulation pump can cause the boiler to overheat and trip the cut-out. Once scale forms, it insulates internal surfaces, forcing the boiler to run hotter and further accelerating wear. These faults are a common sign the system has reached the end of its reliable lifespan.

Kettling noises (All models, particularly Mk 1 60–80)

A loud rumbling or boiling sound during operation is caused by limescale buildup inside the heat exchanger. The Mk 1 range is especially prone to this, as its original copper exchangers were less resistant to scaling. The noise indicates localised boiling, which puts stress on joints and seals. A system flush can reduce the sound, but the buildup usually returns within months, especially in hard water areas.

Leaking or corroded components (Mk 1 and Mk 2 – 70 and 80)

High-output models frequently develop small leaks around the pump housing, heat exchanger joints, or the pressure relief valve. After decades of heating cycles, gaskets and seals lose elasticity, and corrosion eats through copper fittings. Once the heat exchanger itself starts to leak, repair is no longer practical, as replacements have been discontinued. Engineers often advise replacement when multiple small leaks appear at once.

Circulation issues and cold radiators (Mk 2 – 50, 60 models)

When radiators warm slowly or unevenly, it usually means sludge has built up inside the system. The Mk 2’s smaller waterways make it particularly sensitive to flow restrictions. A partial blockage in the pump or return pipework can cause the boiler to short cycle, running inefficiently and failing to heat the property evenly. Power flushing may provide short-term improvement, but without new components, it rarely solves the issue long term.

Thermostat and control faults (Mk 1 and Mk 2 – 80 models)

The mechanical thermostats fitted to the higher-output models often lose calibration or fail entirely after decades of use. This results in overheating or inconsistent temperature control. Original thermostats are obsolete, and retrofitting newer digital controls usually requires full rewiring and adaptation of the boiler’s internal circuitry, which is rarely economical.

Gas valve sticking or failure (All models, more common in Mk 2)

Gas valves across the Fuelsaver series can become stiff or fail to open fully due to internal corrosion. The Mk 2 models, which use electrically operated valves, are especially prone to coil failure from moisture and age. This leads to intermittent ignition or weak flame output. Replacement valves are now discontinued, leaving refurbishment as the only option, which is unreliable and not recommended for long-term use.

Flue and ventilation issues (All models)

Every Fuelsaver Mk 1 and Mk 2 uses an open flue, which depends on proper airflow from the room. Blocked or partially obstructed flues cause incomplete combustion and increase the risk of carbon monoxide. Corroded flue pipes are also a common safety concern. Even if the boiler operates safely, the open-flue design is far less efficient and more vulnerable to draught-related faults than modern room-sealed systems.

General age-related decline (All models)

After decades of use, wear on internal wiring, seals, and connections leads to ongoing faults that are costly to isolate. Replacement parts for these boilers are increasingly unavailable, meaning engineers often use refurbished components that provide only temporary relief. For most households, replacing the Fuelsaver Mk 1 or Mk 2 with a new A-rated boiler is the only practical and cost-effective option.