Pipe size when adding bathroom?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by mc510, Jul 1, 2015.

  1. mc510

    mc510 Member

    Joined:
    Dec 18, 2009
    Location:
    California
    Previous owner of my house added a second bathroom, with permits. They did not, apparently, re-do the original plumbing (which looks to be 5/8 copper) but just tapped-in with ½ inch copper to go to the new bath. I'm guessing that this is the reason that the shower temp (either one) goes off wildly if any other water is turned on. Or is what they did proper? (It's an older home and does not have pressure balanced shower valves, but probably needs them.)
     
  2. Terry

    Terry Administrator Staff Member

    Joined:
    Aug 17, 2004
    Occupation:
    Plumber
    Location:
    Bothell, Washington
    You can run two fixtures on 1/2"

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    Last edited: May 22, 2016
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  4. mc510

    mc510 Member

    Joined:
    Dec 18, 2009
    Location:
    California
    Thanks Terry, that's good to know. Do I have some other problem, or is it just normal that shower temperature goes way off if water is used elsewhere?
     
  5. Terry

    Terry Administrator Staff Member

    Joined:
    Aug 17, 2004
    Occupation:
    Plumber
    Location:
    Bothell, Washington
    You may have four hot fixtures on hot. And the kithchen sink may be near too. Even so, without pressure balancing, it can happen in a home that is plumbed correctly too. Adding an expansion tank helps with that.
    https://terrylove.com/watersize.htm
     
  6. mc510

    mc510 Member

    Joined:
    Dec 18, 2009
    Location:
    California
    Thanks for pointing me to those "fixtures" tables; good information!

    Most common way we experience this is running the shower in the original bathroom, 5/8" (or maybe it's 3/4") copper piped. Then turn on the shower in the added bath (1/2" copper piped) and the temperature drops in the original shower. No other fixtures in use. Eight hot fixtures in total, but only four on the 1/2" branch.

    Also happens when only running fixtures on the original size of the house. Running original shower, then turn on kitchen sink hot water = temperature drop in the shower. In this case, 5.5 hot fixtures on 5/8 or 3/4 diameter pipe; that shouldn't be a problem but is.
     
  7. Terry

    Terry Administrator Staff Member

    Joined:
    Aug 17, 2004
    Occupation:
    Plumber
    Location:
    Bothell, Washington
    1/2" copper is 5/8" OD
    3/4 copper is 7/8" OD

    1/2" PEX is 5/8" OD, and a smaller ID than copper.
    3/4" PEX is 7/8" OD, and a smaller ID than copper.
     
  8. dj2

    dj2 In the Trades

    Joined:
    Aug 13, 2013
    Location:
    California
    Just to clarify:

    ID = inside diameter.
    OD = outside diameter.
     
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