Installed too small of an expansion tank

Discussion in 'Water Heater Forum, Tanks' started by Austin83, Mar 31, 2013.

  1. Austin83

    Austin83 New Member

    Messages:
    15
    Location:
    Texas
    One more expansion tank question, I swear! I just found out my expansion tank is too small. I installed a Watts PLT-12 expansion tank (4.5 gallon tank) a few months ago, and I decided on that one based on what I was told at the plumbing supply store and the Watts sizing chart for the PLT and DETA expansion tanks on their website. The chart says that 40 PSI air pre-charge is standard, and the value 40 PSI is automatically placed on the chart (even though it can be changed). The problem is, all of the PLT tanks have an air pre-charge of 20 psi. Using the 40 PSI value my tank is adequate, but the 20 PSI pre-charge makes my installed tank too small for my system. The only tanks they have with 40 psi pre-charge are the DETA (commercial) series even though the chart says 40 is standard! :mad: Instead of my $40 expansion tank, it looks like I need a $150 tank just because of the pre-charge. Why does the pre-charge make such a difference when you just end up adding air to it to equal the incoming PSI from the water supply line? Can anyone help me make sense of this?

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  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,804
    Location:
    New England
    You're missing a very simple, but important point...prior to installation, you are supposed to inflate the expansion tank so its pressure matches your normal house water pressure. It has a fill valve (Schrader valve) just like the one on your car tires. Since the volume is fairly small, you may want to use something like a bicycle pump to inflate it, but once you've done that, all should be well. You do not have to remove it to add air, but you MUST shut off the water supply to it and open a faucet to drain off any pressure, otherwise, the air in it will compress and match the water pressure, giving you a false reading of its static pressure.
  3. Austin83

    Austin83 New Member

    Messages:
    15
    Location:
    Texas
    I did adjust it according to my incoming pressure, but why does a tank with a pre-charge of 20 psi vs a tank with a pre charge of 40 psi affect the performance of the expansion tank? If I'm just going to increase the pressure to 60 psi anyway, why does it matter? It makes over a 2 gallon difference in how much the tank will take in according to the chart.
    Last edited: Mar 31, 2013
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,804
    Location:
    New England
    When the air pressure isn't high enough to start with, the water can compress it easier, stretching it so there's very little room to expand when the water is heated. Blowing it up to the static pressure of the water supply means the bladder will be bigger, just like a balloon, and thus, have more room when things expand from heating. Think of your tire with 10# of air in it...very little room to compress if you hit a bump...fill it up to the 'proper' pressure, and it gets bigger, holding up the weight (in this case, the water).
  5. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,251
    Location:
    Cave Creek, Arizona
    They are NOT charging you more for the "precharge", there has to be a difference in the construction of the tanks to warrant the difference in price. Once the water is turned on the "precharge" in ALL tanks equals the water pressure, regardless of whether you add or remove air from the tank. I am curious why since both charts seem to have the same numbers how you got a different tank size with the second one?

    It is just a matter of equilibrium. When you start with a "precharge" equal to the water pressure, the diaphragm is basically balanced with the maximum amount of water AND air. If the pressure is less the air is compressed into a smaller space and you then have more water in the tank. If the pressure is higher, then the water cannot start to compress the air until it reaches that pressure so you have an excess of air and less water. As a practical matter, however, if the pressure starts low it will IMMEDIATELY equal the water pressure when the system is turned on, and if it is higher, but less than 100 psi, it will still absorb expansion when the static pressure exceeds that amount. Personally, I do not believe there is such a thing as a "undersized" tank.
    Last edited: Apr 1, 2013
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