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Thread: Boiler Expansion Tank

  1. #1
    DIY Junior Member jmasone's Avatar
    Join Date
    Oct 2012

    Default Boiler Expansion Tank

    About 8 months ago, the Extol diaphragm style expansion tank on my boiler failed (diaphragm failure) and needed to be replaced. Now, I think I might be having the start of the same problem again. Symptoms include:

    1. Pipes leading to hydro-air handlers sometimes make a gurgling noise, leading me to believe air got into the closed system
    2. Bottom of expansion tank (air side) felt warm (though nowhere near as hot as the top portion).

    When I tap the bottom half of the tank, it still makes a hollow tinging sound leading me to believe it's not flooded yet, but the other two symptoms worry me a bit.

    I checked the air pressure on the expansion tank and it looks to be between 10 and 12 psi give or take. It's a car tire gauge I'm using, so not sure how accurate it is.

    Lastly, The pressure on the boiler seems okay and the temp maxes out at 170.


    1. Should I be concerned at this point and call in a pro?
    2. If this is a problem again, is it just bad luck or could there be other causes for this repeated failure?

    Thanks for any input.
    Last edited by jmasone; 10-12-2012 at 10:40 AM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    First, the only way to check the pressure in an expansion tank that measures the static air pressure, is to relieve all water pressure from the other side first; otherwise, it will just read the water pressure. The static pressure should be about the same as the boiler's water pressure. That way, the diaphram doesn't get stretched too far and it will last longer. If you didn't get any water out when you checked the pressure, the bladder is intact - it might not have the proper pressure in it, but it is intact.

    If the pump is not moving the water properly, it can stay IN the boiler too long and intermittently flash to steam, which will makes some sounds. What you are hearing, is hard to tell without being there, but it could be bubbles of steam collapsing.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member jmasone's Avatar
    Join Date
    Oct 2012


    Thanks for the reply. The gurgling/sloshing noise only seems to happen when the zone has been sitting idle for a while and now is calling for heat. The furnace has just kicked on so the water is fairly cool still. Could the circulator pump be faulty and introducing air temporarily into the system, which eventually gets purged out as the system runs? If not the expansion tank, I'm just stumped as to where else this air is coming from.

  4. #4
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Mar 2011


    Air in the boiler/hydronic system comes from poor design or improper purging. Both are often found on the same problem jobs and calling in a smart guy is a good idea since pumps will not move air and the cost of pump failures, poor flow and discomfort can be significant. Diaphragm expansion tanks should last for decades.

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