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Buffer Tank Vs Indirect Tank

Le John
Le John Member Posts: 197
I often hear the term buffer tank which I understand helps to minimize short cycling. Doesn't the indirect do that? Can someone explain this?

So I would have the mod/con and an indirect and a Buffer Tank?. What happens when the install is in a 3X4 closet?


  • Cletus
    Cletus Member Posts: 1
    Buffer Tank vs Indirect

    An indirect is for your domestic water. It has a coal or tank inside a tank so that there is separation between your heating system and domestic system. A buffer tank has no separation in it, just a stored mass of hot water.
  • SWEI
    SWEI Member Posts: 7,356
    Better yet

    Size the mod/con properly and get one with the lowest minimum firing rate you can find.  This will prevent short-cycling.

    Both the Viessmann 222-F and the Rinnai Q Premier have onboard DHW tanks and a 2' x 2' footprint.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    Yeah but...

    I have used a reverse indirect as a combination buffer tank/DHW generator, but you have to understand the major difference in a reverse indirect, that being that boiler water is sent through the tank, and not the coil. In this situation, your minimum tank temperature would typically be 130 to140 degrees F, and that may require another means of being able to mix down for lower temperature loads. I used it on a baseboard application, so the higher low temperature setting was not an issue.

    Attempting to do this with a conventional indirect probably won't work due to limited coil pipe size and associated pressure drop of the heat exchanger. Plus, the coil is in the bottom of the tank, which is not conducive to proper operation for heat extraction..

    Obviously, using a reverse indirect as a buffer will work, but if your design temperature is high, you MUST have an ASSE 1017 compliant mixing valve on the potable side to avoid scald issues.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • hot_rod
    hot_rod Member Posts: 15,066
    In some cases

    a reverse indirect tank can do double duty. I think some brands TurboMax, Ergomax added extra boiler connections to serve as DHW, buffer, and separator in one tank.

    At my place I added a flat plate HX to the buffer/ separator tank to generate DHW instantly. With a properly sized HX you can produce 120F DHW with 125F tank temperature. I used a 30 plate 5X10 to get 3 gpm, DHW flows.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Robert O'Brien