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Where should the expansion tank go and why?

ChrisJ Member Posts: 15,496
I'm reading Dan's book "Pumping Away" and at this point one thing confuses me.  Where should the expansion tank go as a rule? 

If the expansion tank goes on the supply as a rule, why?  WIll it work just as well on the return assuming the pump is on the return right after the expansion tank?

Being the book talks about the best place for the circulator being on the supply I have to assume the expansion tank also belongs on the supply, but I don't understand why.


Chris J
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment


  • Zman
    Zman Member Posts: 7,551
    I think it is all in the book.

    The air scoop, which is often attached to the expansion tank. It will function better with hotter water and at lower pressure.

    The T&P on the boiler does not want the extra pressure.A high head circulator pumping into the boiler could set off the T&P. Picture a system pressure of 18# with a 13# circ head pressure. It would trip a 30# T&P.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • alotlikeearl
    alotlikeearl Member Posts: 68
    edited October 2012
    Check Out

    Here is info from Burnham's Heating Helper that may help.

    "Pumping Away"...but from what?

    The question still at hand is where to place the circulator in the system

    so it reaches its maximum effectiveness. This is where the term “pumping

    away” sometimes gets misconstrued. Many believe that the term

    “pumping away” means to pump away from the boiler and therefore

    they end up mounting the

    circulator directly off the top

    of the boiler on the supply

    pipe. While this may seem

    to be somewhat correct, in

    reality it is wrong. The real

    meaning of the term “pumping

    away” means pumping away

    from the system expansion

    or compression tank. The

    reason for this has to do

    with the functions that the

    circulator and expansion tank serve. The function of the circulator is

    to create a pressure differential and the function of the expansion tank

    is to regulate the system pressure as close to static fill as possible. If

    the tank encounters a drop in pressure it will try and fill that pressure

    void and if it encounters a pressure increase due to mechanical means

    or thermal expansion, it will try and absorb it. When a circulator is

    mounted on the boiler supply but is situated before the expansion tank,

    nearly all of the extra discharge pressure that the circulator is capable

    of developing will be absorbed by the expansion tank resulting in static

    fill pressure or less being exerted through the system piping. The water

    will still move through the system but there will be relatively low system

    return side pressures and more than likely, captive air and noise will be

    encountered in the farthest circuits of the system. When the circulator

    is mounted on the system supply directly after (as close as possible)

    the expansion tank you will now be exerting the extra pressure it is developing

    where it is needed most or to the system supply piping. Now it

    will more effectively move the water and keep high enough pressures

    in the remote portions of piping to prevent air from accumulating or

    coming out of solvency. The lesser pressure that pump is developing

    on its suction side will be in contact with the expansion tank. The action

    now will be that the expansion tank diaphragm will move into place to

    fill that pressure void causing an increase to static fill pressure at the

    suction side of the pump.

    The end result will be a well working, quiet and captive air free system

    with no consumer complaints or callbacks. In final review, the term

    “pumping away” means pumping away from the expansion or compression


  • hot_rod
    hot_rod Member Posts: 21,882
    check out this article

    from the Plumbing & Mechanical website. Siggy does a great job of explaining the concept with both cast iron boilers and hydro separators.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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