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Location of microbubble resorber relative to circulator

Gordan Member Posts: 891
Is there anything about how microbubble resorbers and circulators work that would suggest that it's better to place one before the other? Note, this is not about the expansion tank. I know that that needs to be close to the suction side of the circulator.

Example: does the circulator create turbulent flow that would interfere with the MBR's function? Does it create a low pressure zone on the suction side that makes air come out of solution? Etc.


  • Tom Blackwell_2
    Tom Blackwell_2 Member Posts: 126

    The lowest pressure makes the bubbles bigger, so it should be on the suction side of the circulator, wherever that is in the system. Bubbles are more easily removed at maximum temperature and minimum pressure.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    The lowest pressure makes the bubbles bigger,

    Also, the hotter the water, the bigger the bubbles, so as close to the boiler output as possible.

    My W-M installation manual has it put in the secondary part of the primaray-secondary piping, along with the expansion tank, just before the two circulators. And that is how I have it. But I think this is a big mistake with a modulating boiler.

    The secondary circuit gets up to 120F on one zone and 135F on the other. But it goes up to 170F for the indirect hot water heater that is across the primary. In my non-professional view, the microbubble absorber should be in the primary loop, before the takeoff to the indirect.

    Piped as it is, it took several months to where one of the circulators quieted down from bubbles, instead of a couple of weeks.
  • SpeyFitter
    SpeyFitter Member Posts: 422

    I could be wrong, but I think in Siggy's book it recommends a minimum of 12x pipe diameters of straight pipe between a circulator and an air seperator. The bottom line is put a little bit of distance between a circ and air seperator.

    Now typically in a mod-con system you'll have your circulator pumping into the boiler (in most cases). In this case I'd pipe my feed valve, expansion tank, etc. into the suction side of the circulator, and put the air seperator on the outlet of the boiler (supply/hottest water). In this case, most of the pumps head has been dissipated by the heat exchanger of the mod-con, and the temperature is highest, so the air seperator will work admirably in this location.

    On a conventional boiler, you typically want your air seperator on your supply side, with your feed valve/expansion tank tied directly into the air seperator if it has a tapping (most do) and then your pump with some distance (see above) between your air seperator and your pump for turbulence reasons (to protect the impellar from cavitation). Now will butting your suction of your circ right up to an air seperator kill it prematurely - in theory, yes, but in practice I'm sure they last longer then you would think. But again it's a recommended practice by many guru's/designers  in the industry.

    The bottom line is the positioning of your air seperator, point of no pressure change (feed valve/expansion tank connection) and pump are critical in relation to each other with respect to the removal of air in a hydronic system. Put them in the proper location and they'll work admirably without issue. Put them in the wrong location and you could be pulling air into the system and causing all sorts of problems.

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  • hot_rod
    hot_rod Member Posts: 22,251
    the hottest point

    is the best location. With a primary secondary sometimes it is a good idea to have an additional one on the secondary loop.

    Most mod cons like to have the circ pumping into them, on the return piping, so the expansion tank would be located there, with the fill valve also.

    On older system retro fit boilers, a dirt separator is a good idea. Consider the new combo air/ dirt for the secondary side.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Brad White
    Brad White Member Posts: 2,398

    The notion of straight pipe of so many diameters is relative to scoop types. The microbubble resorbers are more forgiving. The Taco 4900 allows you to place them right at an elbow. Those PALL rings do scrub out excess air and if you fill your system at or upstream of this, it does mean less venting in the end.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    The reason for having the pump as close to the PONPC as possible is...

    As you have more separation down stream of the PONPC, the pressure actually "droops" below the static fill pressure, which is conducive to getting air out, except that it is occurring AFTER the MBR or scoops location. This is one of those concepts that is hard to see in your minds eye, but it does happen, hence the reason for stacking the pump as close to the PONPC as possible.

    And as Brad stated, the X times pipe diameter assumes the use of a scoop, and flow at theoretical maximum velocity. In reality, maximum flow rarely occurs in a conventional multizoned residential space heating application. Finding the theoretical room to put the scoop in with straight pipe before and after the scoop is hard to find in todays mechanical closets.

    Also, numerous manufacturers now make a vertical MBR .


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This discussion has been closed.