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Thread: 3/4 inch supply line to master bath, but 1/2 inch line at shower valve?

  1. #16
    DIY Junior Member barbourdg's Avatar
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    3 minute shower doesn't sound good. LOL

    By running only 2 items at once we would bounce between 5 gpm and 7.5 gpm. If we upgrade to a 75 gallon tank, perhaps we can get 7-10 minute showers.

    So I guess my final question is do we stick with 1/2 all the way through, or install a 3/4 valve on the 1/2.

    Thanks again for all the info, great website!

  2. #17
    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, industry says that 5-6fps flow velocity is about the fastest you can run water through a 1/2" supply line without having long-term problems. That amounts to under 4gpm for a single 1/2" supply line. Since you have both hot and cold, a max might be around 8gpm. Short sections of narrower restrictions cause the water to speed up (venture effect) to compensate, and the flow rate doesn't change much unless there are a lot of them, or they're longer. How a 1/2" valve can claim over 10gpm is not very realistic..I'd like to see their test results. You might be able to do that if your pressure was excessive, but the flow velocity would be beyond maximum for industry standards...it's not like they can reinvent the rules here.

    Most reliable, realistic 1/2" shower/tub valves are rated 6-7gpm. And, you'll only achieve that when your water is using almost a 50-50 mix of hot and cold which is unrealistic, especially in the winter. When your mix ends up more like 80-20 hot/cold in the winter, the max flow rate goes down.

    A 1/2" valve is typically only good for two heads max, and depending on your supply lines (pex or cpvc would be pushing even that) and pressure, may not provide great results even with that.

    As is discussed a lot here, if you have the vertical height to install one, a waste water heat recovery system could extend your shower with the same WH at a significant energy savings verses a bigger WH, and lengthen the available shower time. A little searching will show you some of this info.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #18

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    Quote Originally Posted by jadnashua View Post
    FWIW, industry says that 5-6fps flow velocity is about the fastest you can run water through a 1/2" supply line without having long-term problems. That amounts to under 4gpm for a single 1/2" supply line. Since you have both hot and cold, a max might be around 8gpm. Short sections of narrower restrictions cause the water to speed up (venture effect) to compensate, and the flow rate doesn't change much unless there are a lot of them, or they're longer. How a 1/2" valve can claim over 10gpm is not very realistic..I'd like to see their test results. You might be able to do that if your pressure was excessive, but the flow velocity would be beyond maximum for industry standards...it's not like they can reinvent the rules here.

    Most reliable, realistic 1/2" shower/tub valves are rated 6-7gpm. And, you'll only achieve that when your water is using almost a 50-50 mix of hot and cold which is unrealistic, especially in the winter. When your mix ends up more like 80-20 hot/cold in the winter, the max flow rate goes down.

    A 1/2" valve is typically only good for two heads max, and depending on your supply lines (pex or cpvc would be pushing even that) and pressure, may not provide great results even with that.

    As is discussed a lot here, if you have the vertical height to install one, a waste water heat recovery system could extend your shower with the same WH at a significant energy savings verses a bigger WH, and lengthen the available shower time. A little searching will show you some of this info.
    Your using the 5gpm in the wrong way. Intermittent use is fine to exceed the 5gpm. Hose Bibbs often exceeds the 5gpm limit without ill effects because its not ran all day everyday.

    Delta faucet co rough valve will accept several different cartridges. If you use the T17253 cartridge you will get about 6.8 GPM.......if you use the T17T253 cartridge you get about 9GPM. The valve has 1/2" inlets and outlets. Check for yurself. Its within their frequently asked questions of custom showers.

    Copper and pex is sized the same way.......pex makes up for the insert fittings by using less fittings in the system. The walls of pex do not build scale like copper either.

    The internet and books usually do not tell the whole story. After about 30 years you tend to know fact from fiction.
    Name:  bodysprays.jpg
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    Thats a Delta system that runs two different functions at once in any combination. The bodysprays run 2.0 gpm @80 psi. The owner loves it and it soaks you.
    Last edited by Hackneyplumbing; 02-13-2013 at 02:23 PM.

  4. #19
    DIY Junior Member barbourdg's Avatar
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    Hey Guys, thanks again for all the info. We did finalize the plumbing this afternoon. We're going with the 3/4 to the valve.

    In the end, we're talking about 2 hours of work, to reroute the 3/4 to the shower. From what I read on this site, it's best to have too much then too little. We will install volume control valves on each device, to regulate the water flow so we don't end up with a 3 minute shower! LOL

    THANKS AGAIN!

  5. #20
    DIY Junior Member barbourdg's Avatar
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    After the 3/4 valve, do we use 3/4 or 1/2 volume control valves? We have to convert our 3/4 line to 1/2 at some point before the device.

  6. #21
    Moderator & Master Plumber hj's Avatar
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    As previously stated, the waterway in a 3/4" valve are larger than in a 1/2" one but not by much, and neither has the area of a 1/2" pipe so you would get SOME increase but not a great deal.
    Licensed residential and commercial plumber

  7. #22

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    Kohler makes a 3/4 valve that flows around 18 gpm. Thats alot of water.

  8. #23
    DIY Junior Member barbourdg's Avatar
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    Still little confused. We have a choice of 3/4 or 1/2 lines to use for the thermostatic valve. If we use the 3/4 valve, we can either go with 1/2 or 3/4 volume controls. All of our devices have a 1/2 intake on them.

    We were thinking that 3/4 would be good, since we have the option for more output. If we want to conserve water, we simply don't open up the volume controls all the way.

    Regardless, we don't plan to use more then 10 gpm in the system.

    Thanks and sorry for all the questions. Really appreciate it though!
    Last edited by barbourdg; 02-14-2013 at 12:27 PM.

  9. #24
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you're going to use individual shutoffs to the heads, a 1/2" one should be fine.

    Make sure that the valve is designed for individual shutoffs, and doesn't want a diverter valve. The difference is, with a divertor, you'd have to turn the main valve off (if it has one) to stop all water. With individual shutoffs after the mixing valve, you wouldn't.. Some thermostatic valves don't like to be 'open' all the time with no flow.
    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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  11. #26
    DIY Junior Member barbourdg's Avatar
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    Name:  virtual_shower.jpg
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    Still tweaking, but here is the basic design I created in Photoshop. Still debating on (4) individual volume control valves or a 3 way valve. In this picture I am using a push button Toto valve on the rain head. They don't come in oil rubbed, so I am stuck with brushed nickel. I originally was going to use an oil rubbed rain head in the ceiling, but it seemed like over kill. The brushed nickel rain head, goes with our recessed lighting and sconces on the other side of the bathroom.

  12. #27
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by pfeinstein View Post
    Go with the 1/2", otherwise you might loose pressure. To compensate you might have to jack up overall pressure to get more pressure through this valve (rather than a 3/4") but then you'll have increased pressure at other points in your system (faucets, other showers, and the like).
    Wrong! the size of the supply pipe has nothing to do with the available pressure..it will be the same whether the supply is 1/8" or 2", but a larger pipe can supply LOTS more VOLUME! It's all about the area of the opening. pi*r^2...it's the squared factor. .5" ID, .25^2=0.0625 .75" ID, .375^2=.140624 or 2.25x more volume available (no need to multiply by the constant of pi, which makes the math easier!). You want to be able to supply more volume than the shower heads can use - this is what allows them to use all of the water their internal restrictions allow. When your supply is smaller than the amount you can use, then you'll find the performance lousy.

    The only downside of larger supply lines is that there's more water to purge when trying to get hot to the valve. This is where a recirculation system can help or keeping the distance from the heater as short as possible.
    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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