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we are installing a shower where two sinks used to be. Is it possible to combine the inbound water (1/2") from each sink into one 3/4" line for hot & cold to feed the Moen U valve we want to use to distribute water throughout the shower?

We will have the following in the shower:

  • 2 or 4 body jets
  • 1 10" jaclo showerhead
  • 1 12" jaclo rainshower
  • 1 handheld wand

Thanks for your help.

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    Have you traced the 1/2" lines to see whether they're supplied by a 3/4" line in the first place? They may just split from a single 1/2" line, whereby your plan would be pointless.
    – isherwood
    Jan 9, 2020 at 14:19
  • I think @isherwood has it correct here, this is likely a 1/2" supply feeding both sinks...
    – Solar Mike
    Jan 9, 2020 at 14:22
  • The surface area of a circle (water supply line) is π r2 so a half inch line provides .2 and a 3/4 line is .44 so two 1/2 inch lines provide enough water to fill a 3/4 inch line. But keep @isherwood's comment in mind because it the two 1/2 inch lines were branched from a 1/2 inch main then combining them is useless.
    – MonkeyZeus
    Jan 9, 2020 at 14:55
  • Agreed @isherwood especially if it is CPVC or copper piping. If it is pex pipe with a manifold system, it is highly likely it is a dedicated run to each sink, If the pex is not a manifold system, then it will be branched of as needed to each sink and as mentioned earlier both sinks could be branched off of a single 1/2" line.
    – Jack
    Jan 9, 2020 at 15:13
  • Hello, and welcome to Home Improvement. Interesting question; if one of the answers is good, please click the checkmark to "accept" it. And, you should probably take our tour so you'll know how best to participate here. Jan 9, 2020 at 16:42

2 Answers 2

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I'm not a plumbing code guru, but I see no problem with what you have planned. The only side effects I can think of are a slight increase in noise (due to the added turbulence) and the possibility of stagnant water in one side, should there be an odd flow imbalance.

Be aware that the two 1/2" lines may not be supplied by a 3/4" line, though. They may just split from a single 1/2" line, whereby your plan would be pointless. A split 1/2" line is probably more common for bathroom sinks, which don't require a lot of flow.

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  • Increasing diamter reduces the velocity which reduces the noise.
    – Solar Mike
    Jan 9, 2020 at 17:10
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    It's not quite that simple. There's disturbance before and after the diameter increase, as well as additional bends.
    – isherwood
    Jan 9, 2020 at 17:20
  • "only stagnant water side effect". I wouldn't treat stagnant water in such a cavalier fashion. I would expect code to forbid it for health reason.
    – Jeffrey
    Sep 10, 2021 at 20:45
  • Stagnant water occurs everywhere a sink isn't used regularly, such as in the bathroom in an empty nester's basement. It's rarely a health concern.
    – isherwood
    Sep 10, 2021 at 20:50
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Yes. You can take a 1/2” cold and a 1/2” hot supply and increase the diameter of both to 3/4” cold and 3/4” hot.

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    I think you've misunderstood the question. It's not about increasing, but combining.
    – isherwood
    Jan 9, 2020 at 14:18
  • thank you everyone, I guess the first thing I ned to find out is if each sink is fed independently or off a single 1/2" line. Walls come down next week, so i will find out then I guess. Best regards,
    – arbtown
    Jan 10, 2020 at 15:05

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