Glow-worm Boiler Not Working — Fault Codes & Fixes
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Glow-worm boilers are reliable workhorses found in thousands of UK homes, but like any heating system, they occasionally throw a fault code and stop working when you need them most. These codes are actually helpful — they're the boiler's way of telling you what's gone wrong, whether it's low water pressure, an ignition problem, or a frozen condensate pipe. Most common faults can be spotted and fixed quickly without calling an engineer, though some do require a Gas Safe registered professional. This guide walks through the four most frequent fault codes you'll encounter, what causes them, and whether it's a five-minute fix or time to pick up the phone.
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Most likely cause & what to check
Glow-worm is part of the Vaillant Group and their boilers (Energy, Compact, Flexicom ranges) share many components with Vaillant ecoTEC models. This means fault codes are similar to Vaillant and parts are widely available. Manuals and fault code guides are at glow-worm.co.uk/support or on 0330 100 3461.
Fault code F22 (low water pressure): the most common fault on Glow-worm boilers. The pressure should be 1–1.5 bar. Repressurise using the filling loop underneath the boiler — open the two valves slowly until the gauge reads 1.2 bar, then close both and press reset. If pressure drops again, there is a leak to find.
Fault code F28 (ignition failure): the boiler could not establish a flame. Press and hold reset for 3 seconds. Check other gas appliances are working. If F28 returns repeatedly, an ignition electrode, gas valve, or PCB fault is likely — a Gas Safe engineer is needed.
Fault code F29 (flame loss): the boiler lit but the flame went out unexpectedly. In cold weather, first check the condensate drain pipe outside isn't blocked or frozen — a frozen condensate is a very common cause of F29 faults in winter. Thaw the pipe with warm (not boiling) water and reset.
Fault code F1 (general lockout): the boiler has gone to lockout. Reset once — if it locks out again quickly, stop resetting and call a Gas Safe engineer to investigate.
Glow-worm offer manufacturer warranties varying by model. Because they share the Vaillant parts ecosystem, repairs and parts sourcing tend to be straightforward. Any Gas Safe engineer can work on Glow-worm boilers.
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Frequently asked questions
Why does my Glow-worm boiler keep showing F22 and losing pressure?
Low water pressure (F22) usually means you've got a leak somewhere in the system — either on the heating pipes, radiators, or internal components. Repressurise using the filling loop first, but if the pressure drops again within hours or days, you'll need to find and fix the leak. A qualified plumber can trace this using pressure gauges and dye tracers if it's not obvious.
Is it safe to reset my boiler if it's showing a fault code?
Yes, resetting once or twice is safe and often clears temporary glitches. However, if the boiler locks out again immediately after reset, stop resetting and call a Gas Safe engineer — repeated lockouts indicate a fault that needs professional diagnosis. Never ignore a persistent fault code, as it can point to a safety issue.
My boiler shows F29 in winter — is the frozen condensate pipe really the culprit?
Absolutely — it's one of the most common winter faults. Modern condensing boilers produce water vapour that drains outside, and this pipe freezes solid in cold weather, causing the boiler to shut down safely. Thaw it gently with warm water (not boiling, which can crack the pipe) and the boiler should restart.
Can I repair a Glow-worm boiler myself or do I need a Gas Safe engineer?
You can safely handle minor checks like clearing a frozen condensate pipe or repressurise using the filling loop, but any work involving gas, electrics, or the boiler's internal components must be done by a Gas Safe registered engineer. Attempting repairs yourself on gas appliances is dangerous and may invalidate your warranty.