What does the Glow-worm F20 fault code mean?

What does the Glow-worm F20 fault code mean?

The Glow-worm F20 fault code means the boiler has shut down on its overheat safety protection. Glow-worm boilers trip the F20 lockout when an internal temperature sensor reads above the safe operating limit (around 95°C). The shutdown is intentional — it protects the heat exchanger and the rest of the system from damage.

F20 is one of the more common Glow-worm fault codes on the Energy, Easicom, Ultracom, Betacom and Ultimate ranges. It's almost always caused by a problem somewhere in the heat-transfer chain rather than the boiler itself being broken.

Common causes of the Glow-worm F20 fault code

An F20 lockout happens when the boiler can't get heat away from the heat exchanger quickly enough. The five most common underlying causes:

• Faulty thermistor (overheat sensor): A failing sensor can incorrectly read the boiler as too hot, triggering the lockout even when temperatures are normal. This is the single most common F20 cause on older units.
• Restricted water flow: A failing or stuck circulation pump, an air-locked pipework run, or partially closed valves all reduce flow rate. With less water moving, the boiler can't shed its heat fast enough.
• Limescale or sludge build-up: Magnetite (iron oxide sludge) or hard-water limescale in the heat exchanger acts as insulation. The boiler heats up but can't transfer the heat efficiently, so internal temperatures climb.
• Low system pressure: If pressure has dropped well below 1 bar, there isn't enough water in the system to circulate properly. The pump cavitates and heat doesn't move.
• Faulty pump: If the pump has seized or is running below its rated speed, water doesn't circulate — same outcome as restricted flow.

Glow-worm F20 vs Vaillant F20 — what's the difference?

Glow-worm is owned by the Vaillant Group, so the two brands share some fault-code conventions, but the codes don't always mean the same thing in practice. On a Vaillant boiler, F20 also points to overheat protection but the diagnostic flow and component naming differ from Glow-worm. If you have a Vaillant boiler, our Vaillant F20 guide is the right page to read.

The fix process is similar on both brands — you can try a reset, then escalate to a Gas Safe engineer if it persists — but the parts and serviceable components inside the boiler differ. Make sure your engineer is checking the right manufacturer's manual for your specific boiler.

How to try fixing Glow-worm F20 yourself

There are two safe steps you can try before calling an engineer. Anything beyond these needs a Gas Safe registered professional — overheating faults can mask more serious problems and you should not open the boiler casing yourself.

1. Check your system pressure

Look at the pressure gauge on the front of your Glow-worm boiler. The needle should be sitting in the green band, typically between 1.0 and 1.5 bar when the system is cold. If it's below 1 bar, the boiler will struggle to circulate water properly.

Most Glow-worm boilers have a filling loop underneath that lets you re-pressurise the system. Open the valves slowly until pressure reaches 1.2-1.5 bar, then close them firmly. After re-pressurising, reset the boiler (see step 2).

If pressure keeps dropping, you have a leak somewhere — call us.

2. Reset the boiler

Hold the reset button on the front of the boiler for 3-5 seconds. The display should clear the F20 code and the boiler should re-fire. Wait 15-20 minutes and check that radiators are warming evenly and the boiler is running quietly without re-locking.

If the boiler reignites and stays running, you're probably fine — keep an eye on it for a few days. If F20 returns within hours or days, the underlying cause is still present and needs an engineer.

When to call a Gas Safe engineer

Stop and call us if any of the following apply:

• F20 returns after one or more resets
• Radiators feel unusually hot to the touch before the boiler locks out
• You can hear loud banging, kettling or whistling from the boiler when it's running
• System pressure keeps falling no matter how often you top it up
• The boiler is over 8-10 years old and hasn't been serviced annually — sludge and limescale are very likely contributors

Our engineers diagnose F20 lockouts as part of a standard boiler repair visit. Typical Glow-worm F20 fixes range from a thermistor replacement (straightforward, usually same-day) to a full power flush if sludge is the root cause. We diagnose first, give you a fixed quote, and only proceed with your approval — no work over budget without you saying yes.

Glow-worm models that use the F20 code

F20 appears across most modern Glow-worm boiler ranges, including:

• Glow-worm Energy (combi, system and conventional)
• Glow-worm Easicom
• Glow-worm Ultracom
• Glow-worm Betacom
• Glow-worm Ultimate
• Glow-worm Compact

The diagnostic process is broadly the same across all of them, but specific component locations vary by model. Your engineer will refer to the right Glow-worm manual for your unit.

Preventing future Glow-worm F20 lockouts

Annual servicing is the single biggest prevention factor for F20 faults. A proper service checks the thermistor, gas pressure, combustion analyser readings, and water flow rate — all the things that, if left, eventually cause an F20 lockout. If you have a magnetic filter like a MagnaClean fitted, we'll clean it as part of the service to stop magnetite building up in the heat exchanger.

If your boiler is more than 8-10 years old and frequently locking out on F20, a new boiler installation may be more cost-effective than ongoing repair. Modern Glow-worm and Vaillant boilers are more efficient and come with manufacturer warranties up to 10 years (brand-dependent).

Frequently asked questions

Can I just keep resetting my Glow-worm boiler when F20 appears?

You can reset it once or twice to confirm the lockout was a transient event. If F20 returns, repeated resets won't fix the underlying cause — and the boiler is trying to protect itself by locking out. Continuing to override that risks damage to the heat exchanger, which is by far the most expensive part to replace. Call a Gas Safe engineer.

Is the Glow-worm F20 fault dangerous?

The lockout itself is a safety feature — the boiler has shut down because it detected an unsafe temperature. However, if you smell gas at any time, leave your property immediately and call the National Gas Emergency line on 0800 111 999. Don't attempt to restart the boiler if you suspect a gas leak.

Why does my Glow-worm boiler keep locking out on F20 in winter only?

Winter usage stresses the system more — longer run times, higher demand, colder return temperatures. If a thermistor is marginal or there's sludge in the heat exchanger, summer use may not trigger it but winter does. The root cause is the same all year; it's just exposed in winter.

How much does it cost to fix a Glow-worm F20 fault?

Diagnosis is free. The fix depends on what's causing it: a faulty thermistor is usually a standard repair within our boiler repair range. A full power flush to clear sludge is priced separately. We diagnose first, quote a fixed price, and only proceed with your approval.

Can I clean limescale or sludge from my Glow-worm myself?

No — opening the boiler casing or working on the heat exchanger requires a Gas Safe registered engineer by law. A proper power flush also needs the right equipment to circulate cleaning chemicals through the system at the correct pressure and temperature. Trying to clean it yourself without the kit risks damaging the heat exchanger.