I have a Gloworm flexicom 30 cx combi boiler and when you run a hot tap or shower it only produces hot water for a minute or 2 then runs cold . Sometimes it error codes F25 . My local heating company have changed the boiler pump and the plate heat exchanger which have made no difference . I thought the pump in a combi boiler was only for the Heating (the heating works fine ) and not the hot water ? can anyone confirm this please ?

thanks in advance


  • Hello, and welcome to Home Improvement. If you think the answer is good, you can click the checkmark to accept it. And, props for taking our tour before posting; few newbies do. Nov 9 '19 at 21:11
  • No: pump is not only for heating in boiler with plate exchanger, those, in DHW production, run to circulate "primary" water between the main exchanger and the plate exchanger. Only in "bithermal" models pump is not needed because DHW pass trough the main exchanger. But those are usually the cheapest option and once clogged usually worth only full replacement.
    – DDS
    Dec 10 '19 at 11:57
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    – FreeMan
    Dec 4 '20 at 12:40

I'd suspect the NTC (negative temp coefficient) thermistors as being faulty or not in good thermal contact with whatever they're supposef to be in contact with. That's a known cause of F25, and most other causes would indeed be related to heating circulation. However circulation problems / pressure / air in the system could still cause this problem even on hot water.

  • Thanks for that . I am pretty sure there is no air in the system , so maybe we need to look at the thermistors
    – Richard
    Nov 10 '19 at 22:10

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