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Viessmann mod-con not modulating for DHW

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sunlight33
sunlight33 Member Posts: 378
in THE MAIN WALL
Recently I got a short cycling behavior for my Viessmann 200-W boiler when producing DHW. For those of you who are familiar with the boiler, it can maintain a steady boiler temperature some degrees above the DHW setpoint, for example if I set the DHW to 120F and address 60 to 20 (K), it will try to maintain 156F throughout the entire the DHW cycle, and the boiler will modulate lower so it won't overshoot during the mid to end of the cycle. However in my case it will overshoot and reach thermal limit before the cycle is completed, so the boiler starts again and reaches the limit half a minute later, and this repeats several times until the DHW setpoint is satisfied. This started to happen about 3 weeks ago, before it was fine because I monitor the boiler temperature on a daily basis during DHW operation.
I verify the voltage on plug 190 (signal plug on the gas valve) and see that the gas valve is not going to low fire mode. What's interesting is that for space heating, the boiler has no issue with modulation, the gas valve does go into low fire mode as indicated by the lower voltage on plug 190. Has anyone experienced this before?

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  • Derheatmeister
    Derheatmeister Member Posts: 1,548
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    Which One WB2A...WB2B...?
    Two things i could think about
    1. What kind of tank is installed? What is the Delta tee between the Supply and Return ?
    Could the coil be isolating itself/ clogging with debrie/Calcium/Hard Domestic water ?
    2.Could it have something to do with your tank sensor drifting off/ Bad feedback.
    Is this a original Viessmann sensor ?
    I would start by ohming out some of the sensors .
    Hope this helps..
    Rich_49
  • sunlight33
    sunlight33 Member Posts: 378
    edited December 2021
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    WB2B, it's a Viessmann indirect tank and sensors are all original. The coil is fine as I cleaned the tank earlier this year. The funny thing is that this short cycling doesn't happen all the time, sometimes it works fine then after a coupe of days it short cycles, so I power cycle the boiler, it went okay again for a few days then goes bad. Occasionally I went from bad to good again without me doing anything. Bad board?

    A couple of days ago the boiler had an EE burner fault, I reset the boiler and it's fine ever since, not sure if it is just a random glitch.
  • Derheatmeister
    Derheatmeister Member Posts: 1,548
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    sunlight33 said:

    WB2B, it's a Viessmann indirect tank and sensors are all original. The coil is fine as I cleaned the tank earlier this year. The funny thing is that this short cycling doesn't happen all the time, sometimes it works fine then after a coupe of days it short cycles, so I power cycle the boiler, it went okay again for a few days then goes bad. Occasionally I went from bad to good again without me doing anything. Bad board?

    Maybe it is a bad board...Did you talk to Viessmanns support team about this ?
    I still would start with the sensor vs a new board..
  • sunlight33
    sunlight33 Member Posts: 378
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    Do you know if the modulation is related to the control console (board) or the Vitotronic 200 H01 (front panel)?
  • Derheatmeister
    Derheatmeister Member Posts: 1,548
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    sunlight33 said:

    Do you know if the modulation is related to the control console (board) or the Vitotronic 200 H01 (front panel)?

    Is this the Black or white display?
    I do not Know but i think it is controlled by the board vs the display..
  • sunlight33
    sunlight33 Member Posts: 378
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    white display, wb2b model.

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