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Thread: thoughts on thermal expansion tanks

  1. #1

    Default thoughts on thermal expansion tanks

    Now that I've installed a few thermal expansion tanks, I'm wondering how successful these things are long term. Since they need to be checked periodically, How many of you are actually following up? Are you finding that they are holding pressure, or are you having to add air to them? What happens when one operates under pressurized? Ever had one fail?

  2. #2
    Moderator & Master Plumber hj's Avatar
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    The usually fail because they rust out. T Unless the pressure is ZERO or extremely high, there is no "under" or "over" pressure, because as soon as the system is activated, the tank pressure equalizes and stabilizes at the system's pressure. The "volume" of air in the pressurized tank will change depending on the system's pressure, but that is also fluctuating with every change in pressure, no matter how slight.
    Licensed residential and commercial plumber

  3. #3

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    So if the shrader valve fails, and the pre-charge leaks out, they are under no more than normal stress?

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    In the Trades Gary Swart's Avatar
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    Everything made by man will eventually fail. An expansion tank is no exception, but they generally will last a very long time without problems.

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    DIY Senior Member asktom's Avatar
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    Put a goopy hock inside the cap that goes on the schrader valve and the schrader valve will never fail.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A cap with a decent o-ring rather than an all plastic cap for the Schrader valve will go a long way towards holding in the air you put in there. The bladder static pressure prior to turning the water on is preset to your actual water pressure. Then, it only moves slightly as the water volume changes from a 'neutral' position. If the static air pressure isn't preset properly, the whole bladder is already being stretched further, and it may not last as long in use. It is flexing each time the WH cycles after you stop your hot water use. As with anything, it can only flex so many times before it fails. The further you force it to flex, the shorter time before it fails.
    Jim DeBruycker
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    Moderator & Master Plumber hj's Avatar
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    quote; If the static air pressure isn't preset properly, the whole bladder is already being stretched further, and it may not last as long in use. It is flexing each time the WH cycles after you stop your hot water use

    Once the system is pressurized, if its initial charge is less than or equal to the system's static pressure, the tank's air charge is at that same pressure, and it ALWAYS flexes with ANY pressure change, regardless of the initial pressurization. The ONLY time it does NOT flex with "WH cycles" is when it is at a higher than system pressure so it is only activated during the times when expansion reaches that pressure. It WILL actuate, however, at anything between 1 psi and 150 psi, (over 150 psi the tank's relief valve will discharge and release the pressure buildup before the tank can absorb it).
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    Retired Defense Industry Engineer jadnashua's Avatar
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    Yeah, I said that HJ! But, if you preset the tank to the water pressure, it sort of sits in the middle. If it is too low, the water will compress it further until the pressures match. If you overinflate it, you're stretching it further and the water may not compress it much. It's better to try to match your average water pressure before you turn the water on, then it sits more nearly in it's 'static' or neutral position. It always moves as the volume changes, which is why you put it in there in the first place!
    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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