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Expension tank too big?

john_58 Member Posts: 37
My expansion tank for my water heating boiler is either leaking air or too small. The pressure is fluctuating from 15psi cold (my min. for rise) and over 30 psi when hot (165F). The auto water supply is shut off, yet the relief valve keeps letting go at its 30psi limit. I had a professional come in and only purge the tank check the gas and leave. My question is can you over size a tank. I prefer the steel type to the bladder type, but am open to suggestions. The current tank is primed steel 14" dia by 57" length. thanks


  • ed wallace
    ed wallace Member Posts: 1,613
    compression tank

    i dont think you can over size it is the valve to the tank open? old steel tanks can go bad the steel can get to the point where air molecules can pass thru the steel might be time to go with a pressurized tank
  • jim_49
    jim_49 Member Posts: 24

    If your relief valve keeps leaking and the feeder is closed. You must have aleaking h/w coil, bad feed valve shut-off valve, and a full expansion tank. Poss. all 3.
  • Boiler Guy
    Boiler Guy Member Posts: 585

    was the tank purged? Was it completely or only partially drained?
  • Uni R
    Uni R Member Posts: 663
    Air Tanks

    As a homeowner who has experienced both, I would go with a pressurized tank. If you truly want a conventional tank, make sure it has a B&G Airtrol fitting on the inlet. This will help stop the air from going into solution and finding its way out of the tank. You cannot oversize the tank, but tanks get really climb in price as they increase in volume, and you aren't really gaining anything by oversizing. One thing I think you can do to help ensure the longevity of the bladder is mount it really low with sufficent drop that it isn't really seeing high temperatures or even much in temperature variations.
  • Ron Schroeder_4
    Ron Schroeder_4 Member Posts: 46

    You can't over size the expansion tank. You can also use more than one. With the pressure on the system bled down to below 12psi, make sure the expansion tank has a 12psi charge.

  • john_58
    john_58 Member Posts: 37
    Thanks for the responses

    Wow, thanks for all the advice. I gather that most of you prefer a bladder system.
    The existing tank is newer and in good shape, but the vent looks corroded. I thought by simply replacing it with a larger tank, I would eliminate the uncertainty of the existing one being too small and/or leaking.

    There is a taco air scoop before the tank and the supply adds to it. The tank was completely drained. I have not lost any pressure since shutting the supply off. I am thinking it is a combination of an undersized tank and a faulty supply. The boiler/ducting was new in 1998, and looks very poor (rotten). The previous owners must have not monitored the supply and relief cycles and just let it go. Any more advice is always appreciated.
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