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Viessmann Vitodens with Buffer Tanik??

Bidding out a job, baseboard with a vitodens 200.  Being that the vitodens does not have a conventional TT to fire the boiler rather it works off of temperature in the LLH sensor,  what is your take on buffer tanks.  What would you consider the smallest btu where you would add a buffer tank to the system and how well would the vitodens 200 work with buffer tanks?  In the process of doing the heat loss but do not have the figures yet. 

Thanks, James

Comments

  • Ironman
    Ironman Member Posts: 7,366
    Buffer Tank

    James,



    I would use a buffer tank if there were a substantial difference between the minimum firing rate of the boiler and the smallest zone connected. You want to design for a 10 min. minimum run time to prevent short cycling. Calculate the difference between min. firing rate and smallest load. You'll need 1gal. of buffer for every 1000 btu's difference at a 20* Delta T for ten min. run time. The volume of water in the piping of the smallest zone can also be factored in. That's why when you have old, over-sized piping, a buffer tank is almost never needed.



    The formula for buffer sizing:

    Run time X (min. output - min. load) / (Delta T x 500). 
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • James Day_2
    James Day_2 Member Posts: 191
    Buffer tank

    smallest zone is around 13000 btu. Boiler min fire rate is around 35000 btu. Soabout a 20 gallon buffer. Wasn't sure how the Vitodens 200 would react with a buffer being that it works off flow in the system.
  • Ironman
    Ironman Member Posts: 7,366
    Buffer Tank

    If your mains are not larger than say 1 1/4 in, you might consider using a 20 gal electric water heater tank. I'd pipe the supply into the top coming from the LLH and then come off the bottom to go out to the zones. This way you'll get the maximum capacity of the tank when stratification has occurred. This is obviously just a 2 pipe tank, just a fat spot in the road of flow. If you choose a 4 pipe tank, then I'd let that become my hydraulic separator in place of the LLH.



    I've got to go make a living for now. Have a good one. :)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Paul Pollets
    Paul Pollets Member Posts: 3,656
    Low Loss Header

    First,, if you want to use a conventional thermostat, consider the Vitodens 100. It costs less and is made for a single temperature heating circuit and an optional indirect tank. The Vitodens 200 uses the Vitotrol  300 remote control, for a remote thermostat and the sensor can be disabled, if you the want to use ODR only.

    The low loss header comes  with a sensor and eliminates the need for a buffer tank. The LLH can be omitted if the system flow rate in under 6.5gpm.  We always use a variable speed pump (Grundfos Alpha) on the system side for maximum efficiency.
  • hot_rod
    hot_rod Member Posts: 22,018
    depends on the cycling

    that you are comfortable with.  Is that blow load figure at design condition?  If so mild weather conditions will present additional short cycling.



    Properly applied a buffer tank can also serve as a hydro separator, air elimination point, and provide some dirt separation.  Most hydronic specific buffer  tanks also supply a temperature sensor port in the mid tank range.



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
This discussion has been closed.