Replacing plumbing to second story

[MKB]

I am getting ready to replace the plumbing in our home. I am planning on using PEX, with manifolds, for the water and PVC for the drains.

Main question - should I run 3/4" up to the bathroom on the second floor, or just run 1/2" up there? I have heard pros and cons on both sizes for the second floor.

[jadnashua]

The internal size of 1/2" pex and 1/2" copper are different - the copper has a larger opening. But, with copper, you'll have elbows and other changes in direction. The pex is smoother. The volume actually delivered to the fixture may be close. Pressure should be the same. A larger pipe means after the hot cools off from the last use, you have more to purge before it arrives at the fixture unless you use a recirculation system (pump, usually). So, it is a tradeoff. If you have a large capactity tub or multi-head shower, you might want the larger pipe. I probably missed some points...wait for some other thoughts.

[Marlin336]

If each fixture is getting it's own line I'd go with half inch unless it requires high flow. If you are going to be teeing off for each fixture by code (your local codes will vary) you can only have one fixture on a half inch line so you would need to bring up 3/4 and tee off it with half inch or even 3/4.