15' branch to lav sink, 2 toilets: 1/2" sufficient?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by speede541, Jun 12, 2013.

  1. speede541

    speede541 Member

    Messages:
    124
    Location:
    San Francisco Bay area
    I'm wondering if the water flow at a lavatory sink will be impacted if it shares a 1/2" copper branch also connected to two pressure-assisted low flow Gerber dual-flush toilets?

    I can pretty well guarantee at least one of those toilets will often be refilling when the faucet is used, and probably at times both.

    The branch will be about 15 feet long in total, with the sink being the first fixture at about 10' on the branch (not sure if that even makes a difference).
    Last edited by a moderator: Jun 12, 2013
  2. speede541

    speede541 Member

    Messages:
    124
    Location:
    San Francisco Bay area
    One of the toilets is currently installed elsewhere, fed by a 1/2" branch off a 1" main, where it takes 50 seconds to recharge. So a real life 2.0 GPM, vs. a "book value" of 3.5 I think I'm supposed to use according to a pressure drop table I looked up.

    Also, the bathrooms being serviced are on a 3rd floor of a house that's already off the street, so I'm assuming a 13 PSI pressure drop for 30 feet of lift. But we have excellent water pressure here -- I believe near 80 PSI at the service entrance.

    So I'm not sure if I should be using 1.5 + 2.0 + 2.0 = 5.5 GPM for sink + 2 toilets which I think is real (and about 7.5 ft/sec flow rate), or 2.0 + 3.5 + 3.5 = 9 GPM which I think is the "book" value, because apparently that bumps me into the "undersized" region of the chart with a 12 ft/sec flow rate.
    Last edited: Jun 12, 2013
  3. Terry

    Terry Administrator Staff Member

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    Bothell, Washington
  4. speede541

    speede541 Member

    Messages:
    124
    Location:
    San Francisco Bay area
    Thanks for the confirmation.

    Re: your link, yeah, I bought the DF's based on CR's review, back well before I discovered your site, and around the time you wrote that post. We bought three. One is still in the box, but the other two have been performing flawlessly. However I did go with a Toto for a fourth recently partly due to your recommendation, partly because the Gerber's being slightly noisy for a guest bath near the kitchen, but mostly because of dimensions.
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