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Thread: flow reduction if 3/8 PEX replaces 1/2 copper?

  1. #1
    DIY Senior Member AcidWater's Avatar
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    Default flow reduction if 3/8 PEX replaces 1/2 copper?

    I need to replace all the copper; currently about 45 feet of 3/4 copper to the place where 1/2 branches go to all the fixtures and run 10-15 feet.

    If I ran a 3/8 PEX home run to near the water heater, how much reduction in flow am I likely to get to
    1) a tap -- might be OK for lav but not enough for kitchen?
    2) tub -- usually used as a shower, but I want to be able to fill the tub fast enough.

    Is there a table of capacity in gallons per inch for copper, CPVC and PEX -- they all have different IDs. I'd like to have an idea of how many gallons I'd be saving when flushing the line waiting for the hot water to get to the tap in return for less flow.

    Currently it takes 2-3 gallons to get hot. We dump it into the washing machine.

    Is 3/8 PEX ok for a shower -- I don't have any use for low flow heads; paying to pump from a well teaches you to control the flow using the valve !

    Is there really any advantage to putting a hot manifold after about 20 or 30 feet of 3/4, when the smaller diameter will run for 10-20 feet each? Seems no different than the way its laid out now with the 1/2 from the tail end of the 3/4 run.

    I don't really have room for lots of pipes, but I want to reduce the waste.

    Maybe three hot home runs:
    3/8 to kitchen

    1/2 for about 45' with 1/2 branches to all 3 lavs (1/2 will clear the line faster than 3/8 ?)

    1/2 that branches at the tail to serve both tub/showers. This is the current situation except the majority of the run is 3/4. Would all 1/2 reduce flow too much if both showers were running at the same time?

    I'll leave the cold as it is.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The OD of pex is the same as the equivalent copper pipe, but because the pex walls are thicker, it has less flow. 1/2" pex is close to 1/2" copper, and if you have lots of elbows the loss of velocity in copper might be overcome by the smoothness of the pex and get equivalent flows, but to use 3/8" pex, you'd notice a lot of loss.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
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    3/8" PEX is fine if it's only serving one fixture.
    Unless you have talked it over with the inspector, he will want 1/2" PEX on the tub.

    I've seen some systems that primarily use 3/8" out of the manifold.

  4. #4
    Plunger/TurdPuncher kingsotall's Avatar
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    Why does 3/8 keep coming into the mix¿ I think 3/8 PEX is geared towards radiant floor heating. And if the branches are more on the 10 feet rather than 15 feet side to fixture, a recirc line at the furthest fixture would solve your wait time for hot water.

  5. #5
    Master Plumber Redwood's Avatar
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    Quote Originally Posted by Terry View Post
    3/8" PEX is fine if it's only serving one fixture.
    Unless you have talked it over with the inspector, he will want 1/2" PEX on the tub.

    I've seen some systems that primarily use 3/8" out of the manifold.
    Probably the washing machine and outside hosebibbs too.

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