Glow-worm F.23 Fault Code: Temperature Difference Too Great
Hertfordshire, Bedfordshire & Cambridgeshire
F.23 on a Glow-worm is a safety switch-off because the temperature difference between the flow and return sensors is too great.
This is a circulation fault. The boiler is heating water that is not moving away fast enough.
What F.23 actually means
The boiler compares the two sensors. On a healthy system the difference between water going out and water coming back sits in a predictable range.
A very wide gap means heat is going into water that is barely moving, which is the signature of a pump or flow problem.
What you can safely check yourself
Do not keep resetting a safety switch-off.
Check the pressure gauge is in the normal range.
Check radiator and isolation valves are open, particularly after decorating or other work.
Bleed the radiators if the system has recently been filled or topped up.
When to stop, and why
If pressure and valves are fine, stop. Pump and blockage diagnosis is engineer work.
What causes it beyond the simple checks
A failing or seized pump, a pump set too low for the system, an air lock, sludge restricting the heat exchanger, a closed valve, or one of the two sensors reading incorrectly. Glow-worm is part of the Vaillant Group and the Energy range shares its fault code structure with Vaillant appliances.
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.23 the same as an overheat?
Related but not identical. F.20 is the temperature itself going too high. F.23 is the gap between flow and return being too wide. Both point at circulation.
Will a power flush help?
If sludge is restricting flow, often yes. Worth diagnosing first.
Could it be a sensor?
Yes, and that gets tested rather than assumed.
Related fault codes
Other Glow-worm codes: F.20 · F.22 · F.24 · F.25 · F.27 · F.28 · F.29 · F.32 · F.61 · F.75 · all boiler advice. See also boiler repair. See also power flushing and radiator not heating up.
In Glow-worm’s own words
The Glow-worm Energy range fault finding table lists F.23 as: Safety switch-off. Temperature difference too great (NTC1/NTC2)
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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