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Plumbing buffer tank setup - HTP vs. John Siegnthaler

In the process of installing an HTP 20 gallons SuperStor Ultra Stainless Steel buffer tank to a radiant system that is powered by a Navien NCB 180E.

In the SSU-20B's product documentation (LP-219.pdf), page 6, Figure 3, it appears that the recommended piping concept has the primary loop's supply connected at the bottom, and the secondary loop's supply connected at the top connection of the buffer tank.
That is contrary to previous setups I have seen, such as the one shown and described by John Siegenthaler, P.E. here: 'Alternate methods to pipe a buffer tank'
In his setup, the supply for both, primary and secondary loop, are connected at the top of a buffer tank, while the returns are connected at the bottom. That will result in hot water staying and moving through at the top, and colder water sliding through at the bottom of the tank. Those effects, according to Mr. Siegenthaler, are desirable because they maintain temperature stratification in the tank.

Is anyone able to share some advice on the differences of those setups and which setup to follow?
I can provide more details on the radiant setup if helpful.


  • bob eck
    bob eck Member Posts: 930

    Check this out it shows how to pipe Hydronic separation

    Looks like to me boiler into top of tank system out top of the tank system return bottom of the tank boiler return bottom of the tank

    The same way that hydro seaporators get piped.

    The tank is just adding water volume to systems with micro small zones and to help keep the boiler from short cycling and to get your P/S piping.
  • bob eck
    bob eck Member Posts: 930
    I would call HTP on Tuesday ask if that piping drawing is correct.
    They may want that piped that way because fittings on this tank are not across from the other one. These fittings are on a 45 from each other.
    Could cause a problem if not piped in the right way.
  • Paul48
    Paul48 Member Posts: 4,469
    edited December 2017
    "Hillbilly Reckoning"........You're using the buffer tank to add volume to the system. Piping it in at the top and out at the top, basically short-circuits the tank, and only uses the top of it.

    I can also see the merits of the other way of installing it, but it would seem that would be more applicable with a very large buffer. And, perhaps as mentioned, one that has connections across the tank.
  • hot_rod
    hot_rod Member Posts: 22,105
    Yet another option that has advantage being promoted in the GEO industry outworks for hot also.

    It's called the 3 pipe or direct-to-load" method. Promoted by engineer John Manning who specializes in GEO design. Proper buffering is critical with fixed speed heat pumps in zoned or small load systems, and the desire to drive efficiency with return temperatures.

    I have installed 4 or 5 two pipe buffers now, several at my own place so i can watch them perform.

    You need some resistance in the heat side piping. The small circ integral checks .35- .50 psi may not be enough check protection. A delta P or zone valve may be better.

    If you have micro loads running a very tight ∆T, high flow rate, that low ∆ can cause some boiler short cycling as it reaches limit quickly. Low ∆ is more common in chilled water designs where this concept originated.
    IF you assure a 15- 20∆ in your design 2 pipe works fine.

    So with this 3 pipe the return must pass through the lower tank assuring some buffer always involved.

    So it kinda depends on your system and goals, 2, 3, and 4 all have some pros and cons.

    Here is John Manning presenting the direct to load 3 pipe concept in this video. He is GEO biased :)


    of direct
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gordy
    Gordy Member Posts: 9,546
    If you have a modulating condensing boiler pipe the buffer as John says to reap the rewards of lower return temps to the boiler.