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Thread: Best way to pipe in PEX/New bathroom?

  1. #1
    DIY Junior Member nola mike's Avatar
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    Default Best way to pipe in PEX/New bathroom?

    Trying to get the supplies together for a remote 2nd home, so I don't know all the specifics I'm dealing with. I will be adding a sink and toilet, and relocating a shower. I was planning on using PEX for the first time, and wondering what size to use. I had planned on branching off of the existing 1/2" line, but researching appears that this won't be adequate, although right now I have no issues with water pressure. I'm not sure what my main supply line size is (obviously, I'll need to figure that out). Regardless, I won't be replacing the main line unless I have major issues--the rest of the pipes are easy enough to get to. Given that, IF I have a 1/2" main, would I be better off installing a PEX manifold with 1/2" ports, or using 3/4" PEX off of my existing 1/2" feeder? Or something different?

    Water pipe sizing
    Last edited by Terry; 11-13-2012 at 08:55 AM.

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    Typically I run 3/4" on the cold for one bathroom and 1/2" for the hot.
    For two bathrooms, 3/4" on the cold and 3/4" for the hot. Or two fixtures, a tub and a lav on a 1/2" line.

    Water pipe sizing

    A one bath home can use 3/4" pipe
    Two bath requires 1" for the main
    Three bath requires 1" for the main, Depending on how many fixtures, this could also be 1-1/4"
    A four bath home requires 1-1/4" for the main.

    Last edited by Terry; 11-13-2012 at 09:15 AM.

  3. #3
    DIY Junior Member nola mike's Avatar
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    So I'm at the second home now. As I suspected, the main is 1/2". Right now that is supplying 3 showers, 1 toilet, 3 sinks, a dishwasher, and a washer. There are zero water pressure issues. I'm wondering if it's partly because the house is one story...
    Anyway, I'll be adding a toilet and additional sink to this arrangement. I'll be cutting into the water line before it reaches 2 showers, 2 toilets, and 2 sinks (adding a sink and toilet to this branch, which is the end of the run). Am I better off installing a 3/4" manifold at this point and running 1/2" pex to each of the 6 fixtures, or teeing 2 new 3/4" pex lines off of the existing copper supply?

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    Forum Admin, Expert Plumber Terry's Avatar
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    1/2" is plenty for one plumbing fixture at a time. If you have two people in the home, that's when you would notice. Or perhaps having the washer cycle on while in the shower.

    Your question? Should your home be plumbed to code?

    I like doing things to code, it removes all that thinking and justifying as to whether I will ever have enough friends and family to justify correct plumbing if they all visit me.

    Water pipe sizing

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    DIY Junior Member nola mike's Avatar
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    Quote Originally Posted by Terry View Post
    1/2" is plenty for one plumbing fixture at a time. If you have two people in the home, that's when you would notice. Or perhaps having the washer cycle on while in the shower.

    Your question? Should your home be plumbed to code?

    I like doing things to code, it removes all that thinking and justifying as to whether I will ever have enough friends and family to justify correct plumbing if they all visit me.

    Water pipe sizing
    Luckily, I don't have that many friends :-)
    No, I understand that doing it to code is preferable. However, with a 1/2" main it just isn't feasable. The question is whether there would be less of a pressure drop using the PEX manifold system v. Teeing off the copper, or does it not matter?

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    Moderator & Master Plumber hj's Avatar
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    IN your situation, it doesn't matter.
    Licensed residential and commercial plumber

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    Retired Defense Industry Engineer jadnashua's Avatar
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    You might consider plumbing the insides for an eventual main line upgrade, should it be an issue in the future. Why do it twice, should it become an issue in the future.
    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 nola mike's Avatar
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    Yeah, that's one of the things I was thinking with the manifold. However, since teeing the new fixtures with PEX is so straightforward, I think I'll go that route initially. If there's an issue, I'll know it right away. Even if that's the case, I can do the upgrade at my leisure and not keep the whole house out of commission. Thanks for the input guys.

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    I&C Engineer (mostly WWTP) Lakee911's Avatar
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    Quote Originally Posted by Terry View Post
    Thanks for the link. I wasn't aware that there was such a guide.

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    DIY Junior Member nola mike's Avatar
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    So I took another look at my main; it's 3/4", but for some reason reduced to 1/2" immediately upon entering the house. So I think that my plan will be to pipe 3/4" pex off of the 3/4" main. I'll tee off the existing 1/2" copper branches. At the end of the run (where the new fixtures will be), I'll run a 5 port 1" manifold with 1/2" pex branches supplying the new and old bathroom fixtures. Sound like a better solution?

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, 3/4" pex's ID is about the same as 1/2" copper. To maintain the same max flow volume of the 3/4" main, you might want to start out with 1" pex. A tub would benefit from 3/4" pex coming in, but most other things could probably be done with 1/2" pex (a multi-head shower might want 3/4" pex, too). Outside hose bibs and the WM, probably run 3/4" pex to maximize flow and speed up filling or cover more area if watering the lawn.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  12. #12
    DIY Junior Member nola mike's Avatar
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    Quote Originally Posted by jadnashua View Post
    FWIW, 3/4" pex's ID is about the same as 1/2" copper. To maintain the same max flow volume of the 3/4" main, you might want to start out with 1" pex. A tub would benefit from 3/4" pex coming in, but most other things could probably be done with 1/2" pex (a multi-head shower might want 3/4" pex, too). Outside hose bibs and the WM, probably run 3/4" pex to maximize flow and speed up filling or cover more area if watering the lawn.
    I couldn't find anything comparing the flow rate/pressure drop/whatever of nominal copper v. PEX sizes. But yeah, I guess I might as well do 1" if I redo it. I'll probably keep the 1/2" Cu trunks off the main line, and maybe connect them with 3/4" PEX adapters/pipe?

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    The OD of pex and copper are the same...the wall thickness varies a lot. Copper is a lot stronger, so it is thinner. Copper comes in various wall thicknesses, but even the heaviest typically used in a home is much thinner than pex. There is some discussion about max flow through copper verses pex where pex can be higher but mostly, that is because you have fewer fittings and changes of direction. Regardless, there is a maximum water velocity that is safe for the pipe (around 5fps or so) that limits the volume available. Depending on the pex and fittings that you use, those can be much more restrictive than an equivalent copper fitting which normally does not restrict the ID of the pipe's diameter (since it fits outside the pipe, not inside like a pex fitting). Then, there's the Wirsbo/Uphonor expansion system/fittings verses the barbed inserts from the others. With Uphonor's system, you cannot insert the fitting without first expanding the tubing, then it collapses back on the fitting to make the seal. With all others, you slide the fitting in, then crimp the tubing between the barbs to create the seal...that means the ID through the fitting must be smaller. So, for practical purposes, IF you need the volume, you need to upgrade the pex tubing size when compared to a CU pipe install. Now, many of today's faucets/valves are flow restricted, so having a lower capacity isn't an issue. It is when you come to filling a tub, a washing machine, or try to water the lawn, or fill a pool - there, you want the biggest pipe you can reasonably use given the supply system's capacity.
    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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