This is less of a question, but more of a list of things we tried to stop the frequent F11, F22 and F25 error codes appearing on our Glow Worm Flexicom HX boiler which would stop it from functioning. We have some big (for UK) rooms with three large radiators in the reception rooms.


  • F11, F22 and F25 error codes
  • Really noisy central heating pump, runnning flat out
  • Cold radiators, especially downstairs, even if upstairs were really hot.
  • Moderately hot tap water
  • Generally a very cold house especially when outside temps approach zero °C

Solutions we tried which DID NOT work

  • Turning up the boiler temp
  • Turning up the thermostatic valves for the radiators
  • Reducing flow for upstairs radiators to increase flow in downstairs radiators
  • Replacing the thermostatic valves with new for the radiators which would not heat up
  • Please note, I don't need a solution, as I posted both question and answer, but if you have an answer that will help the community for similar issues, please go ahead.
    – Dib
    Commented Dec 22, 2018 at 7:50

1 Answer 1


The Solution which DID work

When our landlord let us have the heating engineers come round to have a proper look, the engineers discovered the system was clogged with rust and muck. They said it was the worse they had ever seen. The house is as around 20-25 years old, and they believed the system had never been flushed. So the heating engineers did the following:

  • Flushed the system
  • Used tools to vibrate all of the crud out of the radiators and flushed periodically
  • Fitted some kind of filter/ioniser which will attract all the particles into the filter and this can be flushed at every yearly service


We now have all radiators really hot. The house is pleasant to live in. I have not even heard the central heating pump running since then, as I guess it needs to run less hard and less often to get the flow through. Tap water is really hot again, so have been able to turn boiler down to 70°C. I may be able to turn it down further over the next few weeks too.

Hopefully some information within this post will help someone who experiences similar issues with their heating system or boiler.

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