1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Viessmann Vitodens 100 WB1A System Boiler Modulation Fault

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

  1. jrling

    jrling

    Joined:
    15 Apr 2019
    Messages:
    2
    Thanks Received:
    0
    Country:
    United Kingdom
    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
     
  2. DavidLewis

    DavidLewis

    Joined:
    8 Mar 2012
    Messages:
    21
    Thanks Received:
    1
    Location:
    Essex
    Country:
    United Kingdom
    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.
     
  3. JohnD

    JohnD

    Joined:
    15 Nov 2005
    Messages:
    57,920
    Thanks Received:
    2,960
    Location:
    21st Century
    Country:
    Cook Islands
    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.
     
  4. DavidLewis

    DavidLewis

    Joined:
    8 Mar 2012
    Messages:
    21
    Thanks Received:
    1
    Location:
    Essex
    Country:
    United Kingdom
  5. JohnD

    JohnD

    Joined:
    15 Nov 2005
    Messages:
    57,920
    Thanks Received:
    2,960
    Location:
    21st Century
    Country:
    Cook Islands
    ah, that one's not open vented, and the pipe layout is different

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

    DavidLewis

    Joined:
    8 Mar 2012
    Messages:
    21
    Thanks Received:
    1
    Location:
    Essex
    Country:
    United Kingdom
    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.
     
  7. JohnD

    JohnD

    Joined:
    15 Nov 2005
    Messages:
    57,920
    Thanks Received:
    2,960
    Location:
    21st Century
    Country:
    Cook Islands
    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

     
  8. jrling

    jrling

    Joined:
    15 Apr 2019
    Messages:
    2
    Thanks Received:
    0
    Country:
    United Kingdom
    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
     

    Attached Files:

  9. JohnD

    JohnD

    Joined:
    15 Nov 2005
    Messages:
    57,920
    Thanks Received:
    2,960
    Location:
    21st Century
    Country:
    Cook Islands
    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.
     
  10. AGAS

    AGAS

    Joined:
    21 Feb 2014
    Messages:
    9,467
    Thanks Received:
    1,648
    Location:
    down south
    Country:
    United Kingdom
    jeez o_O
     
  11. ianmcd

    ianmcd

    Joined:
    15 Aug 2010
    Messages:
    10,976
    Thanks Received:
    3,118
    Location:
    Lanarkshire
    Country:
    United Kingdom
    You dont have a system boiler, you have a heat only boiler, they are not the same
     
  12. JohnD

    JohnD

    Joined:
    15 Nov 2005
    Messages:
    57,920
    Thanks Received:
    2,960
    Location:
    21st Century
    Country:
    Cook Islands
    Perhaps you have the Compact model. Is it 350mm wide?
     
Loading...

Share This Page