Size of PEX

[Nam 69]

i am the new guy, I have a question about PEX. Can 3/4 be used for the house fixtures. I did not buy a manifold yet. I see most are 3/4 supply and then 1/2 for the fixtures. My well water is very hard and my copper pipes are always springing a leak.
I am electrician by craft, so I know very little about plumbing. I did read my monthly handyman magazine and they say to run 3/4 to all the fixtures. I am thinking most fixtures have 1/2 connections, so I am confused about this part. I did have one plumber run a 1/2 line of PEX to my basement sink and the pressure is terrible. I mentioned this to my guy and he said half inch will be fine as he would run a 3/4 in. Supply line to the manifold. Will this work or is he just telling me a line here. I am going with a new plumber and I would like to have a little correct info to tell him what I want. Right now my well pump is set for 60 pounds and it goes to a device that kills the bacteria in my water. It has a blue light of some sort and it does work as I have my water tested and it is ok now. That fixture does have 3/4 lines in and out. From there to the manifold will be around 35 foot of pipe. Reason is I am trying to place the manifold in the center of the distributaries . I think I may have a plumber coming next week. I have been using 5/8 radiator hose with clamps to repair the holes in the copper pipe after I cut the bad piece out. I am no good at plumbing so this works in a emergency. If someone can advise I would appreciate all feedback. Thank You, Pete

 

[Reach4]

I did have one plumber run a 1/2 line of pex to my basement sink and the pressure is terrible.

A laundry sink? Flow is low on cold, hot, or all including warm? Maybe there is some restriction other than the piping. If part of that path is shared by the washing machine, you could put a pressure gauge on a laundry connection, and see if the restriction is before or after where the washing machine and laundry tub faucet water paths diverge.

I would not run 3/4 to the hot on the lavatory. The reason is that would increase the amount of time that you wait to get hot water there. Same for the kitchen sink hot. 3/8 would be nice for the hot on a lavatory, although it is not often used. It probably costs more to do than 1/2.

3/4 to a toilet would make it harder to adapt to the normal 3/8 stop valve. Those things are often fed by 1/2.

 

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[Terry]

I normally use 1/2" to most fixtures, unless it's for a multi-head shower system or Roman Tub with a 3/4" Grohe fill valve.
I've seen some plumbers run 3/8" with PEX to their fixtures, though most manifolds I'm seeing are for 1/2".

 

[Jadnashua]

Is the existing PEX where you can see the whole run? The reason I ask is if you can look closely and verify that there isn't a kink somewhere, restricting the flow.

A laundry sink may not have a flow restricted valve. A few things in a home are allowed unrestricted lines: tub, hose bibs, and maybe a laundry tub (not positive on that last one). Other than that, the flow is restricted by the fixture, and most of them, that's <2.5 GPM. A 1/2" PEX line can provide that fairly easily.

Also, there are two ways to make connections with PEX...a cinch or clamp fitting, or an expansion fitting. The later has less restrictions through the fittings since their ID is larger. One of the benefits of running PEX verses copper is if you do it well, you need almost no mid-stream fittings, and every fitting drops your dynamic pressure and therefore, also the volume available.

The ID of PEX is smaller than the equivalent copper, but because you can run it and bend it, you don't need as many fittings and changes of directions, so there's less internal friction, and the smaller ID isn't as big of an issue as you might otherwise think.

You can run PEX or copper, either in a trunk and branch system, or use a manifold(s) where you use a larger line to an area, then a manifold to distribute it to individual fixtures.