I’d like to understand the PEX to tub spout issue a little better

[Brandon Weiss]

I know that there are a seemingly infinite number of threads on this forum about why not to use PEX to connect the mixing valve to the tub spout, but I’ve read a lot of them and while the explanations sort of make sense, I feel like I don’t understand them fully, and I’d like to understand the issue(s) a little better.

If someone wouldn’t mind checking my understanding on these two points I’d very grateful.

Common wisdom is not to use PEX to the tub spout because it’s “floppy” and the tub spout won’t be well-supported. Is that because it’s normal to use just a 90° elbow to mount the tub spout to? I’ve only ever seen the drop-ears that screw to some wood backing. I assume if you used one of those that would mitigate the issue?

Common wisdom is not to use PEX to the tub spout because the smaller ID relative to copper pipe will cause water to flow up through the shower head. Is that only when using a tub spout with a built-in diverter? If you used a mixing valve with multiple outlets that has a knob to control which outlet is open (so the diverter is in the valve itself), I assume that would mitigate the issue?

Is there anything else I missed? Some other reason why you wouldn’t want to use PEX to the tub spout?

Thanks so much!

 

[Reach4]

Is there anything else I missed? Some other reason why you wouldn’t want to use PEX to the tub spout?

Yes, and yes.

If your tub spout has a "diverter" function, concern is that pex would cause too much backpressure, and that backpressure would cause water to dribble out of the showerhead when you are trying to use the spout.

If you had a separate diverter valve, or if there was no showerhead, this consideration would not apply.

 

[Sponsor]

 

[Terry]

Kohler

MOEN

FOR TUB SPOUT INSTALLATION: Refer to the installation instructions supplied with your spout. Do not connect deck mount spouts to in-wall valves. Do not use hand showers connected in lieu of a tub spout to a tub/shower valve. Do not use PEX tubing for tub spout drop.

Delta

Glacier Bay

 

[wwhitney]

I assume if you used one of those that would mitigate the issue?

I would say that if you use a drop ear elbow and rigid pipe (copper or brass) for the tub stub-out, the pipe material between diverter and tub would not matter as far as the stability of the stub out.

If you used a mixing valve with multiple outlets that has a knob to control which outlet is open (so the diverter is in the valve itself), I assume that would mitigate the issue?

I would agree that if the water path to the showerhead is separate from the water path to the tub spout, the possible restriction of PEX to the tub spout wouldn't cause any issues.

Cheers, Wayne

 

[Terry]

Any restriction to the tub spout causes problems.

That is why Delta, Koher and Moen all have instructions prohibiting PEX to the tub spout in any way.
It forces water up to the shower head while filling the tub.

Now maybe the homeowner doesn't mind water pouring out everywhere while filling a tub, but most homeowners would want the plumber to come back, cut the wall open and fix it.

 

[wwhitney]

Any restriction to the tub spout causes problems.

That's for the majority of shower valves where the water path to the shower is to first go down, then get diverted at the tub, and go up to the shower head, right?

If the shower valve has an internal diverter and separate non-communicating outlets for the tub and shower, then the pathway to the tub won't be under pressure when shower is selected.

Cheers, Wayne

 

[Terry]

If the shower valve has an internal diverter and separate non-communicating outlets for the tub and shower, then the pathway to the tub won't be under pressure when shower is selected.

Cheers, Wayne

The Hansgrohe i-Box has a separate diverter in the valve. It can have PEX to the drop ear.

These basic valves, NO
Moen Chateau
Moen Moentrol
Moen M-Core
Moen Posi-Temp
Delta R10000
Kohler K-304
Kohler K-8304
Phister
Glacier Bay

All of these that use a tub spout diverter can't have PEX to the spout.

About 95% of valves can't have PEX, so I'm not going to blanket statement say that it might be alright for that slim chance 5% out there.
The average person will just think, sure why not try it with my valve? All valves come with instructions, well except for when I buy wholesale, which I do and they just hand me a valve without any paperwork with it. You're supposed to be educated about what you're installing and look up the paperwork.

I've had to go out and cut up walls for just this very reason. And the showroom selling the hardware had no idea what was in the instructions.
PEX is new so why wouldn't it work? Heck, CPVC doesn't work either and that has been out for a long time now, and I still see it installed where it doesn't belong.

Maybe make a list of which valves you think would work, and let's go over that list.

Phister shows threaded pipe or copper.

 

[Jadnashua]

Most people's goal when using a tub is to get it filled as fast as possible...that won't happen if you've used pex to the spout. It is smaller in ID, and it will restrict the flow.

 

[Brandon Weiss]

Thanks for confirming what I suspected about the drop ear and diverter!

The last thing I don’t understand is the flow restriction to the tub spout. I mean I understand that the ID of PEX is smaller than copper, so flow is technically restricted, but I thought the limiting factor would be the tub spout and not the PEX/Copper? My tub spout is 10 GPM at 60 PSI and I thought 1/2" PEX was more than that, so wouldn’t I end up with 10 GPM regardless? I don’t have a great grasp of flow rates and whatnot so maybe I’ve completely misunderstood

 

[wwhitney]

Flow rate into the tub is going to be determined like this:

If you're on a public water main, it's going to provide some static pressure at the elevation of the tub spout when no water is flowing in the system. [Which static pressure might change by time of day or other factors but should be locally constant.] Additional water flow of 10 gpm through the water main should have a negligible effect on the water main pressure. Every downstream component from the main (meter, water lateral, etc all the way to the tub spout itself) is going to be characterized by some generally (always?) non-decreasing curve of flow rate through that component vs pressure loss at that flow rate. So as you open the tub spout, the flow rate through the system will increase, with increasing pressure loss on each component. When the total pressure loss of all the components adds up to the static pressure, that's the steady state flow rate.

So if you had a tub that was plumbed with 1/2" copper from shower valve to a (non-diverter!) spout, and it got 8 gpm, and you then replaced that 12" of copper with 1/2" PEX, you'd get a little less flow. Maybe 7 gpm? 7.5? 6.5? The pressure loss would be greater through the 1/2" PEX (and particularly the fittings), so the portion of the pressure loss attributable to that segment would go up, and the flow rate through the whole system would have to go down to compensate.

Cheers, Wayne

 

[Terry]

Delta sells their R10000-PFT-MFWS for cold expansion PEX with the tub drop already installed for those that can't or won't solder the copper drop to the tub spout.

I guess this was their solution for those that don't read the instructions.
Give them the option with the tub drop already installed.

It's a hell of a restriction going to PEX.

 

[Trump the Plumber]

Thanks for confirming what I suspected about the drop ear and diverter!

The last thing I don’t understand is the flow restriction to the tub spout. I mean I understand that the ID of PEX is smaller than copper, so flow is technically restricted, but I thought the limiting factor would be the tub spout and not the PEX/Copper? My tub spout is 10 GPM at 60 PSI and I thought 1/2" PEX was more than that, so wouldn’t I end up with 10 GPM regardless? I don’t have a great grasp of flow rates and whatnot so maybe I’ve completely misunderstood

I like more water always and don't get why some plumbers out there want larger pipes for things like tub spouts when you can use most anything and save some time and money. If I'm running a job, I'm thinking about how much money and time I can save. It's all about the money is what I'm saying. Copper cost a lot, I mean really a lot. And there are scrappers that come in and cut the pipe out and then get money for it. They don't get anything for plastic pipes. And I like the colors of the pipes. I can get red, red is for hot, and blue, blue is for cold, don't mix that up in your head though. And white, or clear. Why clear, what's the point in that? Am I supposed to guess on that?

I would plumb it how you like and don't worry about if water comes out both places. It all winds up in the tub anyway. Some people are so picky and worry about dumb stuff. That's all I'm saying.

 

[Terry]

Same for the new Moen M-Core tub/shower valve.
Copper or IPS to the tub spout.

 

[Jeff H Young]

I like more water always and don't get why some plumbers out there want larger pipes for things like tub spouts when you can use most anything and save some time and money. If I'm running a job, I'm thinking about how much money and time I can save. It's all about the money is what I'm saying. Copper cost a lot, I mean really a lot. And there are scrappers that come in and cut the pipe out and then get money for it. They don't get anything for plastic pipes. And I like the colors of the pipes. I can get red, red is for hot, and blue, blue is for cold, don't mix that up in your head though. And white, or clear. Why clear, what's the point in that? Am I supposed to guess on that?

I would plumb it how you like and don't worry about if water comes out both places. It all winds up in the tub anyway. Some people are so picky and worry about dumb stuff. That's all I'm saying.

To hell with Manufacture instructions and pickey people ! we want mo money!

 

[Jadnashua]

Have you measured the output of your tub spout, or is it not installed yet?

FWIW, if you're using 1/2" valves, those rarely can flow more than around 6gpm. If you have some 1/2" copper further back, keep in mind that the Copper Institute calls for a maximum velocity of the flow on hot to be 4gpm, which equates to only 4gpm with 1/2" copper. Exceeding that velocity can cause some both short and long-termed problems for the pipes and the sounds the water flowing can create.

If you're using a diverter tub spout, unless it is used with a valve designed for a twin-el, if you use PEX to the tub, you WILL get water out of the showerehead when filling the tub. If you look at the ports in the valve, the upper one going to the tub spout already has a restriction in it, while the lower one is full diameter. Any restriction will then force some of that water to flow up, versus all of it going down into the tub through the spout. TO get more than that generally requires a valve with 3/4" or larger input piping.

 

[Brandon Weiss]

Flow rate into the tub is going to be determined like this:

If you're on a public water main, it's going to provide some static pressure at the elevation of the tub spout when no water is flowing in the system. [Which static pressure might change by time of day or other factors but should be locally constant.] Additional water flow of 10 gpm through the water main should have a negligible effect on the water main pressure. Every downstream component from the main (meter, water lateral, etc all the way to the tub spout itself) is going to be characterized by some generally (always?) non-decreasing curve of flow rate through that component vs pressure loss at that flow rate. So as you open the tub spout, the flow rate through the system will increase, with increasing pressure loss on each component. When the total pressure loss of all the components adds up to the static pressure, that's the steady state flow rate.

So if you had a tub that was plumbed with 1/2" copper from shower valve to a (non-diverter!) spout, and it got 8 gpm, and you then replaced that 12" of copper with 1/2" PEX, you'd get a little less flow. Maybe 7 gpm? 7.5? 6.5? The pressure loss would be greater through the 1/2" PEX (and particularly the fittings), so the portion of the pressure loss attributable to that segment would go up, and the flow rate through the whole system would have to go down to compensate.

Cheers, Wayne

That’s really helpful, thanks!

 

[Brandon Weiss]

Have you measured the output of your tub spout, or is it not installed yet?

FWIW, if you're using 1/2" valves, those rarely can flow more than around 6gpm. If you have some 1/2" copper further back, keep in mind that the Copper Institute calls for a maximum velocity of the flow on hot to be 4gpm, which equates to only 4gpm with 1/2" copper. Exceeding that velocity can cause some both short and long-termed problems for the pipes and the sounds the water flowing can create.

If you're using a diverter tub spout, unless it is used with a valve designed for a twin-el, if you use PEX to the tub, you WILL get water out of the showerehead when filling the tub. If you look at the ports in the valve, the upper one going to the tub spout already has a restriction in it, while the lower one is full diameter. Any restriction will then force some of that water to flow up, versus all of it going down into the tub through the spout. TO get more than that generally requires a valve with 3/4" or larger input piping.

The valve I’m using is actually 3/4", although the supply to it will be the standard 1/2". I chose the valve primarily because I’m looking for a specific trim/design, not because of the size. I think the reason it’s 3/4" is it supports using a cartridge that allows two outlets (e.g. shower head and wand) to be on at the same time, and so the additional flow from 3/4" would be desirable, but since one of my outlets is a tub spout I don’t really need that feature.

I think I could probably use 3/4" PEX from the valve to the 1/2" tub spout and get great flow, but I might wind up pressing some copper anyways just to be safe/have fun.

 

[oldVermonter]

I see the Moen Brecklyn specifically shows PEX as an allowable option, including (if I read this right) from mixing-valve-to-spout.

https://www.moen.com/shared/docs/instruction-sheets/ins10795.pdf

What is the difference between this model and the Moen models Terry names above? (Sounds like PEX isn't the best choice in any case....)

 

[WorthFlorida]

Brandon, you hit the point on your first "common sense statement".

One thing most everyone misses about a tub spout needing copper or iron pipe, and that its strength. The spout is only supported by the pipe and PEX or CPVC is just too soft or flexible other than having more restriction. If you have kids, bathtub time they can get crazy and playing around the spout can get banged into.

 

[WorthFlorida]

.............I would plumb it how you like and don't worry about if water comes out both places. It all winds up in the tub anyway. Some people are so picky and worry about dumb stuff. That's all I'm saying.

You be surprise how many posts this forum has had in the last ten years that do not like to have water coming out if the shower head when filling the tub. Sometimes it's a bad cartridge, a bad diverter or it's a DIYer not knowing.