Glow-worm Fuelsaver Complheat Boiler Guide

The Glow-worm Fuelsaver Complheat range was designed for durability, but with most units now over three decades old, mechanical wear, corrosion, and scaling are widespread. The range includes several models, such as the Complheat 40, 50, 60, 70, and 80, each with slightly different outputs and component layouts. While their basic operation is similar, certain problems are more common in specific models due to design differences in the gas valve, flue system, and heat exchanger construction.

Pilot light not staying lit (Complheat 40 and 50)

The smaller Complheat 40 and 50 models often develop pilot light issues. This usually happens when the thermocouple wears out or when carbon builds up on the pilot jet, reducing gas flow. Even a small amount of debris can cause the flame to go out as soon as the boiler heats up. In some cases, draughts through the open flue disturb the flame. Replacing the thermocouple or cleaning the pilot assembly can restore operation, though repeated failure often points to a deteriorating gas valve.

Ignition and flame failure (Complheat 60 and 70)

Mid-range models like the Complheat 60 and 70 tend to suffer ignition issues caused by corroded spark electrodes or loose wiring connections. Over time, the electrode insulation breaks down, producing a weak spark that struggles to light the gas. Occasionally, a worn gas valve coil causes intermittent ignition or short cycling. These components are now discontinued, meaning engineers often rely on refurbished parts that do not last as long as new ones.

Overheating and shutdowns (Complheat 70 and 80)

The larger output models, particularly the 70 and 80, operate under higher temperatures and are more prone to overheating. Scale buildup inside the heat exchanger restricts water flow and traps heat, causing the boiler to shut down repeatedly. Faulty thermostats or circulation pumps can make the issue worse by preventing heat from being dispersed through the system. In some cases, the overheat cut-out trips so frequently that the boiler becomes unreliable to use.

Rumbling or kettling noises (All models)

A loud boiling or kettling noise is one of the most common complaints with older Fuelsaver units. It is usually caused by limescale accumulating on the heat exchanger walls, which creates hot spots where water turns to steam. The result is a bubbling or rumbling sound during operation. While a chemical flush can quieten the noise for a while, the underlying corrosion and scale tend to return quickly, especially in hard water areas.

Leaking pipe joints and components (Complheat 70 and 80)

After decades of use, leaks are common across the larger 70 and 80 models. Heat stress and corrosion gradually weaken soldered joints, pump seals, and the pressure relief valve. Even small leaks can lower system pressure and trigger lockouts. Once the main heat exchanger or manifold begins to leak, repair becomes impractical, as replacement parts are almost impossible to find.

Sludge and poor circulation (Complheat 50 and 60)

When radiators heat unevenly or the boiler takes longer than usual to warm up, it usually indicates sludge buildup in the system. Iron oxide and debris restrict water flow, especially in the mid-sized 50 and 60 models that rely on smaller internal waterways. Power flushing may improve circulation temporarily, but if the pump impeller is worn or the system continues to corrode, the problem will reappear within months.

Thermostat and control faults (Complheat 80)

The mechanical thermostats used in the larger models often become unreliable after years of operation. Faulty thermostats can cause short cycling, continuous firing, or failure to maintain a steady temperature. The original thermostat assemblies are no longer manufactured, and retrofitting modern electronic controls usually requires full rewiring, which is not cost-effective for a boiler of this age.

Gas valve or PCB failure (Complheat 60, 70, 80)

The gas control valves on the higher-output Complheat boilers occasionally stick or fail due to internal wear. This results in intermittent ignition, flame dropout, or unstable burning. In some models, early printed circuit boards (PCBs) that control ignition sequencing also fail because of heat stress or moisture damage. Neither component is still available new, which makes repair risky and often short-lived.

Flue and ventilation problems (All models)

All Fuelsaver Complheat boilers use an open-flue design that draws combustion air from the room. Any obstruction in the flue outlet or poor ventilation can lead to incomplete combustion and a risk of carbon monoxide. Over time, flue components corrode internally, and deposits can partially block exhaust flow. Annual safety inspections are essential, but even with regular checks, the open-flue design is far less safe than the sealed systems used in modern boilers.

General age and parts scarcity (All models)

Most Fuelsaver Complheat boilers have now exceeded their design life by more than a decade. Corrosion, scaling, and ageing wiring are unavoidable at this stage. Many key components, such as gas valves, pumps, and thermocouples, are discontinued. Even where replacements exist, they are often refurbished or unbranded, with no manufacturer warranty. As a result, repairs provide only temporary relief before new faults develop elsewhere.