Glow-worm Space Saver Mk 1 & Mk 2 Boiler Guide

The Glow-worm Space Saver Mk 1 and Mk 2 ranges were known for their compact size and simple operation, but with most units now installed over 30 years ago, reliability has declined sharply. These boilers use basic open-flue systems, and many components have long exceeded their expected service life. While the Mk 2 models were a mild improvement on the Mk 1 design, both generations share similar fault patterns, with some issues more common on specific versions.

Pilot light goes out (Mk 1 and Mk 2 – 40 and 50 models)

The smaller Space Saver 40 and 50 models often experience pilot lights that fail to stay lit. This happens when the thermocouple no longer responds properly to the pilot flame or when dust or carbon buildup restricts the pilot jet. In some cases, draughts through the flue or room ventilation cause the flame to flicker and extinguish. Replacing the thermocouple or cleaning the pilot assembly usually resolves the issue temporarily, but frequent failure often indicates a worn gas valve or a weakened flame pattern.

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

The Mk 2 introduced a more advanced ignition sequence, which can fail over time due to corroded electrodes or weak spark generators. Poor electrical contact between the ignition lead and electrode is common after years of vibration and thermal cycling. When this happens, the boiler clicks repeatedly without lighting or lights briefly before cutting out. In many cases, these ignition components are now unavailable as new parts, forcing repairs to rely on refurbished alternatives.

Overheating and cycling (Mk 1 and Mk 2 – 70 and 80 models)

The larger 70 and 80 models produce more heat, which increases the risk of overheating as scale builds up inside the heat exchanger. Restricted water flow, a sluggish circulation pump, or a faulty thermostat can all cause the boiler to run too hot and shut down on safety. Overheating also leads to expansion noise and additional stress on pipe joints, which can eventually result in leaks. These symptoms are a clear indicator that the system is nearing the end of its serviceable life.

Kettling or boiling noises (All models, especially Mk 1)

A rumbling or boiling sound during operation indicates limescale buildup on the heat exchanger surfaces. The Mk 1 models are particularly prone to this because of the exchanger’s narrow waterways and less efficient heat distribution. The noise is caused by localised boiling where water flow is restricted. While descaling treatments can reduce the sound, the effect is short-lived once internal corrosion and scale are established.

Leaks and corrosion (Mk 1 and Mk 2 – 70 and 80 models)

With age, the copper joints and pump housings on the larger Space Saver units often corrode, leading to small but persistent leaks. The heat exchanger itself may also develop hairline cracks due to thermal expansion over decades of use. Once the exchanger starts leaking, replacement is not viable since genuine parts are no longer produced. Even minor leaks can lead to electrical faults or pressure loss if left unresolved.

Poor circulation and cold radiators (Mk 2 – 50 and 60 models)

When radiators heat unevenly or the system takes longer to warm up, it usually points to sludge buildup or a failing circulation pump. The mid-sized Mk 2 models rely on small impellers that lose performance over time. Sludge from internal corrosion restricts flow, causing the boiler to short cycle or overheat. Power flushing can provide short-term relief, but repeated circulation problems are a sign of ageing components throughout the system.

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

Mechanical thermostats used in the larger Space Saver models often drift out of calibration with age, resulting in irregular temperature control. Some units short cycle, while others run continuously. Original thermostats and control boards are no longer manufactured, and most modern controls are not directly compatible with the boiler’s wiring. Replacing them is possible only through complex rewiring, which rarely makes financial sense given the boiler’s age.

Flue and ventilation problems (All models)

Every Space Saver Mk 1 and Mk 2 boiler uses an open-flue design, drawing air from the surrounding room. Over time, flue terminals corrode, and internal deposits can restrict exhaust flow. Blockages or poor ventilation cause incomplete combustion and can create a carbon monoxide risk. Many older installations also lack modern safety sensors. Regular checks are important, but even a clean flue cannot bring these systems up to current safety standards.

General age-related deterioration (All models)

After decades of use, internal wiring, seals, and joints deteriorate, leading to intermittent electrical faults, leaks, and reduced efficiency. With key components like gas valves, pumps, and thermocouples discontinued, even straightforward repairs are increasingly difficult. Engineers often recommend full replacement once multiple minor issues appear within a short period.