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Primary secondary ?

Will primary secondary work if the secondary circulators are on the returns?


  • hot rodhot rod Member Posts: 6,997
    the question becomes

    where would you connect the expansion tank? Ideally you want all the circs to pump away from the PONPC, or the expansion tank connection.

    A drawing of your piping would help. Is it a primary loop with boiler and loads connect in to a primary loop with a pump in that loop?

    Or is the boiler within the primary loop, eliminating one circ.

    Consider a hydraulic separator instead of a primary loop. With a separator you get P/S in a box or "tube".

    And you get the additional benefits of air removal, dirt and magnetic particle separation, and only two pumps may be required, both pumping away and adding their delta P to the boiler and load side.

    Looks like this:
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • MullhausenMullhausen Member Posts: 2

    Ideally I would pump away from point of expansion but in this case the existing pumps are on the returns. Customer wants to keep cost down and pumps are new. I'm installing an alpine and was curious what would happen if the system circs were left on the return. Wasn't sure if my boiler pump would pull water from a zone that is not calling for heat. In the piping schematic for the boiler the boiler pump is set on the return of the primary loop. With the properly spaced tees I was wondering if I would not have the correct pressure zone with the secondary circs on the returns or if it would function properly. Just curious to know. I would put spiro vent and expansion on supply header past tees.
  • IronmanIronman Member Posts: 3,403

    When the expansion tank is connected to the primary loop (PONPC), the secondary loops "see" the entire primary as the point of no pressure change. Therefore, the secondary loops (including the boiler loop) should pump away from the primary.

    With the scenario that you're describing, the secondary circ's would be adding their pressure to the primary which could cause unwanted forced flow into another loop. Flow checks should always be used on any loops coming off the primary, particularly upward vertical ones.

    The main problem with the circ on the return, in addition to increased energy consumption, is that the pressure difference that the circ creates is deducted from the static fill pressure. If it's on the supply, the pressure difference that the circ creates is added to the static fill pressure.

    What this translates to practically is this: if you have a static fill pressure os 12 psi on a two story house, you'll have about 5psi on the top floor. Now, turn on the circ. If it's on the supply, its pressure will be added to the 5psi already there. If it's creating 12ft. of head, that's 5.2psi for a total of 10.2 psi. This is good since the higher the pressure, the more air will be absorbed and carried back to the SpiroVent. But, if the circ's on the return, that 5.2 psi will be deducted from the static fill pressure of 5psi and you'd actually be in a vacuum at the top of the loop.

    It doesn't matter if you have the best air separator made if you can't move the air back down to it, then it can't remove it. It takes positive pressure to get it back down. And, the more, the better.

    More air binding problems are caused by the circ being on the return than any other issue. Put where it belongs: on the supply. In the long run, you'll save yourself and your customer more $$ and head aches.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • ChrisChris Member Posts: 3,056
    Your Hydraulically Separated

    The systems pumps have no influence on the primary pumps! The expansion tank should be piped on the primary/boiler side before the boilers circ so you would be pumping away!

    You can place your system pumps where ever you want!
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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