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Pumping away, circ location and direction

PM_2 Member Posts: 38
I was speaking about the pump in the diagram directly over the boiler. This certainly isn't on the secondary.


  • PM_2
    PM_2 Member Posts: 38
    Circ location and direction

    I just read John Siegenthaler's article referencend in another thread.


    The piping diagram contains is an illustration of a question I've had for a while. The boiler circ is pumping away from the boiler, but toward the expansion tank. Wouldn't it be better to pump away from the point of zero pressure change?
  • Barbarossa
    Barbarossa Member Posts: 89

    I concur that it is better as you describe it; as the expansion tank is the pivot and why use pump energy to compress the tank you might also question the location of the cash-acme valve. I would try to contact the author for his opinion I know many times I welcomed such comments and question regarding my articles. This way we all get better at what we do. If you do get a reply share it with us.

  • How would the pump energy compress the tank?

  • The boiler pump

    is really on a secondary system; it will add it's differential pressure, but it won't be much since it's usually a very small pump.

    The primary circulator is positioned properly, i.e. pumping away from the x-tank, adding it's differential pressure and pressurizing the system.

    But it's a very complicated set-up when you're trying to find the PONPC; I think there are many of them.

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  • PM_2
    PM_2 Member Posts: 38
    pumps compressing

    I don't think the pump will compress the tank, what will happen is that the pressure at the pump inlet will be noticably lower than system pressure. Under some conditions there can be cavitation of the pump which will destroy it. Either way, the actual flow rate may be different than the designers intent.

    In the system shown, we can assume that the boiler has little flow resistance so the difference in inlet pressure (assuming the piping is generously sized) between the two pump positions should be a small effect. This was more of an accademic question.
  • hr
    hr Member Posts: 6,106
    I agree the pump

    should be pumping away from the PONPC. It could be a mistake in the drawing. BUT, with those small circs, I'll bet the system would work fine, as shown in the drawing.

    Then again, I can't tell from the pictures EXACTLY where the expansion tank is connected to the piping. Looks like the spirovent is in a different location from drawing to actual pic, also. I'm not sure Siggy did the drawings on site?

    The secondary pumps use the common piping between the primary and secdondary zones as it's PONPC or compression tank. Page 87 of Dan's "Pumping Away" book.

    hot rod

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  • Dan_8
    Dan_8 Member Posts: 56
    Two Spirotherms?

    The Spirotherm that you can see in the pictures is definitely in what he's calling the 'utility loop' in the schematic and it's not shown there. The expansion tank is hanging off of something hidden behind the flue. You can see the makeup water heading on over that way as well so perhaps the contractor installed two Spirotherms so he didn't have to worry as much about purging the injection loop? Just a guess but that's the way that it looks to me.
  • hr
    hr Member Posts: 6,106
    Two purgers

    is a very good idea with injection mixing. It can take forever to get that air across the small injection bridge.

    If I have the distance, a basic cast air scoop works well in the primary loop. A Spiro, or similar, grabs those "micro bubbles" quickly from the thousands of feet of radiant tube. Saves a lot of manual purging time. As long as you establish flow, regardless of how noisey, the Spiro will handle the small details :)

    hot rod

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