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Thread: Copper Diameter and Pressure

  1. #1
    DIY Junior Member
    Join Date
    Jan 2012
    New England

    Default Copper Diameter and Pressure

    Have a really old house. It's plumbed with copper, 1/2" throughout. I don't see a tad of anything larger.

    The pressure is OK if one thing is open. Two things and it drops. Three, well, that's not good.

    I think the house is getting good pressure from the street - my unscientific observation is that a hose connected just after the meter has more pressure than anything in the house.

    In the basement, there is a run of 25' of the 1/2" copper after the meter and before any splits to the risers. If I increase that run to 3/4" or even 1", will there be a more than a dribble to the fixtures if multiple things are on?

    Is that increasing the pressure, or the supply? What impact will increasing the pipe diameter have?

  2. #2
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    The flow of water causes a drop in pressure due to friction. The loss is a function of distance.
    1/2" copper should support 5 to 6 gallons per minute. Above that flow, losses become noticeable.
    Increasing pipe size will minimize losses. Nothing can increase your pressure, but reducing losses might help!

    Water pipe sizing
    Last edited by Terry; 01-14-2012 at 09:39 AM.

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    quote; the galvy pipe being fulla rust. Rust will cause friction that will reduce pressure but at the same time its restricting the volume. This is why you can run one fixture without any issue.......

    I do not see anything in the original posting which even mentions galvanized pipes. The problem is undersized main lines. 3/4" copper will probably give you the best results. 1" would be much more expensive and while it has the potential to deliver more volume, you probably do not have any application which would need it, or could use it.
    Licensed residential and commercial plumber

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Changing the internal piping might help a little, but what is the supply pipe from the street? If it is small, you may not gain much. Friction is cumulative...the more small pipe you have, the lower the flow will be with increased demand. You can't fill a fire hose from a soda straw quickly, but once full, they'd both have the same pressure until you asked the supply for more water than the soda straw can provide.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Forum Admin, Expert Plumber Terry's Avatar
    Join Date
    Aug 2004
    Bothell, Washington
    Blog Entries


    1/2" pipe is only good for two plumbing fixtures.

    Any plumbing code book will have a chart that list the sizing you will need, and then you just work backwards.

    In Washington State, using the UPC code book, you would need at least 3/4" pipe for a one bath home.
    A two bath home would require 1"
    With some 3 bath homes, depending on distance and the quantity of plumbing fixtures, the inspectors may make you run 1-1/4"

    It's all about friction loss.
    I have never plumbed a home in 3/4" and can't even imagine how anyone would run 1/2" unless they were a carpenter pretending they were a plumber. A two bath home should have 1" from the meter.

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