Custom Shower Considerations

Custom Shower Considerations
When designing a custom shower, it's all about flow.  Flow in and flow out.  Lets start with flow in.  Lets say you want one shower head, one hand held sprayer, and three body sprays.  This is a typical set up.  If you wanted to be able to run all simultaneously, that would be 12.5 GPM.  12.5 gallons per minute is no small amount of water.  Can your pipes deliver this?  Most custom showers require 3/4" hot & cold feed lines.  You may only have 1/2".  Even if you have 3/4" feed pipes to the new shower, can your incoming water pipe from the street deliver 12.5 GPM?  I have seen 3/4" incoming lines connected with 1/2" valves at the curb.  This is common in Cleveland Heights.  Remember those homes were built 80+ years ago.  People were just happy to have indoor plumbing.  The only way to tell is to do a flow test in the basement on the incoming line.  You will have to have a tee put into the incoming pipe on your side of the meter, then a 3/4" full port ball valve facing down.  Now you can test flow by either filling up a 5 gallon bucket and timing how long this took and extrapolating data from this.  Or using a big Rubbermaid type container, actually run the water into it for exactly one minute, then measuring how much water is in the container.
This test will tell you how many GPM you can even get. Period.  And it gets worse.  That's in the basement.  For every 2.3 feet of elevation your shower head is above the test point in the basement, you'll lose 1 PSI of water pressure.  What does that mean?  It means if you wanted 12.5 GPM on the second floor, and you successfully test at 13 GPM in the basement, you'll think that you're okay.  But you're not.  Because of elevation loss in pressure, you'll have less GPM on the second floor than you do in the basement.
Of course, you don't have to do this test in the basement.  You can do it on the second floor.  That will make the results much more accurate.
Does this mean that you can't have the shower of your dreams?  Of course not.  You can have your shower.  You'll just to add some volume controls (shut off valves) that you may not have originally considered adding.  You will have to perhaps shut off the main head and hand held in order to run the body sprays.  Is this a big deal?  No.  But it's hard to add the volume controls after the walls are up and tiled.  It's something that has to be considered in the design phase.
Alright, so now you have all of the water and all of the valves you wanted.  Now you have to think about flow out.  What are you going to do with 12.5 GPM of water?  It has to go somewhere, can your 2" floor drain take all of this?
Drains are designed to flow in a half full condition.  The pipe shouldn't be too big, and it shouldn't be too small.  What we want is air to be above the flow of the liquid in the pipe.  So we design for half full flow.  At 1/4" per foot slope, a 2" pipe can handle about 10 gallons per minute (there is some variance here depending upon whose chart you use).  Some charts say 9 GPM, some say 11 GPM, so I say 10 GPM.  Now, having been a plumber for decades, I can tell you most shower drains in reality are around 1/8" per foot slope.  This is done to try to keep the strainer from tilting.  So at 1/8" per foot slope, you can get about 7 or 8 GPM thru that 2" pipe.
So if you have 12.5 gallons per minute or the capacity to deliver that, you really should have a 3" floor drain.  Can you install a 3" floor drain?  I'm thinking not.  Maybe you can.  Are your joists deep enough to handle a 3" p-trap under the floor?  If so, you are okay.  If not, you'll need two 2" floor drains.  And the 2" floor drains tie independently into a 3" branch.  Please don't ask for both 2" drains to tie together.  That would kind of defeat the whole purpose.  See, what goes into the shower has to come out.  It's going in under pressure.  It's leaving by gravity.  You have to consider this.  You can still have the shower of your dreams.  You may have a bigger drain, or more of them than you thought about.
"You want hot water with that flow?"  So you just spent thousands of dollars on the shower of your dreams.  How was that 5 minute shower?  You will probably mix 70% of that 12.5 gallons per minute with hot water.  How big is your water heater?  If you have a 50 gallon tank that has about 80% usable hot water, (the rest is too diluted to be considered hot) that gives 40 gallons of hot water - 70% of 12.5 is 8 3/4 gallons per minute of hot water needed.  That's about a 5 minute shower.  So another consideration is hot water production for that deluge you want built.  You may be thinking of the 2nd floor bathroom, but we will inevitably be down in the basement.  So plan accordingly.  You'll probably end up with at least two 50 gallon high recovery tanks.
You can still have that shower tho, just don't get blind sided by the other things that need consideration.