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Viessmann boiler doesn't like sunshine

I have a Viessmann Vitodens 200 WB@ 6-24 combi boiler (not included in the recall) that will loses boiler water pressure every time the sun comes out and the outside air temp rises above 50 F. The Viessmann heats our radiant floor system and provides domestic hot water to a storage tank. When it is cloudy and cool, the pressure maintains at 10-12 lbs. and the boiler contently fires for days. As soon as the sun comes out, and air temps rise, you can watch the boiler water pressure meter drop to 4-6 lbs., and the boiler will not fire as it requires a min. of 8 lbs. of pressure. I have to manually boost the water pressure to 10-14 lbs. using the incoming Taco adjustable pressure regulator, and then keep an eye on it until sunset when it works fine again. I replaced the boiler water pressure sensing switch last fall when it failed and would not allow the boiler to fire, as well as the air vent; that was when the storage tank and recirculating pumps were added to the system. The Viessmann outdoor sensor is mounted on the south side of the house in the sun, contrary to Viessmann’s directions, but it has always worked to the system’s satisfaction. To experiment, I have covered the sensor to mimic it being on the shaded north side of the house but there has been no change. Viessmann technical says there are no components or software in their product that would cause the boiler to lose pressure in this way. There are no leaks that I can find anywhere. The incoming water bladder tank has air pressure. Our house main line in has 150 lbs. pressure adjusted down to 50 lbs. My heating contractor is stumped and no one in my area speaks Viessmann. Any thoughts?


  • EricAune
    EricAune Member Posts: 432
    sounds like an expansion tank issue

    have you checked the pressure of the expansion tank when the system is at zero?  Checking it with the system pressurized will not give an accurate reading of the actual charge.  Has the system content been calculated to properly size the tank according to the system volume and operating temperature parameters?

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  • Yardman
    Yardman Member Posts: 3
    Expansion tank check

    I can check the expansion tank pressure next time the boiler pressure drops to almost zero again. As for the tank, it was sized by my heating contractor, a 2 gal. Amtrol with a 17psi. pre-charge rating, 100psi. max.; 240 F. max. temp. rating inline feeding a Triangle Tube 60 gal storage tank. What I don't understand is why my problem only shows up when the sun comes out and outdoor temperature rises?
  • jp_2
    jp_2 Member Posts: 1,935
    sounds like leak

    sounds like a leak to me.

    Is there an auto feed on the system?  maybe some weird solar thermal expansion is causing a leak in the house..

    pressure is dropping cause the system volume appears to be expanding, as in a leak.
  • Yardman
    Yardman Member Posts: 3
    Viessman continues to puzzle

    Hi guys

    I've been monitoring this problem since I first posted it. There are no leaks in the system anywhere that I can tell- no water drips, no meter running. As spring has come on, this issue has stopped. Pressure is maintaining at all times. But now that it IS warmer outside, occasionally the boiler somehow will decide to not produce hot water to keep the storage tank at temperature! This is new and exasperating. I'm starting to think the problem is in some kind of program or sensor in the Viessman, or possibly in the Taco controllers my plumber added to the system last fall.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Sounds like a leak to me too...

    and it could very well be within the heat exchanger of the boiler.

    You'd expect to see water coming from the condensate discharge tube...

    Simple test, which it sounds like you've almost already done. Charge system up to 20 PSI, then turn the make up off and monitor it over time. If it drops significantly and continuously, there is a leak, somewhere.

    Find it and fix it quick before it limes up your HXer, which is not covered by warranty.


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  • CMadatMe
    CMadatMe Member Posts: 3,086

    Do you have pics of the install? I'd be interested to see the piping arrangement. Did you use the expansion board to run the zone control? A pic would speak volumes.

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