Glow-worm F.20 Fault Code: Overheating Safety Switch-Off
Hertfordshire, Bedfordshire & Cambridgeshire
F.20 on a Glow-worm is a safety switch-off because the overheating temperature was reached.
Nearly always a circulation problem rather than a boiler problem. Resetting without fixing the flow gets you the same lockout next cycle.
What F.20 actually means
The boiler monitors its own temperature and cuts out if it exceeds the safe limit. That cut-out is a safety device doing its job.
The reason the temperature ran away is what matters, and it is usually that water is not moving through the boiler fast enough to carry the heat away.
What you can safely check yourself
Do not sit and reset an overheat lockout repeatedly.
Check the pressure gauge is in the normal range, around 1 to 1.5 bar cold.
Check radiator and isolation valves have not been left closed.
If the system has recently been drained or topped up, bleed the radiators, because air will do exactly this.
When to stop, and why
If pressure and valves are fine and it still cuts out, stop and call an engineer.
What causes it beyond the simple checks
A failed or seized pump, sludge or scale restricting the heat exchanger, an air lock, a closed valve, or a temperature sensor giving a false reading. Glow-worm is part of the Vaillant Group, and the Energy range shares its fault code structure with Vaillant appliances. If you have a Vaillant showing a similar code, the equivalent page is linked below.
How Vantage can help
We are Gas Safe registered under 956889 and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day. Call us on 01462 506374. See also boiler repair and boiler servicing.
Frequently asked questions
Is F.20 dangerous?
The switch-off is the boiler being safe. The cause needs finding.
Will a power flush fix it?
If restricted flow from sludge is the cause, often yes. It is worth diagnosing before spending.
Why does it come back after a reset?
Because the reset clears the lockout, not the reason it overheated.
Related fault codes
Other Glow-worm codes: F.22 · F.23 · F.24 · F.25 · F.27 · F.28 · F.29 · F.32 · F.61 · F.75 · all boiler advice. See also boiler repair. The same fault on other makes: Vaillant F20 · Ideal L1 · Worcester Bosch E9. See also power flushing.
In Glow-worm’s own words
The Glow-worm Energy range fault finding table lists F.20 as: Safety switch-off. Overheating temperature reached
Glow-worm use different code sets across their ranges and generations. The definition above comes from The Glow-worm Energy range fault finding table. If your boiler is a different model and the code behaves differently, check the manual for your appliance or ring us and we will tell you what it means on your unit.
When to call an engineer
If the checks above have not cleared it, the next steps are engineer work and the boiler should not be reset repeatedly in the meantime. Call Vantage Plumbing and Heating on 01462 506374. We are Gas Safe registered under 956889, the line is answered by a person at any hour of any day, and you get a price before anyone starts.
Need help today?
Call and speak to a person, any hour, any day of the year. Get a fixed price quote or call 01462 506374.
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