Theory: Do PEX home runs only benefit the hot water distribution?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by jefff, Mar 25, 2019.

  1. jefff

    jefff New Member

    Mar 25, 2019
    Cleveland OH
    I've been doing my research to repipe some or all of my fixtures to home runs as I renovate the basement. Part of which included this presentation:

    I have a 3/4" copper trunk branching mostly to 1/2" copper and some galvanized remnants. There is a high flow shower feed that is 3/4" hot and cold pex that runs through uninsulated space.

    My priorities include:
    • Fast delivery of hot water to fixtures
    • Tempering valve to prevent scalding temps in pipes while letting my heater run at ~140
    • Not much energy waste
    All of these effect the hot water distribution only. Nobody cares about fast delivery of cold water, or letting excess cold water sit in pipes, etc. It seems like you could daisy chain 1" or larger pipe to each cold fixture and toilet in series and have no effect on performance.

    My plan for the hot side is existing 3/4" copper to the heater, run the HW tank at high temp to increase capacity, this honeywell mixing valve to bring supply down to about 120F, to 3/4" uponor PEX, and through two of these Uponor logic manifolds on a tee. Then 1/2" shortest-distance runs to each hot water fixture. I wan't to bring pex out of the wall to a pex stop valve to eliminate connections in the wall.

    When I do my work, should I bother replacing any of the cold, or have a simple branch system to save complexity? I don't see a benefit to the home run system for cold supply except to have matching hot/cold lines that are easy to trace. Prove me wrong?

    Additionally - what is the best way to attach the stubbed out pex at the wall without tearing into plaster/lath? I'll have a bend support, but I'm looking for something like an old-work electrical box that would attach to the plaster itself, not a supporting strap that I'll have to cut out a stud bay and patch.
  2. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    If your tempering valve could be at the bathroom instead of at the WH, and if you run 1/2 inch PEX for the hot, instead of 3/4, you will get hot quicker. If the tempering valve is at the WH, running a 1/2 inch to the shower and home-run 3/8 to the lavatory will get warm water to the shower and lav quicker.

    Of course a recirculation system can get hot/water quicker still.

    Not sure of your situation or objective. You want PEX stubbed out of a wall for a shower, a lavatory, a toilet, or what? What is your access below? Crawl space? I ran long screws into the wall through escutcheons to provide mechanical support at the wall. I fed with both 3/8 OD soft copper and 1/2 inch PEX as I tried various things. I think there would be room for a product made for this purpose.
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  4. jefff

    jefff New Member

    Mar 25, 2019
    Cleveland OH
    Sorry, the idea was 3/4 to the manifold, and then 1/2" home runs on the hot side.

    I like the idea of running 3/8" to fixtures that it would be appropriate for, but the availability of 3/8" makes it a bit impractical.

    The objective would be PEX through the wall to stop valves for lavs, kitchen sink, toilets. Shower / tub would be retrofitted with sweat connections as needed to not tear everything out.

    Basement has access for running pex up to 1st floor, but there are fixtures on the 2nd floor with little access.

    I think my solution might be a bend support in the wall, pex through the wall with a bushing to protect it, then a escutcheon /sleeve combo to "snug up" the bend to the wall. I just don't want it to flop around.

    Something like this:
  5. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    3/8 Aquapex is easy to get in 400 ft rolls, but not in smaller lengths. A bigger problem is the fittings in Aquapex. They are much easier to get in fittings that can be used with stainless clamps.

    You can use clamp fittings and clamps on Aquapex / PEX A if you choose to mix.

    I agree with your hot vs cold theory from a practical point of view. The mixed branch cold and home-run hot might bother some people's sense of desire for symmetry. Mixing expansion and clamp might bother others.
    Last edited: Mar 25, 2019
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    If you're repiping, add in a return line for hot water recirculation. You don't have to use it.

    Any pipe will have a maximum recommended flow rate and your design should not exceed that. That recommendation varies based on the ID of the pipe involved and it's type. Pex allows a higher flow rate than copper. This limitation is based on minimal dynamic pressure loss...the faster you try to flow the water through the pipe, the more friction, and the lower the pressure will be at the outlet. The size of the pipe does not change the static (no flow) pressure, only the dynamic pressure when water or any liquid (air is considered a liquid for this exercise), is flowing through the pipe. For copper and hot water, the maximum velocity is 5-fps which equates to 4gpm with a 1/2" pipe, and 8gpm with 3/4" pipe. When feeding a manifold, you need to consider the maximum number of branches that may be in use at any one time and what their typical draw will be.

    The ID of pex is smaller than the equivalent sized copper, but because (it should be) run with fewer connections, and is potentially smoother inside, the pex manufacturers allow higher velocity, but that is tempered by the smaller ID, but even with that smaller diameter, pex allows slightly more volume, but potentially with a slightly lower dynamic pressure. Excessive velocity can also result in higher water flow noise, and on copper, literally erode the pipe over time from the inside out.

    Both the copper institute and the pex people have free design handbooks. If you follow the recommendations in there, it will provide a reliable, satisfactory end result.

    As to terminating things in the wall, if the wall is open, they make brackets that attach between studs that are designed to accept copper stubouts from the pex supply line. This keeps things nice and rigid and straight. Omitting them works, but my recommendation would be to use a transition and bracket if the valve might be used a bit. It's easier to keep things from being slightly skewed and annoying you if it's visible.

    If you've got a bunch of high flow devices, feeding it's manifold may not be sufficient with a 3/4" inlet.

    If you live in an area that experiences water rationing or shortages, seriously consider a hot water recirculation system. WIth it, and proper design, you'll have essentially NO delay getting hot water. Any other system will have some, and in the first draw in awhile, it could still be significant, depending on the run length, pipe diameter, and the design.

    Properly insulating the lines and good management of the recirculation system can mean lower energy costs overall versus a system without one, especially when you consider the water and sewer costs many people have.

    A manifold system often allows easier maintenance and convenience if it is installed with individual branch shutoffs at the don't have to shut the entire water supply down to perform work on a branch. While it's common to install shutoffs to things like faucets, it's not as common for things like a tub or shower valve. Having the water off while you try to hunt for the proper replacement cartridge can be a big inconvenience.
    Last edited: Mar 27, 2019
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