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Looking for suggestions in tweaking the design of a buffer tank:

optimist55116 Member Posts: 12
I put in a buffer tank a few years ago to help lengthen the burn of my conventional boiler and it helped. I am now replacing that buffer with a slightly smaller, converted stainless steel BoilerMate. Presently I still have the conventional boiler, but next year I am thinking about switching to a modcon boiler.

I have an older house in Minnesota with three different temperature zones: basement embedded in cement PEX (10-12K BTU design); main floor, PEX on under floor aluminum plates (38-42K BTU design); and top floor cast iron radiators (15K).

I have been reading about buffer tanks and hydraulic separators for the last few years and started thinking of a way of borrowing features from both. With that in mind, I thought of placing inflow and outflow tubes in my new buffer tank within inches of each other. (attached drawing, with apologies for development disablement regarding my drafting skills).

My questions:

1. With a condensing boiler and circulators running from the boiler supply (BS) and out to manifolds (MO), if the return from the manifold (MR) is close enough to boiler return (BR), shouldn't the return temp to the boiler be close to the temp returning from the manifold?

2. What distance between the inflow and outflow is optimal? When I struggle with this question, I think of about a half dozen engineering formulas that I don't know how to properly use. I still want to heat the tank, even with the modcon. The main floor is hardwood and keeping the pump on most of the time, and modulating the water temp with a mixing valve, is much better for the floors--and quieter.

Any suggestions or questions?


  • hot_rod
    hot_rod Member Posts: 19,260
    edited May 2014
    use the whole capacity

    It looks like the flow would just go from supply to return without blending the entire tank, short cycling through the core of the tank? It is tough to make a tank be an ideal buffer and a separator, sometimes.

    Look at a chilled water buffer tank, for example, and they often have a vertical baffle to assure the entire tank is used. But with only one in and out port it would not be suitable for separation.

    As a separator you sometimes get flow directly across if both sides have the same GPM, not much buffering until the load drops off. Or you may want to blend the return to a condensing boiler as low as possible.

    You can calculate that "mixed temperatures" but it will change when flow rates change as zones open and close. So there are some trade-offs.

    One of 3 conditions is present in a multi (four or more) port tank or separator;

    equal flow in boiler and distribution circuits

    flow in boiler greater than distribution

    flow in distribution greater than boiler

    Every condition has different, but calculate-able results.

    I have seen exotic Euro tanks that have an active, external pumped, v/s system to constantly adjust the tanks functions and blending. Possibly a bit to complicated and expensive.

    The mixed temperature formula you would use for P/S is the same formula you use for separator calcs,

    This link has some good number crunching formulas. Idronics 15, due this fall will look at separation again after 10 or so years worth of experience in separator use in America.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • optimist55116
    optimist55116 Member Posts: 12
    Thanks hot rod

    i wrote a nice long reply last night, but it seems to have gone into the ethers, It never made it here.

    Thanks for the comments Hot Rod. With regards to the mixing temps, I have worked through those same calculation in the Caleffi reference you gave (BTW, great article), the same ones in Siegenthaler's books.

    That still leaves me scratching my head on how far apart to place the inflow and outflow within the tank: 2 inches? 4 inches? 6 inches?

    When the boiler fires, the buffer water temp is still probably above an efficient condensing temp for the boiler return. For this part of the cycle, i would want pumps running on both the return from the manifold and the return to the boiler, minimizing any buffer mixing. Later, the manifold pump(s) would be off and the buffer would be heating up. During this part of the cycle, i would not capture any of the efficiency of a condensing boiler .

    Are you concerned about the water in the tank not mixing very well?
  • Tim Potter
    Tim Potter Member Posts: 272

    Fellow HomeOwner here, I too added buffers to my CI Boilers (2 houses) to eliminate short cycling. Boy It worked like a charm. but remember, the whole point of a buffer is to add volume (mass) to the system, a 'battery' to store hot water if you will. In order to do that, you must use as much of the tank as you can, filling it up with hot water while taking cool water out to the boiler. So hot from the boiler in at the top, cool back to the boiler from the bottom so when the buffer is 'full' of hot water the load can use that water in the buffer to satisfy the heating demand without constantly firing the boiler.

    Winter Park, CO & Lenexa, KS
  • hot_rod
    hot_rod Member Posts: 19,260
    To Tims point

    if the main goal is to add buffer capacity, supply from the boiler to the top of the tank, cold return to the bottom. Use as much of the tank volume as possible.

    Tank manufacturers use a number of methods to aid in buffering efficiency. Lochinvar has a baffle assembly in the bottom of their Loc-Temp. Others use a special "sparge' tube, some weld a baffle plate inside the tank.

    The Euro tanks I have seen have multiple side taps to gain access to the stratification layers, keep in mind as you pump through a tank, you will blend to some extent. With this Swiss design www.jenni.ch the heat input can be via the inside coil, so the tank can "layer." Then they access those layers with multiple side taps.

    Notice how they avoid top taps that could "ghost" or thermo-siphonh unless protected with zone valves or heat trapper nipples.

    I used the mid port connection on a ThermoCon tank with a double tapped bushing to build a tube to do the same as this Swiss tank.

    I know one design engineer is working on an "active" layering concept to buffer pellet boilers. Like a wood fired boiler you want a long, hot burn cycle to keep them at best efficiencies, but also return boiler temperature min. requirements need to be watched.

    With condensing equipment designs, the opposite, keep that return temperature as low as possible. Design with low temperature emitters to leverage condensing equipment, radiant or large sized panel rads, for example.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Tim Potter
    Tim Potter Member Posts: 272

    Wow, HR, that is a crazy tank. One thing that helped my systems, by adding the ESBE Thermic using a 140* stat element. It loads the tank with steady supply of 138* water (no need to go higher in my system using ODR & injection for in floor radiant). Before the ESBE, the 007 over pumped & the oversized boiler ran cool for 90% of the heat calls. Now during shoulder season, the boiler might only fire a couple times A WEEK......

    The OP wouldn't need the ESBE if converting to a modcon I believe, but with a CI boiler they are simple, effective & bullitproof
    Winter Park, CO & Lenexa, KS