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Pipe sizing to sub-manifolds

artisanfarms | Posted in General Questions on

I’m planning two sub-manifolds for the house I am currently renovating.  One will supply the plumbing core of the house – the kitchen, guest bath and laundry room which are adjacent to each other and the other will supply the master bath and adjacent plant room.  

My question is how should I size the pipe from the main manifold adjacent to the hot water heater in the basement to the two sub-manifolds?  The water heater will be ~20′ from the plumbing core and ~35′ from the master bath.  I’m planning to use PEX A, so will not lose capacity through the fittings.

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  1. Expert Member
    BILL WICHERS | | #1

    What kind of fixture load are your sub manifolds expected to see? You need to size the feeder for the flow you need. If you want to play it safe, just use the same size "main" as you use to feed the main mainfold. If you have several small loads on the sub manifolds, like a bunch of toilets or something else that won't present a large load, use a smaller line. You're probably fine with 3/4" in either case, 1/2" for smaller loads. I wouldn't go smaller than 1/2", and I wouldn't go larger than 1".

    I'm assuming you are using sub manifolds to limit the number of branch runs to fixtures? You may find it's cheaper to just use more PEX than to use more manifolds and associated fittings. PEX itself is a pretty cheap material. You can use the multi-run support brackets to keep things neat.


  2. artisanfarms | | #2

    To answer your questions, one hot water sub manifold will supply 2 sinks, and a shower. The second hot water sub-manifold will supply 3 sinks, a shower, dishwasher and washing machine. It seems like it will be more energy efficient to run one hot water line to each sub manifold which will be emptied several times a day, rather than 3 to the first and 6 to the second, where it is possible that some lines will only be used a few times a week.

    My question was really a guidance question - 1/2" seems too small, especially for the line that supplies 6 end points, 3/4" will probably be sufficient, but would 1" make sense? I'm looking for possible rules of thumb, given the expected use at the actual fixtures.

  3. Expert Member
    BILL WICHERS | | #3

    You won't see any real difference in "efficiency" between the two ways of piping things. You might actually see less "wait for hot water to get hot" waste running seperate, smaller lines to each fixture since that will mean a smaller total volume of water to drain before you get hot water. Larger supply lines both hold a larger volume of water that needs to be drained before you get hot water, and they also have a larger surface area so more heat loss while running.

    With runs as short as yours, I don't really see a need for 1" lines. I'd probably run 3/4" lines, unless I was already working with 1" lines and had the material onsite anyway (basically I'd used 1" if it meant I didn't need a third size (3/4") of pipe on the jobsite).

    I'd consider small lines to each fixture from a single manifold for least energy waste waiting for hot water though. If efficiency of water use and/or water heating energy is your goal, seperate, small lines are probably your best bet.


    1. artisanfarms | | #4

      Thanks for the thoughtful response.

      My goals are a simple and "clean" or "elegant" installation and ultimate energy efficiency, including the energy embodied in the materials used for construction. Also, it offends my sense of order to run 6 pipes to one location, when I could do it with one. That said, you have given me real food for thought and perhaps I need to set aside my biases.

      1. Expert Member
        NICK KEENAN | | #6

        The cheapest solution is usually the lowest embodied energy as well.

      2. Expert Member
        BILL WICHERS | | #7

        Running multiple runs is "plumbing like wiring". My mechanical contractors will tell you that manifolds make service work easier, since you can shut off individual fixtures and leave everything else running. One of my guys likes to say "it lets plumbers work like electricians!" :-)

        It's really a balance. If you have a lot of long lines, then a tapped main arrangement probably makes more sense -- it's less work to install. If the runs are relatively short, then a central manifold offers some advantages. Exactly what consitutes "long" and "less work" depends on your specific project, and how you'd rather do it.

        If you do end up running multiple lines, use the multi-pipe support brackets, or space your single-pipe supports evenly. You end up with all the pipes running in parallel groups this way, which will give you a clean, professional appearance to your completed installation. If they're all hot water lines, be sure to leave enough space between pipes for you to install pipe insulation after installing the pipe.


  4. johngfc | | #5

    Gary Klein has done a lot of work on evaluating and optimizing plumbing layouts, systems, and pipe sizing. I think you'd find his works very information. His three-part article on "hot water distribution systems" is just right for someone asking your questions. Check it out here:
    If the link is deleted, a Google search will quickly locate his articles and videos.

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