Glow-worm [F.23] fault codewhat it means and how to fix it

F.23 is "Safety shutdown: Temperature spread too great (NTC1/NTC2)" in Glow-worm's tables, and the two-digit Ultracom2 and Betacom2 manuals say the same thing in plainer English: "maximum temperature difference reached between return and flow heating — water circulation fault". The flow sensor sits where hot water leaves the heat exchanger and the return sensor where cooler water comes back; the difference between them is a measure of how fast water is moving, and when it opens beyond the board's limit the boiler concludes the water is standing still.

Glow-worm's causes on the current ranges are "pump blocked, insufficient pump output, air in product, flow and return NTC sensors connected the wrong way round"; the Ultracom2 and Betacom2 add "check the pump speed", and the homeowner page adds a defective expansion vessel and fault in the sensor connections. That last cause — sensors swapped — is why F.23 sometimes appears on a boiler that has just been worked on with nothing mechanically wrong.

Glow-worm's homeowner page calls F.23 "a case for a Glow-worm installer in the first instance, as it can relate to how the boiler is working with the heating system", and gives the owner no checks beyond the general note that a reset can sometimes clear a fault; this page follows that and leaves the pump and sensors to the engineer. On the Ultracom, Flexicom and Ultrapower there is no 23 — their circulation faults are F25 and, on the Flexicom, F24 for a return above 90 °C.