Glow-worm Ultimate Boiler Guide

After twenty years or more, even the hardiest boilers start to show their age. The Glow-worm Ultimate range has proved durable, but like all non-condensing systems from the 1990s and early 2000s, it now faces wear that no amount of servicing can fully prevent. Below are the most frequent issues homeowners encounter, what causes them, and what can realistically be done.

Boiler not firing up
This is one of the most common faults in the Ultimate FF range. Homeowners often notice the fan starts, but the burner never ignites. In most cases, the cause lies with a failed ignition PCB or worn spark electrodes that can no longer create a consistent spark. Low gas pressure or faulty air pressure switches can also stop the boiler from firing. A Gas Safe engineer will usually test the ignition circuit and check gas supply pressure at the inlet. Replacing the PCB or electrodes can cost £250–£400 if parts can still be sourced. Given that most are discontinued, this is often the point where replacement becomes the more practical long-term fix.

Pilot light keeps going out
Older BF models like the 30BF, 50BF, and 80BF use a standing pilot flame that needs to stay lit. If it keeps going out, the thermocouple is usually to blame, it senses the flame and tells the gas valve to stay open. Over time, it gets coated in carbon or simply wears out. Another culprit is a dirty pilot jet that restricts the small flow of gas needed to maintain the flame. You can sometimes relight it successfully, but if it happens repeatedly, it’s a sign the parts are near the end of their life. Most original thermocouples and pilot assemblies are now obsolete, so engineers often have to rely on refurbished components.

Kettling or whistling noises
That loud bubbling or whistling sound isn’t just annoying, it’s a sign of limescale or sludge build-up inside the heat exchanger. In hard water areas, mineral deposits cling to the internal metal surfaces, creating hotspots that cause the boiler to overheat locally. An engineer can attempt to flush the system or treat it with descaler, but the results are rarely permanent. Once limescale takes hold in an older heat exchanger, efficiency drops sharply, and the problem tends to return. Many households choose to replace the unit rather than spend on chemical cleans each year.

Overheating or constant cycling
Some Ultimate boilers start cutting out after running for only a few minutes, or they click on and off repeatedly. This constant cycling wastes gas and strains components. The main causes are usually a blocked system filter, air trapped in the heat exchanger, or a faulty thermostat that no longer reads temperatures correctly. A professional service might restore function for a while, but when overheating keeps returning, it’s often down to general system corrosion and poor water circulation. It’s not dangerous if handled early, but continued overheating can damage the PCB or fan.

No heating or hot water
If radiators stay cold or taps run lukewarm, the issue might not be inside the boiler itself but within the wider system. Common culprits are failed pumps, jammed diverter valves, or airlocks in pipework. Older Glow-worms rely on mechanical valves and pumps that simply wear down after decades. These parts were widely available ten years ago but are now difficult to source. Reconditioned pumps sometimes offer a short-term fix, but when essential components are obsolete, replacement becomes the only cost-effective route.

Fan failure (FF models)
Fan-assisted versions of the Ultimate rely on a small motor to expel exhaust gases. Once the bearings seize or the windings burn out, the boiler locks out immediately for safety. Replacement fans are among the most expensive parts still occasionally found, typically £250–£350, including labour. We’ve seen cases where homeowners replaced the fan, only for the PCB to fail shortly after. When two or more components begin failing close together, it usually signals the boiler has reached the end of its service life.

Flashing lights or error codes
Later FF models came with basic indicator lights that flash when a fault occurs. These codes are limited compared to modern diagnostics, often pointing only to “ignition failure” or “overheat.” Engineers must test individual circuits manually to confirm the cause. If the fault light relates to ignition or fan failure, expect a repair quote of £250–£400 depending on part availability. At this age, repeated faults are a sign the boiler’s control board or sensors are deteriorating.

Leaks around the boiler casing
Small drips beneath the boiler are common once the seals and joints inside start corroding. Over time, these leaks can spread to the heat exchanger or pump body. Even tiny leaks can damage electrical wiring or lead to erratic pressure readings.
Some engineers can temporarily reseal joints, but corrosion usually returns. Once water starts escaping from multiple points, replacing the boiler is safer and more economical.

Low pressure or airlocks
Pressure loss happens naturally over time, but if you find yourself topping up the system weekly, there’s likely a leak somewhere. Common causes include failing expansion vessels or pinholes in pipework. Trapped air can also cause noisy operation and cold spots in radiators.
Topping up may help short term, but pressure should hold steady once the system is healthy. When it doesn’t, that points to deeper corrosion or a leak that isn’t visible. Engineers can add sealant to the system, but again, it’s a temporary measure.

Soot or black marks near the flue
This is one of the few faults that should be treated as urgent. Black marks or soot indicate incomplete combustion, meaning the boiler isn’t burning gas cleanly. It can result from a blocked burner, damaged seals, or poor ventilation. These signs can also mean there’s a risk of carbon monoxide. If you ever notice this, turn the boiler off and call a Gas Safe engineer immediately. They will carry out combustion tests and check for CO leaks. In most cases, repair is not worth attempting, and replacement is strongly advised.

Noisy operation
As the years go on, vibration and humming become more frequent. It’s usually due to worn pump bearings, loose mounting brackets, or fan imbalance. While the noise itself isn’t always dangerous, it indicates mechanical wear that can worsen suddenly.
Sometimes a simple service quietens it down, but when noise keeps returning, it’s a clear sign that key components are nearing failure.

Parts discontinued
This is the underlying problem with every Glow-worm Ultimate still in use. Major components such as PCBs, fans, thermocouples, and thermostats are no longer manufactured. Reconditioned or salvaged parts keep some units alive, but there’s no guarantee of reliability or warranty. We’ve seen many homeowners spend £300 here and £200 there, only to face another breakdown months later. Because the Glow-worm Ultimate is a non-condensing boiler with discontinued parts, upgrading through the ECO4 scheme with Free Energy Savings is usually the most sensible long-term decision.

Summary
While the Glow-worm Ultimate was built to last, age and efficiency standards have caught up with it. Most faults can be repaired once or twice, but constant part failures and low efficiency make replacement the smarter option. ECO4-funded installations provide a new, A-rated boiler at no cost to qualifying households,  avoiding the cycle of expensive, short-lived repairs.