1/2" copper to 1/2" PEX Upstairs

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Bender1524

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Hi! Sorry if this topic already exists, I tried searching and didn't find anything.

I am replacing my shower/ tub upstairs. I plan to use the Sharkbite connector to transition copper to pex, then do the rest with pex. (Besides the tub spout) The current copper pipe is all 1/2, and has worked fine. However, since the inside diameter of pex is smaller than copper, am I ok just moving from 1/2" to 1/2" without messing up my pressure? I've watched a dozen YouTube videos and no one ever mentions that. I read one comment on the video that said this could cause a problem if the job is upstairs, and it got me worried enough to post here.
 

Jadnashua

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It somewhat depends on what's upstairs!

It also depends on how you run your pex and what method you use to make connections. If you use the crimp system, the entire fitting is inside the pipe's normal ID, and will create more 'friction' for the water flow than by using the expansion system which literally expands/stretches the pipe to insert the fitting (it won't go in until this is done), and then the natural ability of the pex pipe shrinks back to its original size as much as it can to literally create its own 'crimp' to make the seal. THen, some people tend to run PEX just like copper, with fittings all along the way to change direction...each one adds 'friction' that can during use, drop the dynamic water pressure available at the outlet. If you regularly use and fill a tub, it will take longer to fill than if you'd used copper all of the way.

If you don't use lots of extra fittings, the internal water flow friction and thus dynamic pressure loss will somewhat offset the smaller ID because the water will be able to safely flow faster through the smaller pipe. The static pressure remains the same regardless of the type of pipe you use, but friction caused by the ID, fittings used, and length all add up to lower the dynamic pressure resulting from using that water.

If you used 1/2" pex to supply a full bathroom, and someone were to flush the toilet or use the sink, you are likely to notice the drop in flow if you were filling the tub or taking a shower.

So, the answer to whether you'd notice or not, it's maybe, but leaning more towards probably. Whether it would be enough to be a major stumbling block, a little harder to tell. SOme of this also depends on what your static water pressure is currently. It would become much more of an issue if you had low water pressure now versus something closer to the maximum allowed (which is 80psi). I lived in the middle east for awhile where we had a storage tank on the roof and our water pressure was in the order of maybe 10 psi if the tank was full - more if you were on the first floor. I lived with it, but a shower wasn't very invigorating. We're spoiled here, but I do like it!
 

Bender1524

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Thanks Jadnashua! I was planning to use the crimping method rather than expansion. I don't forsee too many fittings or very much length. I was planning on cutting the copper just below the current connection for the old shower, then using the shark bite connector to switch to pex, then one elbow on each side. There would also be 1/2"x1/2" female adaptor for each of the 4 valve body fittings. They make the adaptors that go from 1/2 to 3/4, so would it just be easier to go with 3/4 pex, or does that introduce other problems? Haha
 

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Jadnashua

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3/4" pex is a bit larger ID than 1/2" copper, and if you went that way, you'd not lose any volume or pressure. Now, whether you need it or not would depend on what and how many outlets were in use at the same time. 1/2" pex is fine for one showerhead, toilet, or vanity at a time since they are all flow restricted that shouldn't impact 1/2" pex. But, if more than one was in use at the same time, you could quickly exceed the limit, try to get more volume out and the velocity goes up while the friction increases, lowering dynamic pressure.

FWIW, the Copper Institute's recommendation on copper is to limit velocity to NGT 5 fps, which, on a 1/2" copper pipe is only about 4gpm on hot, and a bit more on cold since the velocity allows is 8fpm instead of 5.

That can be an overall problem for a home, depending on how many fixtures can reasonably be used at a time. Normally today, the supply line would be larger. Exceeding the recommended velocities can introduce erosion on the pipes, reduced pressure, and flow noises.
 

Jeff H Young

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Big differance between connecting a tub shower valve with a foot of pex and running 6o foot across a house and a whole bunch of fixtures . If your piping is of ample size and pressure than I dont see how a few inches of pex is going to be a problem. sounds to me like you said you were replacing a valve?
 
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