Question on rough in for Moen shower/tub valve

[DuaneK]

I will soon be installing a new Moen valve for our shower/tub. The instructions for a PEX install show the down pipe as a copper sweat joint and the other three as PEX. I'm not sure why they show this. The Spigot for the tub is a thread on. It would be easier for me if I could use a PEX to NPT drop elbow for this, like I plan to use on the shower head. Is there a reason why this is a bad idea and I should use straight copper instead. Then transition to NPT somewhere? I have a good PEX crimp tool and can frame a solid backing for the drop elbow.

 

[Jadnashua]

If you want the tub spout and shower head to both flow at the same time...use pex to the tub spout!

They call for copper there for a reason...the ID of pex to the tub spout will create back pressure and force some water up to the shower head regardless of where the diverter is. You need copper all the way from the valve to the tub spout. Well, if the connection is threaded, you could use a couple of brass nipples and some fittings. Using the much larger ID copper will make that path allow all of the water to go out the tub spout rather than also ending up coming out the shower head as well.

 

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[DuaneK]

Thanks, that makes sense. Only a few feet of atmosphere keeping the water going out the right pipe. I think the ID of PEX is the same a copper, but the fittings do have a smaller ID. A short fitting distance of smaller ID is not adding much restriction under pressure, but this situation is relying on gravity. I'll copper it so I don't get water on my head when giving the kids a bath (although they would think that is hilarious).

 

[Terry]

The ID of PEX is smaller than copper. The OD is the same.
And then add the tiny little opening of the fittings, and it's a lot of restriction. On a Moen it pushes a lot of water out of the showerhead while filling a bath.

 

[DuaneK]

So since I need to run some copper anyways. Would I see much improvement in shower flow if did it all in copper? Also any harm (besides cost) of using an NPT female to socket on the 4 valve ports. I'm a little scared to sweat so close, not sure why I've done plenty of joints the past. Just not next to a $60 valve...

 

[DuaneK]

The valve was delivered today and I figured out how to remove the cartridge, not scared anymore! It is just a chunk of brass now, I'll just do it all in copper. Thanks

 

[Jadnashua]

Many of the valves these days will take either a threaded fitting, or you can solder to it directly...soldering is probably the better way.

As Terry and I said, the ID of pex is smaller than copper. The OD of them both is the same, but to achieve enough strength, the pex wall is considerably thicker, then throw in the fitting, you've got some appreciable restriction compared to copper.

 

[DuaneK]

I was always under the impression that pipe size was ID and OD was determined by wall thickness. I guess pipes (PEX) are now like lumber where you need to figure out if it is a 4" board at 3.5" or a 10" board at 9.25". Makes me so frustrated sometimes, I work in precision metrology measuring things to a micron (0.001mm). Its like some idiot in accounting came along and said a millimeter is now 0.9 millimeters, but if you are measuring over 200 millimeters then a millimeter is 0.8 millimeters.

That said I ran a bunch of PEX to a wet bar in a basement remodel and it would have been a nightmare without it. That is working out great (not a big flow use and we have 50-60psi), but this knowledge is good for future reference to understand. I am going to sweat this shower/tub in copper everywhere. Thanks again for enlightening me!