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Viessmann Vitodens 100 WB1A System Boiler Modulation Fault

Discussion in 'Plumbing and Central Heating' started by jrling, 15 Apr 2019.

  1. jrling

    jrling

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    I have a Viessmann Vitodens 100 WBIA open vent system boiler (not combi) 24KW Natural Gas running CH only with a dozen rads, so a high heating load. About 10 years old. Until recently it worked perfectly and the system is unchanged.

    I run the boiler thermostat at just above the 60C mark on the dial and an external room stat controls the temperature in the conventional way. Nothing clever.

    Recently, it only fires at two bars max on the modulation scale even from cold and therefore the system is taking a long time to heat up to temperature, but it does reach 62C eventually. Even if I turn the boiler dial up to max 80C, it only goes to three or four bars for a short time and then back down to 2 bars. It is condensing fine but only operating on low power.

    The Grundfos 15-60 circulating pump is new. I have a MagnaClean filter fitted with Fernox X-100 Inhibitor in the system and it does not trap much on the magnet when I clean it. I have flushed the system with Fernox.

    The boiler temperature thermostat tests at correct resistance on multimeter. I have tried pushing the ‘Reset’ button on the boiler, but no change.

    There are no Fault Codes being displayed.

    Any advice what to look for or how to test for fault would be appreciated.

    Many thanks
     
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  3. DavidLewis

    DavidLewis

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    I too have a WB1A, and I see the same symptoms. My experience is that to fix it, every year or two I have to remove and flush through the rubber hoses because they become blocked with black "granules". Known as "crunchy hose syndrome" I believe. This leads to much reduced circulation, causing the boiler to modulate down.
     
  4. JohnD

    JohnD

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    My WB1A doesn't have rubber hoses.

    I've heard that they can collect scaly deposits on the sharp bend in the metal return pipe that enters the heat exchanger, top left as you look at it.
     
  5. DavidLewis

    DavidLewis

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  6. JohnD

    JohnD

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    ah, that one's not open vented, and the pipe layout is different

    from part 21 I see it is also a combi.
     
  7. DavidLewis

    DavidLewis

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    True, mine is sealed. I'd overlooked that the OP's was open vented. I hadn't realised the boilers were so different.

    Mine's a system boiler, not a combi, but the rubber pipes are those in the diagrams I linked.
     
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  9. JohnD

    JohnD

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    here we are

    this is an 11-year-old photo.

    I believe it's the brassy looking bend LHS of the round canister that holds the burner and coiled HEX. It does look surprisingly tight and small.


    and here's a wider shot

     
  10. jrling

    jrling

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    Thank you both JohnD & David Lewis.

    As you say, I have the system boiler (actually with the CH pump external not in the case) and it does not have the rubber hoses, but all copper tubing. See attached picture. I have read the flow sensor (black plastic fitting top left), can get gunked up and slows the flow and the boiler modulates to compensate. But I drained the boiler and took out the sensor and it was clean as a whistle. I had used Fernox flushing solution in the boiler a few weeks before. So I don't think the copper tubing is the cause.

    In the second picture - the back of the control dials etc. which folds down, there is an adjustable pot (marked R14 and the left of the two); it was adjusted with the arrow fully clockwise at 5 o'clock. I had read that it could control modulation and wound it a little anti-clockwise to 2 o'clock as in the picture. When I turned on the boiler, it immediately fired up at 4 bars and kept there until the temperature was in the 50s when it backed of down to 2 bars as it approached target 60C setting. Magic!

    I don't know what that pot controls. Do you? Anyhow up, so far so good. 20190423_194221.jpg

    20190423_194344.jpg
     

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  11. JohnD

    JohnD

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    Seems a bit rash to be opening it up and adjusting things without knowing what they are. You should be using an RGI with Viessmann experience.
     
  12. AGAS

    AGAS

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    jeez o_O
     
  13. ianmcd

    ianmcd

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    You dont have a system boiler, you have a heat only boiler, they are not the same
     
  14. JohnD

    JohnD

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    Perhaps you have the Compact model. Is it 350mm wide?
     
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