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
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?
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.
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.
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.
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.
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
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:
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.
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