There's a lot of misinformation and incomplete information in this
thread. Let's bring some citations and facts to the game. You wanted
details and facts, here you go.

At least part of the mess here is due to confusing static pressure and
dynamic pressure. In this context, static pressure is the pressure at
a specified point in the system when flow is zero. Dynamic pressure
is pressure at a specified point in the system when water is flowing
at some non-zero rate.

What you asked about is the dynamic pressure at the showerhead when
the water is flowing at whatever rate it flows when you have the
shower on. The greater the dynamic pressure at that point, the more
forceful the shower stream will seem, for a given showerhead. Yes, as
many posters have pointed out, different showerheads will feel better
or worse all else being the same, but for a given showerhead, the
higher the dynamic pressure at the showerhead, the more forceful feel
the water streams will have.

So first a word about static pressure. The static pressure at the
shower head is determined by the static pressure at the point your
water pipe enters your house, minus 0.433 lbs for every foot of
elevation difference between the entry point and the shower head,
assuming your showerhead is higher than the entry point and you don't
have a pressure regulator valve. So if the static pressure at the
entry point is 50 lbs, and your showerhead is 10 feet higher than the
entry point, the static pressure at the showerhead will be 45.67 lbs.

Note that pipe size (within reason), number of turns, fittings, etc.,
does not enter into this. So when a previous poster says pipe size
doesn't affect pressure, they are talking about static pressure (or
they are wrong, see below).

For a good explanation of static pressure, see:

http://www.nationaldriller.com/CDA/A...00f932a8c0____

On to dynamic pressure. When water flows through a pipe, a pressure
drop occurs along the pipe due to the energy that is required to
overcome the friction between the pipe and the flowing water. Pressure
drop in a pipe is affected by flow rate, the viscosity of the liquid,
the diameter of the pipe, and how smooth the pipe interior is
(corrosion and deposits increase pressure drop).

Fittings such as els, tees, shutoffs, etc., also cause a pressure
drop, and for the same reason. Pressure drop for fittings is often
expressed as the length of equal diameter pipe that would cause the
same pressure drop at the same flow rate.

For an example, a 1/2 inch copper el has a pressure drop equivilent to
2 inches of 1/2 copper pipe. A 3/4 copper el has a pressure drop
equivilent to 3 inches of 3/4 copper pipe.

Pressure drop is usually expressed as lbs per 100 feet of pipe at a
given flow rate. For an example, water flowing at 2 gpm through 1/2
copper pipe will have a pressure drop of 2.6 lbs per hundred feet. For
3/4 copper pipe under the same 2.0 gpm flow, the pressure drop is 0.5
lbs per hundred feet.

For a good explanation of pressure drop and tables for pipe and
fittings see:

http://www.nationaldriller.com/CDA/A...00f932a8c0____

What does all this mean for your shower?

Let assume the pressure at the entry point to your house is 50lbs.
Lets assume you have 4 elbows and 100 feet of total pipe length (not
counting the elbows) between the entry and your showerhead, and that
the showerhead is 10 feet higher than the entry point. Let's also
assume copper pipe and a flow rate of 2 gpm. What will be the dynamic
pressure at the showerhead with both 1/2 inch and 3/4 inch pipe?

Dynamic pressure at showerhead equals static pressure at entry, minus
static pressure drop due to change in elevation, minus dynamic
pressure drop in pipe, minus dynamic pressure drop in fittings.

For 1/2 inch this is:

50 - (.433 x 10) - 2.6 - (((4 x 2) / 1200) x 2.6) = 43.0527lbs

For 3/4 inch this is:

50 - (.433 x 10) - 0.5 - (((4 x 3) / 1200) x 0.5) = 45.165

All the figures come from the cited references.

The complicated term is the number of ells times the equivilent length
of pipe in inches divided by the number of inches in 100 feet, times
the pressure drop of pipe per 100 feet.

So for the above example, the dynamic pressure will be a little over
two lbs greater for 3/4 pipe compared to 1/2 pipe. Will you notice it
standing in the shower? I doubt it.

You will notice that the pressure drop due to elevation change is the
major factor, followed by the dynamic drop of the 100 feet of pipe.
The elbows contribute only a tiny amount.

Sticking a valve in the pipe will add a dynamic pressure drop term to
the calculation. The magnitude of the term will depend on the valve
and the flow rate but it will be the same for the two cases, so you
will still be better off (albeit only a tiny bit) with 3/4 pipe.

For completeness I will mention one complicating factor. The above
calculations assume the same flow rate in both cases. However, in
practice this won't quite be true, because the dynamic pressure is
higher in the 3/4 case. This will mean the flow rate will be slightly
higher, which will increase all the pressure drops slightly. The net
affect is that the difference in practice will be even less than shown
above.

I hope this helps clear up some of the confusion.

Paul Franklin