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Thread: 3/4" or 1/2" pex branch line - which is best?

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    DIY Member closer's Avatar
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    Default 3/4" or 1/2" pex branch line - which is best?

    I was about to install my water feeds with copper as usual but in conversations with several who have used PEX more recently they suggested going the PEX route for my new plumbing requirements, especially as I had some very tight areas to work with that may require several smaller joints.

    In the attached drawing the two green lines represent the existing 3/4" copper lines coming from our basement (fed from a 42' dug well). The branch lines are the red and blue lines that will feed to the tub and the shower. All faucets have 1/2" connections. I know the ID of PEX is smaller than 1/2" copper so should I be running 3/4" PEX in my brach lines as far as I can then switch to 1/2" at the T for the tub and at the final wall connections for the shower unit (Grohe)?

    What is the best connection I can make at (A) - left side - to go from the 3/4" copper to the PEX? Space is very tight in here with both copper lines and the 3" drain/vent stack being somewhat in the way of each other. There were three 5" heat ducts in this closet as well, one of which has been removed as it is no longer necessary in the new bathroom (there were three in the area we are converting, one of which will remain. zThe other is at the far right of the closet and is also in the way, thus the switch to PEX.

    In this installation I show a 45 drop at the left end of the ABS (which will have a slope of 1/4" per foot)... The distance between the 45 and the Waste Y at the 3" horizontal Stack is less than 3'. Is this ok?

    Thanks again for any assistance you can provide folks. The diagram is not to scale, but it shows the general idea... ie. the Tub drain is actually within 6' of one Vent stack and just under 7' from the other. The entire length of the room however is actually 17'.

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  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    You could run 1/2" to the tub and the shower. Or run 3/4" until you branch off for the shower.

    The waste and vents are not right. The tub trap and shower trap are not right in elevation with venting. They will siphon.

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    DIY Member ImOld's Avatar
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    I am not a plumber but...........

    I have used pex only in the last year after many decades of copper, pvc, steel, etc.

    The last place I would want to use pex is
    especially as I had some very tight areas to work with that may require several smaller joints.
    Unless you assemble everything and then put it into place.

    Perhaps the working plumbers here would care to comment on this.

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    Quote Originally Posted by Terry View Post
    The waste and vents are not right. The tub trap and shower trap are not right in elevation with venting. They will siphon.
    thanks Terry... I'll probably run the 3/4" as the shower is capable of running more than one feature at a time...

    not sure what you mean by siphoning though, unless you mean the drop from P to drain would take with it the water in the bottom of the P?...

    not sure if I drew it quite the way I thought it would work... but the way i showed it in the elevation is probably the only way I could run the drain... (ie through the valance) without cutting through every floor joist along the way... the joists have 9" of space while the valance below would have another 9". the run from the shower 2" PTrap to the 2" drain in the valance would be about 30" with a vertical drop of about 5 inches inches. the run from the bathtub 1 1/2"" PTrap to the 2" drain in the valance would be about 20" with a vertical drop of about 5 inches inches... iff i made these two drops more gradual with 45 bends just after the P traps and another 45 just before the drop to the Drain in the Valance would that negate the effect of potential syphoning?

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    Forum Admin, Expert Plumber Terry's Avatar
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    Here is a nice link to Bert Polk's plumbing tips

    The vents come off the top of the trap arm, not below.

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    Quote Originally Posted by Terry View Post
    Here is a nice link to Bert Polk's plumbing tips

    The vents come off the top of the trap arm, not below.
    Thanks again terry... The link to Bert's PDF is great. I notice on page 12 that they talk about a flat vent which looks somewhat similar to the setup I was trying to achieve... Is the difference that in mine I drop down 5" before hitting the horizontal run that also connects with the vertical vent stack?... In that case if I revert to blowing holes in the 9" joists to run the pipe on the same level as the PTraps will that work?
    Will putting the holes in the joists be code ok... Ie. will they still offer the strength required for supporting the upper floor? What size should I make the holes... I got a 3" hole saw for the 2" drain when I thought I was not drilling through a joist and also to give some wiggle room... Is this too big?

    I noticed in Bert's book on page 8 that he says a santee cannot be used for drains, that a Y and an 1/8 are code... Then on page 12 he shows a shower drain setup with a santee! Which one is correct?

    If I end up blowing through the joists, I will still have to drop the line down about 12" at the far left to make contact with the 3" line. Is that ok?

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    Forum Admin, Expert Plumber Terry's Avatar
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    a santee cannot be used for drains, that a Y and an 1/8 are code... Then on page 12 he shows a shower drain setup with a santee! Which one is correct?
    Code wants a wye on it's back. Old code used to allow it. Not now.

    Drains are at 2% grade, 1/4" per foot.
    You need 2" on the to and bottom of the joist to retain structure. You can have a hole 1/3 of the height. (most of the time)
    I don't like running 3" across floor joists. I have done it at times. I prefer going under floor joists with 3" and running with a joist at times. I sometimes will run in the corner, where it can be boxed out, and jumping up between the floor joists.

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    I can only go in one direction and that is perpendicular to the joists , which is why I was hoping to run through the valance just below. I think in an earlier post though you, or someone said I couldn't run a straight drain the 20" (tub) or 30" (shower) to the valance level and put the PTraps there... They would have then been in the same horizontal run as the vent stack as well. Is this because the PTraps has to be directly below the drain it is servicing?

    When we took the old tub out it drained sideways to a PTrap that was in the 6" wall... It then ran horizontally to the vent stack. Except for the 5" drop is this not like what I was trying to do?
    Last edited by closer; 12-08-2013 at 12:58 PM.

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    here's a different sketch of the modified layout... where I have dropped a short line from the tub and shower to the level of the valance where the PTraps are... both would be in line with the Vent Stack and still have the 2% drop over the 17' of run... I've included a top view as well to show that where, in the side view they tend to look like they head to the left, in fact the lines coming from the tub and the shower come out more perpendicular to the 2' drain main horizontal line
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    just got back from speaking with another plumber and he said there wasn't anything wrong with the first setup... something about the right side vent stack negating any effects of syphoning... so now I'm in a real quandry as to which model (if either) I can use... and open to suggestions...

    I've always come to this forum for advice as I think you guys are the best, and everything has worked out great each time... many thanks... including a small bathroom reno (our first plumbing project), a laundry room built into the wall plumbing reno and a rather large kitchen plumbing reno which included a tricky island setup... the other bathroom reno was pretty straightforward and I was able to figure that one out without much guidance...

    but I really need to get this plumbing project under way and am looking for a solution... besides ripping holes in the joists... btw... there is a window above the tub so putting in another vent stack directly above the tub is out of the question

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    Retired Defense Industry Engineer jadnashua's Avatar
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    As I understand it, neither one is properly vented (this is somewhat hazy depending on what codes you use where this is located). As Terry said, the vent connection must occur before the drain line goes from horizontal to anything other than the standard 2-degree minimum slope. Also, by not putting the traps directly below your outlets, you have the opportunity of crud buildup in the pipe that will not be protected by the trap's water seal. And, the distance from the trap along the trap arm to the vent allowed is dependent on the diameter of the pipe - the goal being that there should still be air space to allow it to vent rather than end up full of water, and creating a siphoning effect.
    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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    Hi Jim.
    I think I'm finally clear on the horizontal venting that you and terry have mentioned will be the problem.
    So... I still need to run the major 2" drain through the valance, which is the 9" space below the floor joists. Anything other than this will mean that I have to blow holes in some major structure. On the same level as the PTrap there is about 6" of space between the outer edge of the joist and the start of the outside 2x6 wall. I also have space to get to it within the space of the joists. Would it work and is it legal if I were to extend a short (less than a foot from the potential drop to the valance main drain below) horizontal drain line and then up about 4' within the outer wall, then across and back up the vent stack I originally showed on the first diagram? Would doing this for both the tub and the shower solve the problem of siphoning and changes of plane? Would I still require a vertical connection from the main valance line to the vent stack or would the two sub-vents coming off the tub and shower work?

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    DIY Senior Member Reach4's Avatar
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    Quote Originally Posted by closer View Post
    here's a different sketch of the modified layout...
    You have the best sketches.

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    Quote Originally Posted by Reach4 View Post
    You have the best sketches.
    thanks... I think! its tough drawing plumbing in 2D! will be posting a photo in a minute to see if that helps

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    ok... enough of the sketches already... this is a real time photo of the space I am dealing with... nothing is permanently placed yet... but the vertical ABS (1 1/2") is about where I intended it to go... inside a 2x4 plumbing wall for the shower.the horizontal 2" ABS is about where the main drain would be (only 12" down in the valance. The hole on the left is where teh Ptrap will be and the edge of the hole is 5" from the 2x6 outside wall stud. on the far right is the 2' Ptrap for the shower, the end of which will be about 24' from teh wall.

    Question (always, eh?!)... can i make this both to code and workable (ie, no syphoning) if I run 1 1/2" ABS where teh blue dimensions are and on the same plane as the joists (which is also the plane that the Ptraps will be on. The 1 1/2" tub line, left, would run the extra 5" to the wall and just under the wall 2x6, then 90, up 36" and then horizontally to the new vertical stack that will run to the attic. The shower stack on the right would do the same only be much shorter. I realize the tub vent may be extending the distance a little. if absolutely required I guess I could move the main vertical vent further left to cut the distance by a few feet and the bring the shower horizontal line to meet it too.

    the main line would still be in the valance level so would I need to ensure it too had a T connection to the vertical stack or would the fact that it connects directly to the Main 3" stack (very far left and out of reach of the camera) ok

    I hope I'm getting closer... would love to start this tomorrow as time is running short...

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