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I've been told that PEX requires home runs to a manifold (or star topology, to borrow from networking) to maintain the same flow rate/pressure as copper. Anyone else heard this?

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4 Answers 4

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PEX uses fittings that have a smaller inside diameter than the tubing, unlike copper where the fitting ID is typically the same as the tubing. That smaller ID is going to reduce your flow rate vs. copper if you have a lot of fittings between the source and your fixture, as you might in a non-home-run situation where fixtures are teed off a main trunk. In a properly installed system, this will be countered somewhat by using larger diameter tubing for that trunk line.

So that's the theory, but in practice I can tell you that I've now lived in two houses that were plumbed in non-home-run PEX and never had problems with reduced flow in the remote fixtures. Both of these houses used 3/4" PEX for the trunk and 1/2" PEX for the branch lines.

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FWIW, I've seen far more trunk-and-branch installs than home-run installs. My current house has a couple fixtures sharing a 1/2" line, and those do compete for pressure, but the ones on the 3/4" trunk don't cause noticeable problems. I do plan on fixing it at some point, but at least the builder saved $10 on that 10 feet of 3/4" PEX and fittings... –  gregmac yesterday

The problem is PEX clamps leak around the two crimp point areas. (Unlike hose clamps that have an overlap and have uniform area on the seal area) This leak can lead to pop offs and thus house interior flooding.

Therefore homerun designs (no crimps behind walls and ceilings) allow for the leak and pop-offs to contain the water damage to a one area.

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I've seen PEX blow apart when you turn on the water and forgot to crimp something, but I've never seen PEX leak -- either with the rings or with the clamps. I've worked on hundreds of water treatment systems done with PEX, and I also personally know of many houses (including my current ~10 year old house) that have no issues. –  gregmac yesterday

PEX is generally run to a manifold, but there are adapters that let you connect it to existing copper pipe. However, it's so easy to run that I'd just do it the nice way.

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As PEX is just the material the pipes are made out of, you should need exactly the same configuration as you would if you were using copper. I would have thought that the only way the flow rate would be affected would be if the tube has a significantly different internal diameter.

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