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Thread: Pex with thermostatic valve/volume valves

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    DIY Junior Member tomtbone's Avatar
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    Default Pex with thermostatic valve/volume valves

    I have heard that I should not use pex between a shower valve and tub spout because of resistance. I will be using a 3/4" thermostatic valve with separate volume controls for the tub and multiple shower heads; all 3/4".

    Having volume controls for everything including the tub, anyone see a problem using 3/4" PEx for everything and no copper as long as its all strapped down?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    You'll lose some of the volume capability of the valve if you use pex. Often, they specifically say not to use it to interconnect pieces. Using it as a supply also means you won't get the maximum volume you could have. in a traditional setup, using 1/2" pex, with a divertor spout, it will back up into the shower. Since most stub spouts are 1/2", using pex to it would be a problem. I'd use copper after the valve for all connections. This will also mean that you can anchor it well.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member tomtbone's Avatar
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    I am not using a traditional setup however with a diverter. The tub spout is basically plumbed like another showerhead, it has its own volume control (on/off). I suppose I could always use 1/2" copper to go down to the tub spout, but I could also use Pex drop ear elbows screwed into 2x4's. and that would be just as solid right?

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    In the Trades Wally Hays's Avatar
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    The reason for not using it between the valve and the spout is because the added restriction can force water up through the shower head when you are filling the tub. The mixing valve should have no effect one way or the other. If you don't mind the shower head dribbling than go for it.
    Perception is 3/4 of reality

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    Plumber jimbo's Avatar
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    The issues you heard about involve using 1/2" PEX on a 1/2" shower valve. I suspect the same issue would apply to using 3/4" PEX on a 3/4" valve. Consider using copper for the drop to the spout. And remember in terms of your overall plan, that 3/4" PEX is more or less equivalent to 1/2" copper.

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    Moderator & Master Plumber hj's Avatar
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    You do NOT have the same dynamics in your design that a diverter spout would. ALL of your devices will have a positive shutoff valve in the line going to them so it makes no difference WHAT you use, or the size, to any of them, including the spout. A bigger issue, if the thermostatic valve has an on/off function would be that you should NEVER have all the volume control valves off at the same time. Even though the valve should have "check stops", you could leave the valve turned on, and stop the flow with the volume controls, which could create a "cross over/backflow" situation between the hot and cold systems.

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    DIY Junior Member tomtbone's Avatar
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    Quote Originally Posted by hj View Post
    You do NOT have the same dynamics in your design that a diverter spout would. ALL of your devices will have a positive shutoff valve in the line going to them so it makes no difference WHAT you use, or the size, to any of them, including the spout. A bigger issue, if the thermostatic valve has an on/off function would be that you should NEVER have all the volume control valves off at the same time. Even though the valve should have "check stops", you could leave the valve turned on, and stop the flow with the volume controls, which could create a "cross over/backflow" situation between the hot and cold systems.
    What I have is a Kohler Thermostatic valve, with no volume control built in. IT is meant to only control the temperature.
    http://www.us.kohler.com/onlinecatal.../1041158_1.pdf

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Read the installation instructions very carefully. Many of them must have the water going INTO them turned off, i.e., not have the water OUT of them stopped. This is why most people use a multiport divertor valve rather than multiple stops. If it isn't obvious from the instructions (it may only be one line or word burried somewhere), I'd call the manufacturer's tech support line and specifically get their okay for your installation plan...
    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 tomtbone's Avatar
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    This is what it says in the directions:

    The K-669 and K-679 thermostatic mixing valves do not contain an integral volume control/shut-off valve. You must install a separate volume control/shut-off valve (K-671 or K-681)
    downstream of any used valve outlet.

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