How to Fix Glow-worm F22 Fault Code (UK)
Glow-worm showing F22 — Low water content. UK diagnosis, likely spare (expansion vessel) and step-by-step reset.
Glow-worm
What the code means
F22 on a Glow-worm indicates low water content. The boiler locks out to protect the heat exchanger and, on gas faults, to comply with the Gas Safety (Installation and Use) Regulations. Always reset to confirm the code is live, not historical.
Likely spare
The most common fix is the expansion vessel. Check the exploded diagram against the GC number on the data plate before ordering — Glow-worm revised parts on Glow-worm units mid-production.
Fix procedure
Check EV precharge.
Log the fault and reset. If the code returns within 24 hours, escalate to full electrical diagnosis with a manometer and multimeter.
Frequently asked questions
- Can I reset F22 myself?
- You can press the reset button up to 3 times. If F22 returns, the boiler needs a Gas Safe engineer — it is illegal to repair gas components without registration.
- Is the part covered under warranty?
- Glow-worm standard warranty is 2–10 years depending on model. Registered installations get the full term. Check the sticker inside the case.
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