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Expansion tank on the supply or return side?

Hey guys, I have baseboard heat in the house with a Weil McLain GV series boiler and zone valves. Currently I have a closed type expansion tank that started leaking at one of the seams. I am going to replace it with a bladder style tank with a Spirovent and an auto fill valve which my system currently lacks. Unless I'm mistaken the boiler manual and the instructions that came with the tank are telling me two different things. One is saying install the tank on the supply side, the other the return side. Am I missing something? Does it matter?


  • JUGHNEJUGHNE Posts: 5,578Member
    Does your GV have a pump inside the boiler?
  • GordyGordy Posts: 9,264Member
    edited December 2018
    You want the x tank piped on the suction side of the circulator.

    I believe the GV has an onboard circulator. So to pump away from the x tank that connection should be as close to the return side boiler port as possible.
  • GordyGordy Posts: 9,264Member
    That's why the x tank literature shows the circulator with the x tank on the suction side. The GV drawing isn't showing the circulator because it's in the boiler.
  • IronmanIronman Posts: 4,964Member
    The key is this: where is the circulator in relation to the expansion tank? It should be "pumping away" from the expansion tank.
    I believe that your boiler has an internal circulator? If that's so, then the tank should be on the return.
    Where's the circulator on your system?
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • NorthwoodsNorthwoods Posts: 5Member
    > @JUGHNE said:
    > Does your GV have a pump inside the boiler?

    Yes, it actually looks like there's two pumps inside the boiler...
  • NorthwoodsNorthwoods Posts: 5Member
    Ok, so pumping away or on the suction side would mean putting the x tank on the return line in my case. Thanks guys! Is that so the pumps don't "overpressure" the x tank?

    Not to get off topic but since I'm going to be changing some plumbing I was thinking of feeding the system with softened water instead of straight city water. Good idea or no? I just figured the less junk in the water the better...
  • GordyGordy Posts: 9,264Member
    Pumping away from the x tank (the point of no pressure change adds the differential pressure of the circulator to the system pressure which facilitates good air removal. Make sure the xtank has the proper charge while isolated from the system. 12-15 psi unless your house is three stories.

    I wouldn't do soft water. Once system is filled there should be no more water introduced unless there is a leak which should be fixed if there is.
  • Jamie HallJamie Hall Posts: 10,279Member
    Don't use softened water. The stuff is amazingly corrosive. Unless your city water is really horrid, it's better.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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