Shouldn't a 6.8 gpm shower valve be able to supply adequately two sprays at 2.5 gpm each?

I have a Grohe pressure balance shower/tub valve (model Grohesafe 35 253 000). My plan is to plug the lower outlet port, and only use the top outlet.
The top outlet (1/2") will come off the valve to a 5 port diverter (Grohe 29 714). This diverter has two inlets and three outlets, since the water is already mixed, I will plug one of the inlets.
The three outlets - two are 1/2" and one 3/4". I will route one line to an overhead shower, one line to a handshower wall union/elbow, and the 3/4" to a "T" which splits off to two 1/2" lines each terminating on a body jet spray. So basically a valve runs to a diverter, then the diverter transfers the flow to an overhead shower, or a handshower or two sprays.
In reading the specs for the valve it says it delivers 6.8 gpm, and the sprays are 2.5 gpm each, so for two it will take 5 gpm, we should be ok, unless I want to use the handshower or overhead shower at the same time which is impossible since the diverter only transfer the flow to one of the three outlets, it does not mix.
So I called up Grohe technical support and presented this over the phone, and the guy says "you cannot use pressure balanced valve with sprays it's not strong enough you need more gpm". I said since the diverter does not allow you to transfer flow to more than one port at a time, if I have 6.8 gpm going into two devices at 2.5 gpm shouldn't it work?" he said no, just thermostatic valves must be used for sprays. I asked why the pressure balance valve at 6.8 gpm is not enough he said what if you want to run the overhead shower and sprays at the same time, I asked him is this possible with the diverter he said no, that the diverter was designed to use one port only at one time, he couldn't answer. He said I also need to use volume controls instead of diverters, that the diverter will hinder the pressure and flow volume - I do not understand again, Grohe's diverter just transfer flow, at the end of the day it looks like a bent pipe inside the diverter, not any less efficient than a 90 degree elbow in the copper line.
So I would like to ask those who knows, is 6.8 gpm enough to deliver flow to two spray jets rated at 2.5 gpm each?
Thanks,
MC
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Might be. Shouldn't be very hard to rig it up and test. I chose to install a valve for each outlet on mine. Valve may allow 6.8 gal to pass thru but not at high enough pressure to be useable. Probably gives 2.5 gallons with pressure for a shower head and 6.8 gallons with no pressure for the tub spout. I wouldn't ignore Grohe's advice without testing first.
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I am not ignoring his advise, I think it was odd he couldn't explain his opinion. I will call back tomorrow and talk to someone else and see what the response is. Just that adding a volume control and trim to each outlet is a LOT of $. Thanks Pat.
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DRIVEL SNIPPED

No, thats the last thing you want. You already asked the manufacturer's tech support folks ("those who knows") and recived a response that does not conform to your ignorant pre conceptions.
Unsatisfied with responses from those who actually build the things, you come here to find an answer that fits your pre conceived notions.
You are a remarkably ignorant fool, who's posting history here on this newsgroup demonstrates taht ignorance day after day after day, post after post after post.
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This is a newsgroup for free exchange of information. This is exactly the place for idiots like me to ask ignorant questions and solicit advise for any home repair subjects.
You seem like a very angry person. I hope you find the inner peace to deal with whatever is causing it.
MC
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The short answer is "no", the math is not "2.5 + 2.5 < 6.8 means we are OK". The long answer is below.
The flow characteristics of a shower valve or other component of a water piping system are not determined by just a single number, like "6.8 gpm". What you really need to know is a curve, pressure drop versus flow rate. For example, it could be that the shower valve will cause a pressure loss of 10 psi at 2.5 gpm, 20 psi at 4 gpm, 30 psi at 5 gpm, 40psi at 5.75 gpm, 50psi at 6.25 gpm, etc. The specification "6.8 gpm" probably reflects the flow rate you get with a pressure drop equal to the full system pressure, maybe 60 psi (I'm not sure what standard is used).
Pipes cause a pressure drop in the same way. As does an outlet like a tub spout or a showerhead. For a 2.5 gpm shower head, again that is the flow rate at full system pressure drop. So the curve for such a shower head might be 10 psi at 0.8 gpm, 20 psi at 1.4 gpm, 30 psi at 1.9 gpm, 40 psi at 2.2 gpm, 50 psi at 2.4 gpm, 60 psi at 2.5 gpm. At the outlet you know the pressure is 0 psi since the water is just falling. The tub spout, with such a large opening, probably causes negligible pressure drop.
Thus given a piping system with a constant pressure source (an approximation), one way to determine the flow rate is: guess a rate. Determine the pressure loss at that flow rate through each of the components and add the losses up. If you get exactly the source pressure you have the right answer. If you get too much pressure loss, guess a lower flow rate; too little pressure loss, guess a higher flow rate.
We can do a quick example using the example numbers above and ignoring the piping losses and assuming you have 50 psi water pressure available at the shower valve inlet. Let's guess 1.9 gpm out of each shower head. Then each shower head has a pressure loss of 30 psi. Then the flow rate through the shower valve is 3.8 gpm, which gives a pressure loss of about 20 psi. This makes a total pressure loss of 50 psi, so we guessed about right.
So in this example, you end up with two shower head as 1.9 gpm instead of two at 2.5 gpm. Since I made up the flow curves, the actual answer might be as low as 1.5 gpm out of each head. This is why the manufacturer says not to use the shower valve for multiple shower heads.
Cheers, Wayne
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wrote:

Wayne. Thank you very much. I was sick a few days and got this reply late. Your explanation made sense, so it's all in the design of the valve and not so much in the rated flow capacity and "optimum" pressure.
The Grohe expert I spoke to basically categorically told me that I could not use a pressure balance valve (must use thermostatic valve) and never use a diverter (must use individual volume controls), I asked him how come other vendors spray jets (rated at same 2.5 GPM per jet) have both? For example the Moentrol system:
http://www.moen.com/products/263
That is a pressure balance valve, with a built-in diverter, and it powers 4 jets at 2.5 GPM each. He could not answer that but said he couldn't comment on other products which I can't blame him. That's why I thought I would ask here, but of course I am not going to pipe a system and "hope" that it works and go against the advise of experts.
Thanks again,
MC
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