.5" PEX ability to supply multiple shower heads

[theunit]

Hi all - Thanks for your insights previously and taking a look at this post. I have read several other, similar posts that deal with this issue, but thought it would be beneficial to share my particular situation with a photo. Appreciate any thoughts you have or your experience if you have a similar setup.

The scenario:

• .75" hot and cold supply to within a few feet of the shower I am building, which is on the second story of the home
• .5" PEX H & C coming off the .75" supply lines running to the shower valve
• .5" PEX teeing off to the bathroom toilet (#1 in the image)
• Shower valve is a .5" Grohe Smartcontrol, for reference
• Three individual .5" PEX lines planned for three shower fixtures - two standard shower heads and one rain shower head (#'s 3, 5 and 4 in the image, respectively)
• All shower fixtures are rated at 2.5 GPM
• Water flow and pressure throughout the home seems strong at every other outlet
• Main water supply is located two floors directly below the shower area
• Two 50 gallon water heaters supply hot water
• As you can see in the image, I still have significant work to do, not the least of which is strapping and supporting the PEX, etc.

The questions:

1. Will this system, as is, provide a decent pressure/flow to the three shower fixtures? Either, one, two or all three of them on/any combination thereof? I am concerned about the ID of the transitions in the PEX, at the valves and transitions to fixtures.
2. Excluding tearing apart the floor to plumb .75" supply to the .5" valve, would I benefit from using .75" PEX from the valve to the three shower fixtures? i.e. after getting to the valve, would this allow more water to 'build up' behind the shower fixtures for better pressure/flow?
3. What else am I not thinking about to ensure a good experience down the road?

Thank you

 

[Jadnashua]

I don't think you'll be happy if you plan to have all heads running at the same time.

FWIW, I don't think you'll find any 1/2" valve that is rated to deliver much over 6gpm, and some are less when 1/2"...7.5gpm is just asking too much for not only the valve but the supply lines. Those ratings are for 1/2" copper...while pex's ID is smaller, it does allow slightly higher maximum flow velocity (on copper, it's 5fps for hot - that equates to 8gpm on a 3/4" copper line, but only 4gpm with a 1/2" copper...pex gets about 5gpm in 1/2" with the higher velocity allowed).

If you want to maximize what's available to the valve, you'd want to use 3/4" pex and reduce it right at the valve, but the valve will be a limiting factor.

 

[Sponsor]

 

[theunit]

I don't think you'll be happy if you plan to have all heads running at the same time.

FWIW, I don't think you'll find any 1/2" valve that is rated to deliver much over 6gpm, and some are less when 1/2"...7.5gpm is just asking too much for not only the valve but the supply lines. Those ratings are for 1/2" copper...while pex's ID is smaller, it does allow slightly higher maximum flow velocity (on copper, it's 5fps for hot - that equates to 8gpm on a 3/4" copper line, but only 4gpm with a 1/2" copper...pex gets about 5gpm in 1/2" with the higher velocity allowed).

If you want to maximize what's available to the valve, you'd want to use 3/4" pex and reduce it right at the valve, but the valve will be a limiting factor.

Thanks, Jim - appreciate your input as always. Since I posted yesterday, I realized I have a separate 1/2" supply (H & C) nearby. Since they are not going to be used for anything else, I plan on piping them over to this area and running a dedicated cold supply to the valve (i.e. not teeing off the line that goes to the toilet as I mentioned earlier) and joining the two 1/2" Hot supplies together prior to the valve.. I figure that will at least give me the best shot based on what I've got to work with. Do you see any issues with that plan?

Secondly, I took a look at the specification for the valve from Grohe. It looks like it is saying it can achieve a 9.5gpm flow rate (at 43.5 psi) in a similar configuration to what I'll ultimately have (see image below). How would you interpret this information from their manual?

Thanks again,
Graham

 

[Reach4]

If you run untempered hot water to the shower area, and have a tempering valve for the bathroom, the hot and cold supply lines to the bathroom will carry a more balanced flow. The hot will arrive sooner at the showers, so less water will be drawn to wait for showering-temperature water to arrive.

 

[theunit]

Thanks, Reach. Do you mean installing a tempering valve on the supply that goes to the bathroom sink/toilet; but leaving the shower supply alone? I'm not sure I understand.

 

[Reach4]

Thanks, Reach. Do you mean installing a tempering valve on the supply that goes to the bathroom sink/toilet; but leaving the shower supply alone? I'm not sure I understand.

I am suggesting a tempering valve for the shower and bathroom sink. Since you will run cold to the toilet, that won't be affected. Feed that with hot water that has not already gone through a tempering valve.

I am not a plumber. I have never used a tempering valve.

 

[Jadnashua]

If you used a 1/2 x 1/2 x 3/4" T, to combine the two 1/2" lines, then ran 3/4" to the valve and reduced it (I'd use copper), you'd get the advantage of higher volume with less flow restrictions. That will still leave you with the bottleneck at the 1/2" valve. Double-check, but I doubt it is rated at more than about 6gpm, and if you really want all three things running at the same time, you will lose pressure because the high velocity required will create more friction, and dynamic pressure loss.

A flow like that generally calls for a 3/4" valve if you want best performance. You may be satisfied with what you get. Look at what happens with a water hose without a nozzle. See how far you can get it to go. Then, start to close off the opening with say your thumb. The actual volume of water is the same, but it goes much further. Take that hose and try to branch it to run multiple sprinklers simultaneously, it generally just doesn't work well. Same issue with your supply line to the valve which is a limit, while feeding multiple outlets. For the heads to generate the desired pressure, they need more supply or at least equal in volume...I don't think your 1/2" valve and supply lines can do that.

Note, while you can get more volume than the industry guidelines, it comes at a cost:
- more friction, thus lower pressure
- more water flow noises
- on your copper sections, potentially literally eroding the pipe from the insides.

FWIW, on most designs, a thermostatically controlled valve can flow slightly more volume than a pressure balanced valve, at least, that's what Delta says on their design. From my limited experience, that seems to hold for other brands as well.

What does Grohe say that valve's maximum flow is?

 

[theunit]

Thanks again, Jim. All very helpful info. If I'm interpreting their specs correctly (like in the image above), other parts of their manual seem to illustrate up to 13.4gpm at 87 psi for various configurations of shower heads, body sprays, etc. The specific configuration I'm pursuing is shown in the image above, so it looks like they show the valve handling as much as 13.4gpm @ 87 psi down to 5.5gpm @ 14.5 psi. Does that seem feasible/reasonable? I'm interested if you would read the page from the manual that I pasted above in the same way.

I did end up adding the second 1/2" supply line with a 1/2x1/2x3/4 tee followed by a copper reducer at the valve. I figured it couldn't hurt. I assume that in practice we will only use all three heads sparingly, and more likely a combination of any 2 of 3 will be much more frequent.

 

[Jadnashua]

FWIW, code in the USA says you should never exceed 80psi and if it is, to use a regulator to limit it to that value. Europe may allow it, thus why they would even consider listing that in their spec sheet.

I think that they sell that configuration with both a 1/2 and 3/4" inlet...that higher output may be for the 3/4" version. Is there another set of charts in the spec? From an engineering viewpoint, running 13+ gpm through a 1/2" valve and pipes would exceed the industries maximum velocity recommendations by over 200%. True, it isn't running constantly, but all of the problems associated with that excessive velocity still occur, but over a longer timeframe.

 

[theunit]

Thanks, Jim. I didn't see another, larger configuration but understand the point you make and that seems to make sense - that the table I found may be for the large valve, or just a maximum that was achieved in test conditions only. In any case, with the two combined 1/2 supplies for hot water and a dedicated 1/2 cold, I'll take my chances and see what we achieve. I'm hopeful that since Grohe designed this 1/2" valve specifically control a combination of three fixtures that it will work moderately well on an average home. We'll see...

I'll aim to share results once I've got this thing done. No, to build the shower floor mortar bed...