Combi Boiler DHW going cold

Discussion in 'Engineers' Talk' started by C3ham, Jan 23, 2024.

  1. C3ham

    C3ham New Member

    I have a Worcester 36cdi compact and have started having issues with the hot water going cold then heating up again this happens about three times a shower very annoying i have attached a graph of my hot water temp and flow temp my flow temp spikes when DHW is demanded is this normal behaviour or is my diversion valve faulty?


    Thank you in advance for any help
  2. MGW

    MGW Screwfix Select

    I had it with mothers boiler, and it was simply a pipe constricting flow. The boiler had two options eco on or eco off, with eco on no problem turning a tap part on, with eco off tap needed to be full on, but with shower eco on it would go hot cold and hot again, and eco off when it went hot it stayed hot.

    My own house found winter OK but summer the shower rose was not big enough and it would switch off during the shower.

    Step one is remove shower head and clean it, make sure no lime built up in the head.
  3. Kiaora oap

    Kiaora oap Active Member

    possibly partial blocked heat exchanger, that would give the symptoms you describe,
    LearnMore likes this.
  4. Bob Rathbone

    Bob Rathbone Screwfix Select

    Had an issue with my Worcester combi, its 15 years old and suddenly stopped giving hot water except for a few litres when the tap was turned on, the it ran cold. The issue was the diverter valve. I looked at the stepper motor when hot water was demanded and it did not move, took the motor off and it moved. Dismantled the diverter valve and the rubber finger that blocks the flow was caked in a hard crust, cleaned it off, re assembled and all is now well. If you decide to repair yours, mark the top of the metal actuating lever so you re assemble it correctly.
    billysloke likes this.
  5. Teki

    Teki Screwfix Select

    @C3ham how did you log those temperatures?

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