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I am redoing a bathroom and while I have access, I would like to do my entire house in Pex. It looks like I will be able to get to all supplies and had a few questions. My main supply is 3/4 and will run to a Viega 30 port system (2-3/4 supply and then 1/2 for all outlets).

  • Should I run each fixture its own line or is that overkill?
  • Will the 1/2" lines suffice for shower pressure?
  • All upstairs will be run through a kitchen soffit, does it matter they will lay in there unsecured?
  • Lastly, do I have to use 90* bends if I have enough room to give it an easy turn into the soffit?

My current plumbing is the standard 1980's copper tubing with 3/4" and 1/2" lines that are branched. I did not know that you could get away with the 1/2" as which most Pex Manifolds come.

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  • Hello, and welcome to Stack Exchange. How long are the runs, especially that 1/2" second floor shower? – Daniel Griscom Aug 9 '16 at 1:36
  • Thank you for the response. The run will be approximately 30-35 feet from the manifold. It is already currently run with about 20 feet of pex from a copper line. If I do the manifold I want to redo everything. I have had no issues with hot/cold water in the shower. – USMCGRUNT Aug 9 '16 at 12:29
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Pex is typically installed as home run, not trunk and branch. The central manifold gives (mostly) equal pressure to all fixtures, so temperature won't change when water pressure changes like on trunk and branch. Because water pressure variation won't scald or freeze you, home run also allows more latitude with pipe sizing.

Home run pex also gives you the other advantage of pex, freeze protection. No fittings in walls, attics, or crawlspaces means no hidden damage if a freeze occurs, that's probably worth the added $100 in pex tubing.

1/2" pex will have no problem delivering enough water to any standard fixture. Test have shown that pex will perform almost identical to copper in standard household systems at 60' or more: http://www.homeinnovation.com/~/media/Files/Reports/pex_copper_pressure.pdf

You may be able to outrun your 3/4" feed though. If you have 20+ outlets you probably have laundry, showers, and hose bibbs all sharing that 3/4" feed. Turn on a garden hose and do a load of heavy laundry on warm and you'll be approaching maximum flow through that 3/4" line, which I would guess also supplies your water heater, with no pressure for your upstairs shower. That's an extreme case but you should be aware of it. I have done retrofits on 2 houses that I lived in where I ended up with that scenario, but I never experienced the problem because I rarely water the garden and shower at the same time. On my most recent remodel (with 34 total hot and cold lines) I have run a 1" trunk to 2 closets with manifolds, and put the hose bibbs and laundry on their own manifold just in case (using 1"x1"x3/4" tees to feed 6 way manifolds). That may be overkill though.

You can combine lavatory and toilet cold lines. You'll notice the water pressure change but it will only affect your hand washing. It only saves 1 line per bathroom though, so may not be worth it.

Pex should be secured every 32-48", but in retrofits you can't always do that. I have not had any tubes that bang around where I've had to run unsecured pex, but you may. Less slack in the line will give it less opportunity to move too. Long sweeping curves are fine, elbows are to be avoided for flow and freezing concerns unless you absolutely must use them. There are 90 bend supports that hold the pex in a 3" radius if you need tight turns, usually used to make the upturn into a wall. In your soffit you should be able to do sweeping turns except for maybe the upturn into the wall.

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There are pros and cons to both homerun and the more traditional trunk and branch systems.

Home Run Systems

Pros:

  • hot water is delivered faster and therefore less is wasted
  • stable water pressure (fixtures have less impact on the pressure at other fixtures)
  • uses less fittings and connections (yes you can and should avoid using elbows wherever you have sufficient space to simply bend the pex)

Cons:

  • uses more feet of pex
  • increased pressure loss at each fixture
  • does not work well for remote fixtures
  • cannot use a hot water recirculation pump

Truck & Brach Systems

Pros:

  • uses less pipe
  • less pressure loss at each fixture
  • can use a hot water recirculation pump

Cons:

  • uses more fittings and connections
  • less stable water pressure (fixtures have more impact on the pressure at other fixtures)

Hybrid Systems

There is also the option to use a mix of both of the above systems to create a hybrid system that can take advantage of the pros of both systems. These systems use remote manifolds or multi-port tees so that there is a trunk section that serves the water to each group of fixtures with each fixture individually piped from that connection.

Here's a link to a PDF from Uponor, a pex manufacture, which provides a comparison of all three systems (they call the hybrid system 'Uponor Logic') See page 10 and 11.

According to them, the hybrid system:

  • provides hot water 45% faster than trunk and branch systems and almost as fast as home run systems
  • provides hot water to fixtures with 33% less pressure loss compared to home run systems but still 30% more compared to trunk and branch systems
  • uses over 65% fewer fittings and connections than trunk and branch and only about 19% more than home run systems
  • uses 40% less pex than home run and about the same as trunk and branch systems
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I never have run a manifold system. I just run 3/4 and branch off 1/2 for each fixture. 3/4 for the Hose bib outside. Everything should be strapped down every three feet or it will clang and bang when you turn the water on and off. As for the 90s it depends how tight of a turn your going to make with the pex, if you go so sharp its going to crimp it then yes use a 90 or you can drill a holes if possible to give yourself more room.

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