On 2008-02-27, MiamiCuse wrote:

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?

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