Glow-worm [F.9] fault codewhat it means and how to fix it

F.9 is one of the more ambiguous Glow-worm codes because different sources describe it differently — some as a water pressure sensor fault, some as circulation, some as a fan or air pressure switch condition depending on the range. In practice on the Energy, Flexicom and Betacom ranges it most often turns out to be pressure-related: either the system pressure is genuinely low, or the pressure sensor has silted up with magnetite and can no longer read properly. That second scenario is the one that confuses people, because the gauge reads a perfectly respectable 1.3 bar while the appliance insists there is a problem. Both are telling the truth. Because the code meaning varies by range, it is worth confirming which appliance you have from the data plate before acting on any advice, including this. If your appliance is a Betacom or Ultracom, cross-check against the F.73 and F.74 pressure sensor codes, which cover related conditions.

1. 1

   Read the pressure gauge cold. If it is under 1 bar, top up to 1.2 through the filling loop and reset — that may well be the whole answer.

2. 2

   Open every thermostatic radiator valve fully and check the service valves below the appliance are aligned with the pipework.

3. 3

   Bleed the radiators from the ground floor upwards, finishing with the highest, then repressurise to 1.2 bar.

4. 4

   Check the data plate behind the front panel to confirm exactly which Glow-worm range you have. F.9 does not mean the same thing across every range and this genuinely matters.

5. 5

   If the fault persists with the gauge reading a healthy 1.2 to 1.5 bar, that points at the sensor. Book an engineer and tell them the pressure is normal — it saves them repeating the check.