Shower mixing valve and continuation of supply line

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Duffman56

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New to the forum and to plumbing. Live in an old house, built around 1900. Three bathroom sinks, two toilets, two shower/tubs, kitchen sink, dishwasher, washing machine, and utility sink. Eventually (maybe in a decade) I hope to add another full bathroom....

Virtually all of the plumbing is 1/2" galvanized. My plan is to tear out all of this and do main runs of 3/4" pex, branching off to 1/2" pex at each fixture (probably just tying in to the existing 1/2" galvanized below each fixture for now). Most of the pipe will be fairly easy to get to if I avoid the short runs where the pipe goes through the floor to the fixtures.

I am going to tile my shower on the main floor and while I've got the wall open I definitely want to replace the galvanized in that wall cavity. There are two pairs of supply lines, one pair for the shower and another that goes all the way up into the attic to service the bathroom on the second floor. My thought here was to take out the two lines, and just have one pair of 3/4" going straight up through this wall cavity. I would branch off each 3/4" with 1/2" for the shower mixing valve on the main floor, the remaining 3/4" going up would service the second floor bathroom. I've uploaded a pretty crude drawing to try and illustrate.

Is this the best way to tackle this?
 

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Duffman56

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One more question - I'm needing a 3/4" npt to 1" pex elbow for where my main line enters the house, but have been unable to find one anywhere. Do these exist? Thanks to anyone who replies.
 

Jadnashua

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The i/d of 3/4" pex is a fair amount smaller than 3/4" copper...personally, I'd prefer a 1" pex main trunk. 1/2" is fine for most fixtures, though. You might prefer to feed your tub with 3/4" pex, though, as it will refill faster even with a 1/2" valve.

You might consider doing a manifold system. THere are at least a few design guides available free on the web when plumbing a house with pex. I'd start there, and ask questions to help select the best design for you once you have the basics down.

One of the things with pex is their fittings. On copper, the full diameter of the tubing is maintained through a fitting since the fitting goes on the outside of the pipe. WIth pex, the fitting goes inside of the pipe, each one adds a bit of restriction, and that can add up over a long run if you use many. Don't plumb it like copper...it bends, so you don't need a fitting to change direction as often. There are two different systems for attaching pex to fittings, crimp or expansion. The expansion method requires type-A pex, which is more flexible (makes a tighter turn without crimping itself) and because you expand the pex pipe to insert the fitting (it won't fit otherwise, unlike the crimp versions), the actual ID remains larger throughout, offering less restrictions.

The pipe's length and flow volume, along with restrictions (fittings) along the way will affect the overall flow and the dynamic pressure drop along the way. Larger pipe, because it can flow slower for the same volume, has less dynamic pressure loss as does using type-A pex with their larger ID fittings.
 

Duffman56

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Thanks Jad. I'd originally thought about using 1" but had seen a few places online say 3/4" would work. I have a cousin that owns his own plumbing business and he'd said 3/4" would work as well. But if you really think I need to go with 1" I can do that.

Would you plumb the wall cavity by the shower in the way I was proposing? If so, how would you work in shutoffs?

Have you ever heard of a 3/4" npt to 1" pex elbow? Can't seem to find one. Thanks!
 

Jadnashua

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The hassle with too small of a pipe is, you can't easily fix it later. There's an issue with a large pipe, too, in that it will hold more water in it and on hot, will take more to flush it out (which is where a recirculation system can resolve that).

It really depends on what your maximum simultaneous use will be. The more you ask it to flow, the greater the dynamic pressure drop.

There are guides from the people that make the stuff that will help you decide rather than a few opinions you can't vet. Keep in mind that the area or cross-section of the pipe opening has a squared factor in it, so a little increase can have a significant effect. For example, a 1/2" copper line can only carry half of the volume a 3/4" line can with similar pressure drops. Bumping it up again, increases it radically again. Now, your question is, is the additional cost worth it for YOUR circumstances. The industry guidelines will help you decide. FWIW, the larger tubing will also have a larger minimum bend radius.
 

Duffman56

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Thanks again Jad. Would you plumb the wall cavity by the shower in the way I was proposing? If so, how would you work in shutoffs?

Have you ever heard of a 3/4" npt to 1" pex elbow? Can't seem to find one. Thanks!
 

Reach4

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Have you ever heard of a 3/4" npt to 1" pex elbow? Can't seem to find one. Thanks!
You will need to use two pieces AFAIK. When you are looking for a part, you need to specify FIP or MIP or FNPT or MNPT. However in this case, I don't know of either whichever you are looking for.

I would use a brass elbow http://www.pexuniverse.com/1-x-3-4-brass-90-elbow-lead-free to do the size conversion and then use a 1 inch MIP to PEX adapter.
b-rl0504lf-02.jpg
 

Duffman56

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Good point reach. Was hoping for 3/4” MNPT to 1” pex. Is MIP male iron pipe? That’s what internet search suggested.
 

Reach4

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Good point reach. Was hoping for 3/4” MNPT to 1” pex. Is MIP male iron pipe? That’s what internet search suggested.


I think MNPT and MIP are used interchangeably when describing elbows. MNPT is tapered for sure. I think MIP is tapered also in practice.
 
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