PEX B for Carwash Shower

[Steve_A]

I am installing a multiple head shower that could use up to 10gpm (not likely but could if everything is on). Currently fed by Navien 240 (may need to add another but that is for another day). I have 3/4" PEX B feeding 3/4 Copper risers then too 3/4" PEX B into the Kohler DTV+ 6 output valve. I'm concerned the PEX B fittings (I've kept them to a minimum but there's still 7 crimps) will cut my flow. I don't have access to PEX A tools.
Two Questions
1) should I replace all the PEX B with 1" now that I have it all open or is it not worth it? I figure it will cost me about $150 to replace all the hot lines that I've already run. The Cold to the hot water heater is all 3/4" PEX A and is fully accessible back to the main (which is larger than 3/4, haven't measured it).
2) Is there a problem leaving the cold run to the shower at 3/4"? I'd expect to use very little cold water on that line.

 

[wwhitney]

The design process goes like this (where I'm omitting the complexity of the two separate water paths, hot and cold):

If your shower heads want 10 gpm max, what pressure do they need at what point to do that? Say they need 10 gpm @ 60 psi into the Kohler DTV. If your static pressure at the elevation of the Kohler DTV (which doesn't depend on pipe sizes, as no water is flowing) is below 60 psi, then you'll never get 10 gpm out of the system. If it's above 60 psi, then the difference is your pressure budget for all your piping from the water main (assuming city water) to your Kohler DTV.

So you can calculate, based on the pipe type, size, and length, and counting each fitting, what the pressure loss through each of those components will be at 10 gpm. And then if the cumulative pressure loss is less than your pressure budget, you'll get at least 60 psi @ 10 gpm at your Kohler DTV and it should work as advertised. If it exceeds your pressure budget, you'll get less than 10 gpm.

Often in the accounting, if there's a significant length of pipe that is smaller than anything else, that pipe's pressure loss will dominate. E.g. 15 psi for that pipe, and 3 psi for everything else. That's the case where upsizing that one length of pipe can make a big difference.

As a reference point, say your Pex-B crimp fittings are ASTM F1807 with brass crimp rings, and your 7 crimps are in the form of 3 elbows (not sure about that leftover crimp, why you have an odd number). Uponor (which might be biased, as they are marketing their Pex-A with F1960 fittings) says that a 3/4" F1807 elbow has an equivalent length of 10.5'. And suppose your actual tubing length is another 30'. That would make the total equivalent length about 60'. The ID of 3/4" pex is 0.681", so this handy calculator:

http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php

says that 10 gpm through 60' of 0.681" ID plastic pipe will drop 11.8 psi. That's on top of whatever losses you have upstream of the PEX you're talking about.

In contrast, say you switch to 1" pipe with 1" fitting, same standards. The equivalent length of the elbow is now 10.1', so we're still at 60' equivalent length, but now the ID is 0.875". So the calculator says the pressure drop will go down to 3.5 psi, again, for that segment only.

Based on the above, you can decide if it's worth replacing it or worth doing more calculations. And you might want to figure out the actual hot water flow rate, since with 8 gpm the pressure drops go down a fair amount.

Cheers, Wayne

 

[Sponsor]

 

[Jadnashua]

Shower heads need velocity to make them feel nice (unless you're talking about a rain head). Velocity depends some on the pressure, but much more on the restrictions in the shower head it self. The nozzles in the shower head cause the water to accelerate as it goes through the restriction. If the path cannot supply more than the heads can use, there's no reason for it to try to speed up going through the heads, and the effect becomes much more like a rain shower head. Think about a soaker hose...you may get the same volume out of it, and the supply pressure is identical, but the outlet sprays is very gentle...people often confuse pressure and velocity when it comes to the performance of a shower head...you need both, but volume is the bigger issue - you really want to be able to supply more volume than the heads can ultimately pass through to get the Bernoulli effect of the acceleration.

FWIW, restrictions in the path to the heads will lower the available volume, which may mean the velocity increase from the nozzle doesn't happen.