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Thread: Question About Water Velocity

  1. #1
    DIY Junior Member keystone183's Avatar
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    Default Question About Water Velocity

    Ok, completely new here. Looking for a little info and help.

    I'm remodeling the two bathrooms in a house built in the early 50's. Completely redoing the water supply lines. My water supply is a well. I ran 1 1/4" pipe from the well about 175' to under the house (over kill i now realize). The pressure tank is set at about 60psi. Today when i finally got the supply all hooked up, i'm getting about 37 gallon/minute, +/- two or three. This will be hooked up to a 1" pex manifold. My concern is, that by my internet calculations, that is about 10 ft/sec, which i have read is too much. Can anybody tell me if i should be worried here, or if i need to modify something? Thanks for all the help in advance. This site is invaluable.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Water pipe sizing

    You can use the chart above for sizing. For that distance, you did need 1-1/4"


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    Retired Defense Industry Engineer jadnashua's Avatar
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    You'd only reach max speed if you were to try to draw that much. Then, your pump may not sustain that for long, either. By having a properly sized supply line, you can ensure you will not notice significant variations when you open more than one fixture. It is highly unlikely that you'd ever be drawing that much at once in the house, and the speed would vary accordingly.
    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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    DIY Junior Member keystone183's Avatar
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    Ok, thanks guys. I guess i wasn't thinking about that quite right.

    How about if i'm running a sprinkler system on this? Need to calculate and keep the velocity down for that?

    I've actually been reading alot this evening, and it seems that velocity might be more of a concern on metallic pipes? Any opinions? A consensus maximum velocity i should be shooting for?

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    Moderator & Master Plumber hj's Avatar
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    You will ONLY use the volume that the device is capable of using, UNLESS the pipe is disconnected and the water runs full flow, in which case you don't have to worry about velocity. How do you know that if you use a larger pipe you will not just get more volume but the velocity will be the same?
    Licensed residential and commercial plumber

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    DIY Junior Member keystone183's Avatar
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    Quote Originally Posted by hj View Post
    and the water runs full flow, in which case you don't have to worry about velocity.
    Little confused about this? Why does velocity not matter in that case?


    Quote Originally Posted by hj View Post
    How do you know that if you use a larger pipe you will not just get more volume but the velocity will be the same?
    Also confused by this. At the same pressure, a smaller pipe would mean less volume and less velocity?

    Thanks for helping me out here guys! I am really trying to understand all this, so sorry if i seem dense...thanks again.

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    Forum Admin, Expert Plumber Terry's Avatar
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    A large pipe reduces velocity and friction.

    That's why homes with more bathrooms need a larger water service. The small pipes have too much friction loss.

    When you have a milk shake, a larger straw makes it easier.

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Terry View Post
    When you have a milk shake, a larger straw makes it easier.
    Ja, but suction limits the pressure differential to less than one atmosphere which then limits velocity. Waiting for the milkshake to warm up increases velocity.

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