Glow-worm Boiler Problems: Common Faults

Glow-worm sits in the value end of the market and shares engineering with its parent group, which means parts availability is good and many components are familiar. These are the faults we actually attend, rather than a list of every possible code.

For the specific fault codes, see our Glow-worm fault code list.

Fault 1: Losing Pressure

The most common Glow-worm complaint, and it has three usual causes.

The expansion vessel

Inside the boiler is a vessel with a rubber diaphragm and a charge of air. It absorbs the expansion of water as it heats. When the charge is lost, pressure swings widely — low when cold, very high when hot, and often discharging through the relief valve.

The tell-tale is a gauge that reads under 1 bar cold and close to 3 bar hot. Recharging the vessel often fixes it without replacing anything.

The pressure relief valve

Once a relief valve has discharged and picked up debris on its seat, it often weeps continuously. The symptom is water dripping from a small pipe outside the property, and pressure slowly falling.

An actual leak

Elsewhere on the system rather than in the boiler. Our guide to a boiler losing pressure covers finding it.

Fault 2: Diverter Valve

Classic combi failure. The valve directs flow either to the heating circuit or the hot water plate, and when it sticks you get one but not the other.

Symptoms: hot water fine but radiators cold, or heating fine but hot water lukewarm. Often intermittent at first, becoming permanent.

Repairable — either a new cartridge or the whole valve, depending on model. A common repair on units around the six to ten year mark.

Fault 3: Ignition Lockout

The boiler attempts to fire — fan spins, clicking — then locks out.

Most often a dirty or worn ignition electrode, which is a cheap part and a routine service item. Other causes are a failed flame sensor, a condensate blockage backing up, or a gas supply issue.

Reset once. If it locks out again, stop and get it looked at — repeatedly resetting an ignition fault is not safe. Our guide to a boiler that will not fire covers the diagnosis order.

Fault 4: Pump Problems

Two versions.

Seized after summer — the pump has stood still for months and sticks. The boiler fires, detects no flow, and locks out. Sometimes freed mechanically, sometimes not.

Worn by sludge — a system with no filter and depleted inhibitor grinds the pump bearings. The symptom is noise before failure, and the fix is a new pump plus dealing with the sludge that caused it. Our guide to draining and refilling covers inhibitor.

Fault 5: Scaling on the Plate Heat Exchanger

In a hard water area this is a matter of time rather than chance. The plate exchanger transfers heat to the hot water, and scale coats it.

Symptoms: hot water that starts hot then goes lukewarm, or takes noticeably longer to arrive than it used to.

Sometimes chemically cleaned, often replaced. A scale reducer on the incoming main slows the recurrence — Anglian Water publishes hardness figures for this area, and it is at the harder end.

Repair or Replace?

A reasonable rule of thumb:

• Under 8 years: repair. Parts are available and the boiler has life left.
• 8 to 12 years: depends on the part. A £120 diverter valve is worth doing; a heat exchanger usually is not.
• Over 12 years, with a major component failed: replacement is usually better value, particularly given the efficiency difference.

The other consideration is repeat failures. A boiler needing its third repair in two years is telling you something.

Keeping One Healthy

• Annual service by a Gas Safe registered engineer, with the electrode cleaned and the vessel checked.
• A magnetic filter fitted and emptied at each service.
• Inhibitor checked and topped up.
• Run the heating briefly in summer to stop the pump and valves seizing.
• A scale reducer in a hard water area.

Frequently Asked Questions

Are Glow-worm boilers reliable?

Reasonably, for their price point. They share engineering with more expensive units in the same group. They have less tolerance for a neglected system than premium boilers, so maintenance matters more.

Why does mine lose pressure every few weeks?

Most often the expansion vessel has lost its charge, or the pressure relief valve is weeping. Both are fixable and neither necessarily means a new boiler.

Is it worth fitting a magnetic filter to an older boiler?

Yes. It stops circulating debris reaching the pump and heat exchanger, and it is inexpensive relative to either.

My hot water goes cold after a few minutes.

Usually a scaled plate heat exchanger or a failing diverter valve. Both are common and both are repairable.

If Yours Needs Looking At

Most of these faults are diagnosable from the symptom before anyone opens the casing. We cover Milton Keynes and the surrounding area for boiler repair. Get in touch and describe what it is doing.