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Vitodens 100 DHW Priority

Hi, since my plumber serviced our boiler, our DHW loses temperature when the heating kicks in. He first thought it had to do with crud in some pipes, then to do with the thermostat setting on the shower (although we told him that it happens on all taps). It only happened since the service, so I wonderd whether he has changed a particular setting.

Any ideas?

Comments

  • Can you be more specific? Is the DHW not as hot? Is there less DHW available?

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    Marie_Ange1
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,699

    Hi, Can you give us photos of the equipment? It would be nice to see how flow is controlled.

    Yours, Larry

    GGross
  • Marie_Ange1
    Marie_Ange1 Member Posts: 3

    When the heating kicks in the DHW will go hot and cold, on and off.

  • GGross
    GGross Member Posts: 1,405
    edited December 2024

    Can you take some pictures of the system as piped? I'm assuming this is a combi, are you absolutely sure the only time the DHW goes hot-cold-hot-cold is when there is a call for heat or is there just always a call for heat and it is a coincidence? Combi units will sometimes experience flow issues, or the little flow sensor can get debris stuck, or the heat exchanger can become scaled (the little flat plate one not the big round one), all of these issues are regular service issues that would cause your issue. As far as I know there is no way to disable DHW priority on those combis unless you turn the DHW off entirely. There is also a diverting valve inside the unit which disables all heating water flow until DHW is satisfied.

  • GW
    GW Member Posts: 4,921

    shower only? If yes, it's not a water production issue, it's a shower valve issue

    such a simple question you pose, but so many variables.

    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
    GGross
  • Marie_Ange1
    Marie_Ange1 Member Posts: 3

    Hi, it isn't only the shower. All taps and happens when the heating kicks in. Boiler is 10 years old, never had a problem. All worked fine till the service. The engineer came back and discoavered some clogged pipes. Some of the pipes were furred up and the engineer cleared them, but that hasn't resolved the problem.

  • HVACNUT
    HVACNUT Member Posts: 6,558

    Its possible a setting was mistakenly changed in the parameters. DHW should be priority. Is it just the boiler (combi), or is there a separate hot water tank?

  • Grallert
    Grallert Member Posts: 895

    My money is on a setting change but the boiler is ten years old and it would be out of the question that the mixing valve in the boiler is faulty. assuming it is a combi boiler.

    Miss Hall's School service mechanic, greenhouse manager, teacher, dog walker and designated driver

  • GGross
    GGross Member Posts: 1,405

    You can't change the DHW prio setting on the combi, it is hard coded into the board. If the board has been replaced it is possible to change the setting, but you will get an error, and no DHW at all because it won't recognize the flow sensor. hot-cold-hot-cold DHW from a combi (on demand, tankless etc) is a symptom of either a plugged DHW heat exchanger, filters leading to it, heat transfer issue inside the DHW heat exchanger (needs clean and flush), possibly the diverting valve is stuck in the center position but even then it must move back to heating position, could have a defective or crudded up DHW flow sensor.

    GrallertAlan (California Radiant) Forbes