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Thread: When and how to test thermal expansion tank?

  1. #1
    DIY Senior Member SteveW's Avatar
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    Default When and how to test thermal expansion tank?

    On another thread there are some videos from Dunbar Plumbing showing the black, stagnant water that comes out of the air side of a failed thermal expansion tank. I am wondering how to test a thermal expansion tank? I tap on mine once a month or so to make sure the air side sounds different from the water side. Is that enough?

    Should we all be letting some air out of the Shrader valve periodically to see if there is water on the air side of the tank?

  2. #2
    General Engineering Contractor ballvalve's Avatar
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    I think the best way to test a expansion tank is to not have one. That black crud is not a maybe, its a WHEN.

    Here are links from another discussion on those guys, take a look at the interesting products that are made specifically to displace the need for expansion tanks... especially the toilet tank filler-relief valve. Thats a unique one.


    So it seems Watts has provided an entire series of options to expansion tanks: here are a few links.

    http://www.watts.com/pages/_products...ls.asp?pid=803

    Here is one that really surprised me - wonder if Terry knows of it:

    http://www.watts.com/pages/_products...ls.asp?pid=855

    And another surprise - combo ball valve and thermal expansion relief valve for water heaters.


    http://www.watts.com/pages/_products...ls.asp?pid=564

    I believe all of those superior to tanks in most conditions, especially the ballvalve with the port for a drain line.

    Here is a US Apollo for $50 plus: http://www.grainger.com/Grainger/APO...XZ8?Pid=search
    Last edited by ballvalve; Today at 01:31 PM.
    Last edited by ballvalve; 11-27-2010 at 03:15 PM.

  3. #3
    In the Trades Gary Swart's Avatar
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    Maybe I'm missing something, but it looks to me like these devices work about like a T/P. I can't see where the water goes that is released when these things open. A thermal expansion tank hold the expanded water until the pressure in the heater lowers. I find it strange that if these things are a suitable replacement for a thermal expansion tank that this is the first time in my memory that anyone has mentioned them.

  4. #4
    General Engineering Contractor ballvalve's Avatar
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    At least for the toilet valve, it goes nicely into the tank. The others can be plumbed to a drain.

    I believe we greatly over estimate the expansion issue, and a tank is better used for water hammer.

    Watts appears to say they are a suitable alternate.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Watts MAKES them so what would you expect them to say, "These are substitutes for an expansion tank but are not as good, and also only duplicate the action of the tank's T&P valve"? They "dump" the water on the floor, except for the fill valve model which uses the "obsolete" tank ball operation. IF the expansion tank has failed, water will come out of the air valve when you depress the stem. The alternate test is to remove the tank, or drain all the pressure out of the water system, and then check for air pressure with a tire gauge.

  6. #6
    Forum Admin, Expert Plumber Terry's Avatar
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    An expansion tank helps to buffer the sudden drops in pressure when someone else in the home uses a faucet.
    I like to have an expansion tank for that reason alone.

  7. #7
    General Engineering Contractor ballvalve's Avatar
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    http://www.watts.com/pages/_products...=64&parCat=325

    They offer tanks on the same page as the alternatives.

    Notice that unlike the Water heater T&P you can buy the valve combo's in 60, 65, 80 etc. preset ratings, so you are not duplicating the water heater valve, but making it the final line of defense.

    If our government knew anything about public safety, they would drop the ridiculous lead standards in valves and fittings and require
    instead a back up T&P with a pressure relief setting far below 150psi. THAT might save a life.

    I am only discussing houses with pressure reducing regulators and standard water heaters. Not closed boiler heating systems.

    I have that situation in a few homes and have no pressure spikes.

  8. #8
    In the Trades Gary Swart's Avatar
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    I can personally verify the value of an expansion tank. When I installed my PRV, my water heater T/P immediately began to trip when the water began to heat. Not knowing about expansion tanks at that time, I replaced the T/P, but the problem continued. After posing the problem on this forum, I received my basic expansion tank lesson. Several years later, no T/P leaks.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    If an expansion tank is leaking internally, eventually, it will fill up with water. In the interim, it will start to get heavier from that water. A properly setup expansion tank has the air pressure preset to your static water pressure, and therefore holds minimal water. So, while it is known to be working, if you knock on it, it should sound hollow. If it ever feels heavy and sounds with a thud when you think to knock on it, it probably needs to be replace. If everything else in the house is working properly, you'd probably notice that the T&P valve weeping after using a lot of hot water.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Master Plumber Redwood's Avatar
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    I see some real problems with the pressure relief ballcock as it can easily be replaced by an unknowing person and remove the system protection.

    Personally I think the failure rate on expansion tanks is low enough that you don't have to worry about it.

  11. #11
    DIY Junior Member jasonbaur's Avatar
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    Quote Originally Posted by SteveW View Post
    On another thread there are some videos from Dunbar Plumbing showing the black, stagnant water that comes out of the air side of a failed thermal expansion tank. I am wondering how to test a thermal expansion tank? I tap on mine once a month or so to make sure the air side sounds different from the water side. Is that enough?

    Should we all be letting some air out of the Shrader valve periodically to see if there is water on the air side of the tank?
    I watched this video concerning expansion tanks a few days ago, I thought it was very informative.


  12. #12
    Engineer Furd's Avatar
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    As is far too common that video (and most others aimed at the DIY crowd) had some good information and a lot of poor or downright erroneous information. That specific expansion tank was on a HEATING system, not a potable water system. The "relief valve" on top of the air separator is NOT a relief valve but an air vent. The air vent has no pressure relieving function.

  13. #13
    DIY Junior Member jasonbaur's Avatar
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    Oops Should I erase the video? I just liked how the video went over how to test for a bad tank.

  14. #14
    Master Plumber-Gas Fitter shacko's Avatar
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    Quote Originally Posted by ballvalve View Post
    At least for the toilet valve, it goes nicely into the tank. The others can be plumbed to a drain.

    I believe we greatly over estimate the expansion issue, and a tank is better used for water hammer.

    Watts appears to say they are a suitable alternate.
    They wont work in the areas that REQUIRE an expansion tank.

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