Copper Diameter and Pressure

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Engine1, Jan 14, 2012.

  1. Engine1

    Engine1 New Member

    Jan 13, 2012
    New England
    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. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    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 a moderator: Jan 14, 2012
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  4. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 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.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    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.
  6. Terry

    Terry Administrator Staff Member

    Aug 17, 2004
    Bothell, Washington
    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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