Glow-worm [F.10] fault codewhat it means and how to fix it

Two generations, two sensors. With a stop — F.10 — on an Energy, Ultimate, Easicom or Compact the code is "Short circuit: Flow temperature sensor", the mirror of F.00, and Glow-worm's manuals give the causes as "NTC sensor defective, short circuit in the cable harness, cable/housing". The two-digit Ultracom2, Betacom 24a/28a and Betacom2 keep that meaning as F10, "flow heating temperature sensor fault — sensor short-circuit".

But on an Ultracom hxi, cxi, sxi, a Flexicom or an Ultrapower, the trade table lists F10 as "Central return flow thermistor" — a fault on the return sensor, the one on the pipe coming back to the boiler — and the 24CI/18SI manual has 10 as "central heating return NTC fault".

So the same two digits send an engineer to opposite ends of the heat exchanger depending on the age of the boiler. The work is the same kind either way: meter the thermistor the code actually names, inspect its lead for chafing on the casing, replace whichever has failed, and reset. Glow-worm's homeowner page carries F.10 with the single instruction to call an engineer, and none of the older user guides gives a householder action for F10 at all.