I am installing a 4 head "vertical spa" w/ 10GPM max system flow, but I expect to usually use it in 2.5gpm mode.

The house is 3/4" w/ 10' 1/2" hot and cold bathroom branch. I would think this is sufficient, since many bathrooms are 1/2" and have tub spouts which can do 10gpm.

However, some pipe charts give a different recommendation. I've seen 10fps, 5fps, and 3fps max velocity recommended?? Now, If 1/2" isn't big enough for this >10gpm room, then 3/4" shouldn't be big enough for a house which already has >20gpm max??

So I'm very curious why there seems to be this conundrum..
1)Why do some refernces say we only do 3-5fps, when a bathroom with a tub can get the room over double this number?
2)Also, am I ok for my application w/ pipe sizing as is?

details:
pipe is 1" service, 3/4" meter&backbone, and bathroom hot and cold branch is 10' of 1/2".
Kitchen: sink & dishwasher
Util: sink & washingmachine
Bathroom1: wc, tub, 1 sinks
Bathroom2: wc, sink, 4headverticalspa

Hi Firstshower:

I see what you are asking... but really not how we usually look at it as plumbers.

What we are more concerned with is if the hot water system can handle the increased VOLUME required for this new shower head and if you will actually run out of hot water after only a five or ten minutes...

Most 40 gallon gas water heaters, for example, will provide about 60-65 gallons of hot water (includes a 20-25 gallon recovery here) the first hour... so if you are shooting at 10 GPM and about 1/2 (or more... ) of that is hot water and we call that 5 GPM then the shower can run in this 10 GPM mode for approximately 13 minutes give or take a couple minutes (and I think this is conservative... more likely 3/4 water from shower head is hot water so could only last 8 minutes or so in the case presented).

If you have an electric water heater then the numbers only decrease from there. Can you tell me what heats your hot water?

So... with that in mind, we usually pipe 3/4" hot and cold water to a 3/4" port (inlet/outlet) shower valve that then feeds out to the multishower controls/heads. But only if your hot water system can handle it or you are going to all the work and expense to simply run out of water after 8-15 minutes (and fact that knowone else can shower for a little while after you have taken your MEGA shower... ;) ). I know you said high volume would not be used often, but when it is you want it to work maximum and full volume if possible).

Now if your shower valve only has 1/2" ports then there is no reason to run 3/4"... stick with the 1/2" and you will still be fine. You won't have the incredible volume output, but also won't run out of hot water quite as fast either.

Anyway.. hope that helped a little! Let me know if you have any more questions and answer mine if you have a chance!

MARK

Mark,

It's a 50gal fast-recovery gas tank, but I keep it on the cool side so your 10-15min is probably pretty accurate..

I just found in one of my references that says that the 4-8fps limit is mostly for copper which is carrying water that is hard, acidic, unusually hot, or will water-hammer.

Thanks

Hey FS:

Yeah... I have never worried about FPS... always worry about volume and recovery time and it has never failed me yet!

Seems to me you will be fine using 1/2" but 3/4" will deliver a more aggressive showering experience... for sure!

If you are still concerned about the FPS then call the toll-free technical number included with the vertical spa and ask them for information... Let me know what you decide.

MARK