Glow-worm Space Saver Complheat & KFB Boiler Guide

The Glow-worm Space Saver Complheat and KFB ranges were known for their reliability and straightforward design. However, most units have now been in operation for over 30 years, and even well-maintained systems are showing signs of wear. Both series share similar construction, with the KFB acting as an updated version that introduced fan-assisted flue technology for improved efficiency. Despite this, the same fundamental age-related faults are common across both.

Fan failure (KFB models)

The most frequent fault in the KFB range is fan failure. Over time, bearings seize, fan blades collect dust, and the motor windings weaken under heat exposure. Once the fan slows or stops completely, the boiler cannot ignite, as it relies on a proving switch to confirm correct airflow. Symptoms include intermittent starting, humming noises, or complete ignition lockout. Replacements are scarce, and refurbished fans often fail again after a short period.

Ignition and flame failure (Complheat and KFB 50 and 60 models)

Ignition faults are common in both series, typically caused by corroded spark electrodes or damaged ignition leads. On Complheat boilers, weak or intermittent sparks make it difficult for the pilot flame to establish. On KFB units, the added dependency on fan operation means that ignition can also fail if the proving switch malfunctions or detects insufficient airflow. A low or fluctuating gas pressure may cause the flame to ignite and then extinguish shortly after.

Pilot light not staying lit (Complheat 40 and 50)

The smaller Complheat 40 and 50 models often suffer from pilot lights that go out repeatedly. The usual causes are worn thermocouples, blocked pilot jets, or slight draughts through the open flue. When the thermocouple no longer senses the pilot flame correctly, it cuts off the gas supply for safety. Cleaning the pilot assembly may help temporarily, but ongoing failure usually points to deeper wear in the gas valve or combustion chamber.

Overheating and frequent shutdowns (Complheat and KFB 70 and 80 models)

High-output versions such as the Complheat 70 and 80 or KFB 70 and 80 are more prone to overheating. The extra thermal load leads to scaling within the heat exchanger, restricting water flow. A worn circulation pump or thermostat can also prevent proper heat dissipation, causing the boiler to cycle on its overheat cut-out. Repeated overheating puts strain on joints and seals, often leading to small leaks or permanent heat exchanger damage.

Kettling noises (All models)

A deep rumbling or boiling sound is a sure sign of limescale and corrosion buildup in the heat exchanger. It affects all Complheat and KFB boilers, especially those used in hard water areas. As scale accumulates, water flow becomes uneven, creating hot spots that cause bubbling noises during operation. While a chemical flush can quieten the system temporarily, the underlying metal fatigue and corrosion make the issue likely to return.

Leaks and corrosion (Complheat 70 and 80, KFB 70 and 80)

With age, heat stress causes the copper pipework, soldered joints, and pump housing to corrode. Larger models, which operate at higher output temperatures, are most affected. Small leaks may appear at the pump or manifold, and once the heat exchanger itself begins to leak, repair is no longer practical. These leaks can also damage surrounding electrical parts, leading to repeated system failures.

Poor circulation and cold radiators (Complheat 50 and 60)

Sludge buildup and weakened pumps are common in mid-range models. Iron oxide and sediment restrict water flow, resulting in slow radiator heating or uneven warmth across the home. The circulation pump may still run but fail to maintain enough flow to prevent the boiler from overheating. A power flush can offer short-term improvement, but ageing pumps and continued corrosion mean the issue usually reappears.

Thermostat and control failures (Complheat and KFB 80)

Mechanical thermostats and control knobs in the larger models often fail with age, causing short cycling or continuous operation. On the KFB, the added fan control circuit introduces another failure point. Once these components fail, maintaining consistent heating becomes difficult. Replacement thermostats are no longer manufactured, and wiring in modern controls requires modifications that are rarely cost-effective for these systems.

Flue and ventilation issues (Complheat open-flue, KFB fan-assisted)

Complheat boilers rely on open-flue ventilation, meaning that any room airflow disruption or flue blockage can cause incomplete combustion. The KFB’s fan-assisted flue improves safety but adds complexity. If the flue becomes blocked or the proving switch malfunctions, the fan fails to start or the boiler locks out. Corrosion within the flue hood or pressure tubing is also common after decades of use.

General age-related faults (All models)

After decades of operation, corrosion, electrical degradation, and material fatigue affect every part of these systems. Gas valves can stick, pump seals weaken, and wiring becomes brittle. With many parts discontinued, even simple repairs are increasingly difficult to complete safely. Once multiple faults begin to occur within a short period, full replacement is the only practical and reliable option.