Thermal Expansion

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by MrBillyd, May 2, 2012.

  1. MrBillyd

    MrBillyd New Member

    Messages:
    10
    Location:
    Scottsdale
    Here is a bit of background of my Thermal expansion problems.
    Here in Scottsdale, my water pressure was hitting about 100 during a 24 hour period.
    I have been seeing a bit of water on my garage floor from the over pressure relief on a daily basis. ( I think since they changed out my meter to a new style, I assume this new one has a check valve?
    The 100psi was the norm and I would see the highest indicator being over 140 150.
    So I sweated a pressure regulator on my line and dialed it down to 70 psi.
    I still get stagnate pressures over 120 psi, after turning on the tap it drops to about 55 and then settles at 70. Then it builds again from there.

    So, now I still see the water about noon time on the floor.
    I have a 80 gallon tank with a SunEarth solary system heating it. I do not use the Electrical back up 11 months out of the year.
    So, my tank probably cycles a temperature delta of about 70 deg. With the highest here in May, 160 F.. Meaning after a family of 4 takes showers, and dishes, clothes ect. it drops to 70-90 F. When it builds temp the pressure of course increases.
    I have a mixing valve that adds cold to the hot water to drop it to about 130 F.

    I think my old meter allowed the pressure back into the citys network.


    So I added a Watts Thermal expansion tank. A Plt 12 last week.
    I followed some info from http://www.griffithengineering.net/services/aspe_newsletters/Thermal_Expansion_Tanks_revised.pdf
    And punched in my info from Watts web site. http://www.watts.com/pages/support/sizing_DET.asp
    That tells you how to size it.
    I set the air pressure to 70psi, to match my water pressure.

    I still get a bit of water on the floor. The engineering link I posted says after the pressure valve has been exercised a few times it becomes leaky. I think I believe him as it seems to blow about 120 psi.

    So, after my long story> I have a question or two.
    I understand water does not compress.
    How does this expansion vessel work? I know it's designed to compress the Air on the other side of the bladder.
    So, I got the pre charge set to 70, and the water set to 70.
    Lets say the water is 100 F. and it temps to 140F.
    As the water wants to expand, the water has a place to go, (not the city this time) and pushes into my vessel.
    So, I monitor the pressure on the clothes washer hot water spigot > I think I should not see the pressure rise on this location.
    I think it should instead compress the air.

    My new Watts PL12 seems to be a dud, as I see the pressure rise on the hot water and pop the over pressure relief at about 120psi.
    I know it's suppose to pop at 150psi, so I will address that later.

    I have not checked it< But if it was working correctly I would expect the air pressure to rise from 70 to the 100's

    How do you check for a dud?

    Any thoughts.

    Bill
  2. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Yes, you are susceptible to "weeping" of a t/p valve anywhere over 100 PSI. 120 PSI will probably weep even on a brand new valve. Remember that it is basically a spring, and at 150 PSI it would be full open.

    You have a solar system, and that is a different animal. You definitely have to be designed for over temps and over pressures. I do not know if a simple XT is enough. I suggest you call your solar installer in to verify your system design.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,048
    Location:
    Cave Creek, Arizona
    When the expansion occurs, the air is compressed, BUT the water pressure will ALSO rise, (the air and water pressure will be equal), but neither of them will increase as fast or as high as it would without the expansion tank. A "good" PRV will maintain the pressure and NOT drop when a faucet is opened OR increase when it is closed. I have not seen any Scottsdale water meters, or any place else in the valley for that matter, which has a check valve in it.
    Last edited: May 2, 2012
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,266
    Location:
    New England
    If the expansion tank is sized properly, with typical volumes of water expanding, the pressure should remain fairly constant...yes, as you compress the air, the pressure will rise, but nowhere near as much as if there was none.

    If the T&P valve has been exercised frequently, replace it and that should stop the weeping if the PRV and tank are working and setup properly.

    Think of the air in the bladder as a spring...properly engineered and setup, you can push the spring a fair amount (compress the air) without changing the pressure much.
    Last edited: May 2, 2012
  5. MrBillyd

    MrBillyd New Member

    Messages:
    10
    Location:
    Scottsdale
    I had the size correct

    Jadnashua, I totally agree with you.
    I used the link in my post above, for sizing the Watts PL Tanks. I played with the automated calculator, that takes into account for range of temp, and number of gallons of the tank. I used 125 psi for my calculations in the macro so be conservative (15% below T&P as Roger Griffen states in the engineering link). The default on the automated link is 150 psi.
    The calculator has my tank volume at 4gal. and my unit is maxed at 4.5. So, I have it engineered well within range.
    I expect my water pressure not to go above +10, and I expect to see my air side take the brunt of the pressure.

    I found my problem, and it is my air gauge
    . I bought a new one, and found my pre charge at 40psi. With this pre charge my max volume is over 9 gallons, so this is why my T&P valve is tripping..
    I now have it matched to 70psi for real this time. I will monitor my hot water pressure, tomorrow and take an air pressure reading when the system is at its max.


    Thanks for your thoughts.

    Bill
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,048
    Location:
    Cave Creek, Arizona
    quote; take an air pressure reading when the system is at its max.

    Unless you have developed a new rule in physics, it will be the SAME as your water pressure. Even when your precharge was 40 psi, it still equlaized to the system pressure once you turned the water on. THe difference the precharge makes is the VOLUME of air in the tank once its pressure equalizes. With a 5 psi precharge the volume will be close to zero and will absorb very little expansion, at 120 psi, the volume will be 100%, but the water expansion pressure will be "rejected" by the tank until it exceeds 120 psi.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,266
    Location:
    New England
    As said, once in the system, the water pressure will cause the air pressure in the expansion tank to be rise to its level. you can only check the precharge when there's no water pressure being applied. If you don't have a good water pressure gauge, you can use a tire pressure gauge on the expansion tank once things are up and running, but it must be moderately accurate!

    Over extending the bladder in the expansion tank can shorten its life and in worst case scenario, cause it to immediately fail.
  8. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Change the t-p valve and put a bucket under the pipe to water the roses.

    Springs are very reliable and do not have a range of 100 to 150, take as an example the adjustable relief valves - they are spot on as to opening pressure.
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