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Thread: PEX Homerun manifold - use 1/2" or 3/8"?

  1. #1
    DIY Junior Member pit monster's Avatar
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    Default PEX Homerun manifold - use 1/2" or 3/8"?

    I will be re-plumbing our new old 1923 home with pex. I have studied some of the installation materials and have decided to use a manifold system. I know it will cost me a few more bucks in pipe and materials but I like the idea of a more professional looking job (done of course by a total newbie), as well as the isolation aspect of each run.

    One of the installation guides I reviewed suggested using 3/8" lines off the manifold to get the hot water to the fixture sooner. Most of the discussion I have seen on the internet forums shows people suggesting 1/2" runs vs 3/8".

    3/8" seems as if it should be a little more flexible and be able to withstand a tighter bend radius, although the way things line up in my basement that really will not be an issue. What am I losing by using 3/8 vs 1/2 lines?

  2. #2

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    I did my house using wirbo and 1/2 inch runs from thje manifolds. Worked fine.. The materials came from fergusons.. An expander tool expands the pipe and it slips onto the manifold and then the pipe shrinkis back down.. Best system for pex I think.. I kept the hot water manifold close top the heater and had a couple cold water manifolds near the joist bay keadiing over/under the wet wall.

    I was surprised how easy it was. I used 1" pipe for the cold waster running to the manifolds and using the expander tool on the 1" pipe was tough the rest was pretty easy..

    If you haven't checked out furgusons plumibing yet, you probably should.. They gave me a big catalog of all the pex stiuff so that I could figure out my plan.. I would question yhour ability to find the components you need for 3/6" ruins..

    Ojh, and I am t a total newbie too.. The tool rental was $25/day and it tool me 3 days as I had to do some extra work making the connection to the line entering the house.
    Roger

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    1/2" pex is about 1.8x larger in diameter than 3/8" pex. I think on long runs and any fixture where you want higher flow (say washing machine, tub, maybe a shower), you would be very dissapointed with the 3/8" run. In fact, you might notice a flow problem with 1/2" on certain fixtures...would depend on the item itself. This is even more true if you don't use the Uphonor system (with the expander tool and fittings) since they decrease the overall diameter even more. With the expansion system, the ID of the fitting is larger - it won't fit into the pipe until you expand it. The other systems you can push the fitting in, then you crimp it to keep it there. On the Uphonor (Wirsbo) system, the elasticity and memory of the tubing holds it in place once installed. 1/2" pex is smaller ID than 1/2" copper. Keep in mind that the ability of the pipe to flow (the area) is the square of the radius, so small differences make really big changes in the area. The good thing about pex is you use fewer connections and direction changes which makes the flow have fewer restrictions, but diameter changes really make a difference.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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