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Thread: Water pressure vs. volume

  1. #1
    DIY Member SH140's Avatar
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    Default Water pressure vs. volume

    How does pipe size affect pressure & volume? If you have 3/4" copper piping, with a section of 1/2" somewhere in the line, what effect will that have? (compared to a 100% 3/4" line)? If you're remodeling and can't completely replace all the 1/2" piping in a circuit with 3/4", is it worth the trouble?
    Last edited by SH140; 09-14-2013 at 12:51 PM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    It depends on how long the 1/2" section is. Until you exceed the flow rate of the 1/2" line, there won't be any excessive friction, and both the volume and pressure may not be affected. FWIW, even a soda straw, with no flow, would have the same pressure, but obviously the volume you could get from it would be different than, say, from a fire hose. What happens when you have a restriction is that the liquid speeds up (think venturi, a jet in a carburetor is an example as is a hose nozzle), then, it can slow back down again if it transitions to a larger pipe. There is a maximum safe velocity of water flow in pipes, and about 5-6fps is the max you want, otherwise, it can create turbulence and pipe erosion.

    All pipe creates friction as liquids flow through them - the higher the flow rate, the more friction loss.
    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 Senior Member Murphy625's Avatar
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    Quote Originally Posted by SH140 View Post
    How does pipe size affect pressure & volume? If you have 3/4" copper piping, with a section of 1/2" somewhere in the line, what effect will that have? (compared to a 100% 3/4" line)? If you're remodeling and can't completely replace all the 1/2" piping in a circuit with 3/4", is it worth the trouble?
    That depends on both how long the 1/2 inch pipe is and where it is located.

    A 1/2 inch pipe located at the source is going to create more problems than the same length of 1/2 inch pipe located at the discharge point.

    It also depends on how many gallons of water is going through it. If you had 10 feet of 1/2 inch pipe connecting to a 50 foot run of 3/4 that was feeding a shower, it would generate enough friction to cause a problem.
    However, if the first 50 feet was 3/4 and the last 10 was 1/2, the effect would be far less noticeable.

    Take the above scenario and apply it to a kitchen sink and you probably wouldn't notice a difference either way until a dishwasher called for water at the same time you opened the faucet.

    As the other poster implied, pressure drops don't happen until you reach a specific flow rate within a specific size of pipe.

  4. #4
    In the Trades Gary Swart's Avatar
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    Keep in mind that a 3/4" pipe hold almost twice the volume of water as a 1/2" pipe. If there is a long length of 3/4" used for hot water, it will waste a lot of water and take time to purge the cool water out of the pipe to get hot water. A recirculation system would eliminate that problem. It's also great for 1/2" pipe too.

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    DIY Member SH140's Avatar
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    Thanks for the info

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    Moderator & Master Plumber hj's Avatar
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    quote; If you had 10 feet of 1/2 inch pipe connecting to a 50 foot run of 3/4 that was feeding a shower, it would generate enough friction to cause a problem.
    However, if the first 50 feet was 3/4 and the last 10 was 1/2, the effect would be far less noticeable.

    It makes no difference WHERE the reduction is, the effect is the same, as long as the flow conditions are equal. BUT, in the real world there would usually be other "devices" connecting to the 50' of 3/4" which WOULD affect the pressure and volume if it were at the end of the line.
    Licensed residential and commercial plumber

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