Pex sizing

[mfarver]

Maybe this is an obvious question, but I noticed that a lot of the manifold/home run Pex systems (like Vanguard Manabloc) say their outlets are for 3/8" or 1/2" PEX. I found some advice online that says since PEX has a smaller inner diameter than copper for the same OD, you should upsize PEX. Some other advice says you should downsize, since PEX can tolerate a much higher velocity than copper without eroding. The higher velocity keeps deposits (Biofilm?) from forming in the tubing. A bonus would be that a smaller diameter tubing means hot water faster.

The local big box stores seem to carry Pex and fittings in 1/2 and 3/4" sizes. They have 3/8" but not in red/blue, just white. Online stores seem to carry a large variety of sizes.

In my case I have high water pressure, (120psi ahead of my regulator, 70psi downstream), and am adding a fullbath on the second story, and a half bath below. They are about 40 feet from the manifold location. I can either do the whole job in 1/2", home-running to the two sinks and toilets, and the shower. Or I can do 3/8" to everything but the shower.

Thoughts?

[Gary Slusser]

PEX has a maximum velocity too; IIRC 5-6 ft/sec but... it and other PE (polyethylene) products are the slickest, meaning they have the least friction loss so smaller ID works were other materials require a larger ID AND if they use branch and tee installation their pressure loss will be much higher. That's why homerun installations can use so much smaller ID and supply the same gpm with the same pressure or in most cases, less pressure.

You should look up the friction loss charts for whatever material you are going to use and then make sure of the maximum velocity/ft and calculate your velocity before you buy anything. Most folks guess at the ID etc. and then buy the next larger ID...

IMO 70 psi is still too high and a max of 60 is more than adequate.

Any plumbing supply house will have red/blue 3/8", or order it over the Internet. If you think about it, most toilets and sink pigtails are 3/8" and the fixtures have flow controls so, 3/8" PEX usually is all you need with a manifold except to a tub/shower or large tub where you might need 1/2". The manifold systems I have seen all used 3/8" for everything.

[sopulon]

If you're doing a fairly standard installation using normal fixtures and runs of ~30' or less, I'd just use 1/2" for all home runs. The manufacturer recommends 3/8", but as you noted, the stores tend to carry 1/2" fittings and tube. I just replumbed my house with 1/2" PEX (rather than the previous galvanized/copper mix) for all fixtures (toilets, shower, sinks, dishwasher, icemaker, washing machine, etc.) and it works great. There's not really much benefit to switching to 3/8" other than a very slight material cost savings. Hot water should reach distant fixtures faster in 3/8" vs. 1/2", but the difference is minimal (as well as any possible energy savings). I did use 3/4" for the hose bibs, however.

Al