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Thread: Expansion Tank and Water Hammer Arrester

  1. #1
    DIY Member themp's Avatar
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    Default Expansion Tank and Water Hammer Arrester

    I just saw a post from Terry that showed a water hammer arrester on the refrigerator ice make supply line. I then went and Googled how do these work and saw that there is a piston and spring that compresses air in the cylinder. My question is geared to my son's water heater expansion tank of 3 years that failed this week. He admits he did not check the charge yearly. It was completely full of water. And it was a Watts. Installed when the water heater was replaced by the plumbing company he hired.

    So, can a water arrester be used as an expansion tank? I remember someone posting that the expansion of water from a 50 gallon water heater was a few cups of water. I know these arresters are small, but they seem to last longer. Or do they fail also and have to be replaced and how would you know if it was behind a wall? I feel water expansion tanks fail to regularly and some kind of piston type device would be much more reliable and last longer. Why do you have to have this huge expansion tank for a few cups of water from thermal expansion? Why cannot the water heater come with one just like the TP valve.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    On average, you may not empty the entire WH, but if you do, the expansion can be more than a few cups. An ET is sized so that it does not stretch the bladder all that much, which also means the pressure doesn't rise much in the process.

    A hammer arrester is designed for a high pressure spikes, and does not actually hold any water, for the most part. They perform very different jobs, and are sized accordingly. No, you can't use an arrester for what you want.

    If the ET was not sized properly, OR if it was not precharged properly to your static water pressure, the bladder will be stressed, and it won't last as long. Or, you could have just had the luck of the draw, but they often last longer than 3-years.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the Trades Gary Swart's Avatar
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    FWIW, I have never checked the pressure in my ET since it was installed. Of course I balanced the pressure in the tank with the PRV pressure setting at install. It's been maybe 5 years or more and no problem. Only a guess, but did he balance the pressures at install?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The tanks seem to come precharged at 40psi. Most homes, unless maybe on a well, tend to have more...meaning, the tank's air pressure should be increased to match the water pressure. FWIW, you can't check the pressure there until you've depressurized the water supply...otherwise, it will always read the water pressure. So, that makes it an easy way to check the water pressure with an tire pressure gauge in case you do not own a water pressure gauge. Just make sure you don't get water out of the valve before putting the tire gauge on it...water into that gauge may ruin it, and water there is an indication of a failed tank.
    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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    Moderator & Master Plumber hj's Avatar
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    The VOLUME of air, (and its static pressure), is what makes an expansion work. A shock arrestor has a VERY limited amount of air, and you have no idea what its initial pressure is, or if it depends on springs what their compression is.
    Licensed residential and commercial plumber

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    DIY Member themp's Avatar
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    So with the new expansion tank, we put a water pressure gauge on the laundry sink that can measure max pressure over time and found that it had reached 100psi at one point. So, now it looks like the expansion tank failed because the pressure reducing valve is failing and letting the city pressure in which runs 90-100psi. Thus, the new PRV and ET failed in 3 years. Someone needs to design a better setup that can be monitored and maintained easier

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    Moderator & Master Plumber hj's Avatar
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    quote; found that it had reached 100psi at one point.

    A "failed PRV" would allow/reach 100 psi ALL the time, not "at one point". A failed expansion tank WILL allow the pressure to reach 100 psi "at one point". You may be misdiagnosing your problem, OR may be ready to replace something, such as the PRV, which may NOT be defective.
    Licensed residential and commercial plumber

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    DIY Member themp's Avatar
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    hj, yes you are correct on this. I went over to look at the pressure gauge on the laundry room sink and saw this. Normal water pressure from the PRV is 60 psi. New expansion tank was set to 60 psi. When the washing machine started to fill the gauge dropped to 40 psi, then when it cutoff the needle bounced back to 60 and hit the max pressure needle and pushed it to around 100psi. So, the pressure gauge with max reading can be incorrect here based on this bounce. Basically like a water hammer, the pressure jumps to 60 psi and messes up the max needle as it hits it. We decided that the PRV is OK at this point and just the expansion tank had failed and my son will check this more often now.

    I took the gauge outside and connected it to a water faucet and it shows the same. When I turn on another outside water faucet the pressure drops to 40 and then recovers when the water is turned off. Is this normal? A 20psi drop in pressure based on water flowing.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Pressure drop is a function of pipe sizing, which is based on how much friction the water sees during that flow (more corners, smaller diameter, longer run, more friction). Pressure is also affected by elevation changes, so if you measured it on the second or third floor (should you have one), it would be lower than in the basement. When there's no flow, the pressure is the same whether you have a fire hose or a soda straw, but volume delivered is a balance between pressure and the size of the pipe - a bigger pipe can flow more water.

    Dropping 20psi could be a pipe size issue, or the PRV may be restricting things. Many of them have an inlet filter, and it may be partially clogged, limiting flow. Filters are another area where you'll get restrictions at higher flow volumes.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10
    Moderator & Master Plumber hj's Avatar
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    A "good" PRV drops very little if any, when a moderate amount of water, such as a bathtub, is turned on. The "bounce" IS caused by water hammer, (and if the gauge had a higher maximum, the pressure might exceed that also), and an expansion tank may not prevent it unless it is installed RIGHT AT the washing machine.
    Licensed residential and commercial plumber

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    Retired Defense Industry Engineer jadnashua's Avatar
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    By virtue of the fact on the supply side, the pressure is higher, unless there's a restriction in the PRV (clogged filter maybe), unless it is defective, there should not be much difference in your outlet pressure. Things that can affect overall flow pressure (not static) were mentioned earlier.
    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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