Connecting 1/2 copper to 1/8 pex?

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Thanks for the response Larry. As Larry noted, I would normally run a larger line for lateral runs and then downsize the fixtures runs. In a manifold system, I believe the downsizing occurs at the manifold.

Main manifolds typically have 3/4" in and multiple 1/2" out. I already have a borderline flow rate (my opinion) with fairly clean 1/2" copper fixture lines, 3/4" laterals and a 1" main. With the inside diameter of PEX being considerably smaller than copper, I'm concerned about a possible flow rate decrease on long (80-100 feet) 1/2" PEX fixture runs.

Thanks for pointing out the 3/8" sink and toilets leaders. There shouldn't be a problem there, but what about showers and baths which have full 1/2" inlets? Should I run 3/4" PEX to those? The ID of 3/4" PEX appears to be similar to 1/2" copper, although I haven't really measured.

Also, the 3/4" shower lines would have to be branched off before the main manifold. That could create a flow rate drop to the...

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For what it's worth:

Although most PEX manufacturers really push manifold systems, could that be because a manifold system uses a lot more pipe?

While a manifold system may be an acceptable choice in some applications, there are several factors which need to be considered. Manifold systems often waste water. This is because of the need to clear the cooled water out of hot water lines when you want hot water. When the hot water has not been used for a while the water in the pipes cools down (unless you have a circulating system of some kind). In order to get hot water, most people just let the cooled water run down the drain until it heats up. With a standard, "non manifold" system this usually only has to be done once at each bathroom group. With a manifold system each fixture has to have it's supply purged of cooled water separately. Even though the individual lines are usually 1/2" instead of 3/4", if you do the math you will see that that is more water...

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