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Thread: 1/2" Pex with 3/8" elbows for shower thermostatic & volume control installation?

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    DIY Junior Member murphy290's Avatar
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    Question 1/2" Pex with 3/8" elbows for shower thermostatic & volume control installation?

    Our alcove-type shower will have a fixed shower head and a handheld on the right wall, and controls on the left wall. The valves used are a Grohe universal rough-in (35 026) with a thermostatic control (19 865) and two separate volume controls (rough-in is 29 273 and trim is 19 182) for the handheld and fixed shower head so that they can be used separately or at the same time.

    Because of the tight space and the lack of detailed instructions supplied by the manufacturer, installation wasn't easy. Will the valve installation shown with 1/2" Pex and 3/8" elbows work properly?

    Thanks

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    Retired Defense Industry Engineer jadnashua's Avatar
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    1/2" pex may not be enough to operate two showerheads at the same time, at least with any useful pressure...the volume is just likely to be too small. It may depend on how hot your supply water is, as you'll probably need a lot of cold to give you enough, and the only way that may work is if the hot is very hot, then a lot of cold would be needed to temper the outlet to a comfortable level. In the winter, if your incoming cold water is pretty cold, you won't be using much cold to bring the temp down, and your outlet volume may go down.
    Jim DeBruycker
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    DIY Junior Member murphy290's Avatar
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    Thanks - are you saying that if we only use either the shower head or the handheld at any one time, this setup would be OK, even with the 3/8" elbows? In order to be able to use both at the same time, would we need 3/4" Pex and 5/8" elbows? Does using elbows cause any problems - further reducing flow or otherwise?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Your results may differ...1/2" pex has about the same ID as 3/8" copper, so you're already restricted some. THen, add in a few fittings, and the ID gets smaller. Each fitting, and the actual length of the tubing, contribute to friction and flow restrictions. A fitting, if it opens up again after it, acts like a venturi...the fluid speeds up going through, then, slows back down again afterwards, not affecting the total flow much. But, add a bunch of them, or no opportunity to stabilize and minimize any turbulence, it reduces the flow. Depending on the showerheads you are using (federal rules limit them to a max of 2.5gpm each), you may have a hard time getting decent volume (and velocity through the heads) for a good shower if they are both running at the same time.

    My comment on winter verses summer and how hot your water heater is set comes into play when you consider you have two supply pipes...make the hot hotter, and you can mix more cold into it to get your desired temp - your volume available goes up. But, in the winter, the cold may be VERY cold, and you could count on much less volume from the cold tap to reach your desired temperature.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member murphy290's Avatar
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    So I guess the question is, should we make changes, and if so, what should we do?

    Taking a closer look, what we have is:

    Both hot and cold water supplies are 1/2" copper, transitioning to 1/2" Pex.
    For both the hot and cold water 1/2 Pex lines, there is one 3/8" elbow halfway to the thermostatic valve, plus a 3/8" elbow at each inlet to the thermostatic valve.
    Distance from copper water supply lines to thermostatic valve is 8 feet.
    From the thermostatic valve, there are two 1/2 Pex lines, each with a 3/8" elbow, that go to two separate volume controls.
    Distance from thermostatic valve to volume controls is less than 1 ft.
    From one volume control, there is 1/2 Pex with one 3/8" elbow to the fixed shower head.
    For the other volume control, there is 1/2 Pex with two 3/8" elbows to the handheld.
    Distance from volume controls to shower heads is 8 ft.
    Both shower heads are 2.5 gpm; the fixed one says recommended minimum pressure is 15 psi.

    Thanks

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I don't have enough real life experience to help you much more on this. I will say that 1/2" pex supply lines for filling a tub does slow it down verses 1/2" copper (real personal experience), and that one showerhead is adequate, but I think two would be marginal to inadequate. But, some of this depends on what your local water pressure is. ANy showerhead starts to lose performance when one of two things happens: the pressure is just too low, or, you cannot supply more water than they want to dispense - IOW, you want more water volume available than the head(s) can dispense to get the designed stream size and velocity. PEX at 1/2" may not provide the flow you need for two great showerheads. You won't know for sure until you try. Changing it to copper or larger pex, and you'd almost certainly be okay.

    WIsh a pro would add their thoughts...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member Reach4's Avatar
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    Would it have been much harder to use 1/2 inch fittings? https://www.argco.com/store/Main.asp...l&item=7016486

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    DIY Junior Member murphy290's Avatar
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    Sorry - I believe they are 1/2" fittings. The inside diameter that fits into the Pex is 3/8". I don't have plumbing experience, so help is appreciated.

    Thanks

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    DIY Senior Member Reach4's Avatar
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    I have never run PEX. I think I would have bent the tubing and used fewer fittings, but maybe there were other factors. Minimum bend radius, is at least 6 times the OD (some allow less). With 1/2 inch, I think ID is about 0.475 and OD is 5/8 inch. So that would be 3.75 inch radius. I would probably sweep wider than that.

    http://pexsupply.blogspot.com/2010/0...ex-tubing.html

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