I'm going to run a water line out to a little outbuilding I have out
near the barn. The line I am going to tap off of is probably a 3/4"
line. If I run a larger diameter, say a 1 or 11/4 inch from there to
the outbuilding would that result in any increase in pressure at the
outbuilding or would the fact that there was a 3/4" feed negate any
Yeah but October a year from now I'll be 70 and right now I feel like
I past that a couple of years ago. It's so hot down here in south
Texas that the damn water moved over under a shade tree. Now I've to
dig a trench over there and lay some more pipe.
On Tue, 10 Aug 2010 16:50:40 -0500, "The Post Quartermaster"
If the valve on the spigot or outlet you'l be using has a greater
diameter than the one on the 3/4" pipe, you'll have LESS pressure.
There is no way to get more in pounds per square inch than the 3/4
Also the extra length of pipe lowers the pressure some, but not very
much, maybe not even noticeable. Using larger diameter pipe will make
the effect of that less, but maybe not noticeably.
How long will the pipe be and why do you need more pressure?
It's only about 40 feet, if that. The place we bought has 3/4 all the
way from the well to the old house, about 125 feet. The pressure is
okay, I just thought I might lose some by extending it. I'm probably
going to upgrade all the pipe in a year or so after I get some of the
really necessary things done. Like a place to actually put a bed to
sleep on, and things like that. <G> They told us it was a 150 year old
farmhouse. Well, really it's a 150 year old woodpile.
Is the pressure tank in the house? If so, that's where the pressure drop starts,
unless you are drawing more water than the pump can supply at pressure. 40 feet
of additional 3/4" pipe shouldn't have much drop unless you are using a lot of
water. Certainly, 1" pipe shouild be more than big enough.
The larger diameter will reduce the additional losses added by the longer pipe,
compared to the same added length of smaller diameter pipe. The larger pipe will
also mean that a large valve at the end of the added pipe can cause a larger
pressure drop to be seen inside the house where the new pipe attaches compared
to a smaller pipe.
On 8/10/2010 4:50 PM, The Post Quartermaster wrote:
you certainly won't gain any PRESSURE by increasing the line. But you
can LOSE pressure if the line is too small. What you will do with a
larger line is eliminate any possibility of pressure drop due to
frictional losses. How far are you going? and do you really need a butt
load of pressure? Are you just filling tanks, or are you hosing out
stalls and need a good jet stream? I personally would not even bother
going up to the one inch, unless you're going over 500 feet, or up a
hill. One inch and one-and-a-quarter are quite a bit more expensive
than 3/4" also.
remove the "not" from my address to email
The way to approach this question is what are the water uses you will
have in the outbuilding and what flow rates do they demand? If you
just have a lavatory, it only needs 1.5 gpm, and since you said it is
only 40', the 3/4" pipe will be fine, and you'll never see the
difference if you upsize to 1" pipe. But if you want to fill a
bathtub as quickly as possible, then yes, the 1" pipe will give you
Here's how fluid flow works:
You can assume the water main or pressure tank is a roughly constant
pressure source. When water is flowing each element (pipe, valve,
water heater, etc.) between and the source and the outlet contributes
pressure drop depending on the flow rate. As you open an outlet, the
flow rate starts at zero and climbs until the total pressure drop
equals the source pressure. That is, once the water droplets are
flying through the air, the pressure is zero.
In the case of a bath tub filler, it is usually designed to have a big
opening with very little pressure drop compared to other components in
the system. As opposed to a lavatory faucet, where it is designed to
have a pretty rapidly increasing pressure drop as you exceed the
federally mandated 1.5 gpm maximum flow.
Back to the case of the OP, he mentioned 40' from the main building to
the outbuilding. For a simple example suppose the interior piping in
the main building was also 40' of 3/4" copper and there are no other
sources of pressure drop. Now the pressure loss per foot of 1" piping
is roughly 1/3 of that of 3/4" piping (it depends on the ratio of the
interior diameters, which for copper pipe depends on the wall
thickness, type K, L, M).
So the difference between 3/4" and 1" outbuilding piping is 2 units of
pressure loss versus 4/3 units of pressure loss, or a ratio of 3:2.
Since pressure drop varies as roughly the square of flow rate, the
ratio of the flow rates is about sqrt(3:2), or 1.2. That is, in this
simple example you'd expect 20% more flow with the 1" pipe.
Actually Wayne saved me from having to develop a pressure drop matrix
as a function of flow vs pipe size . :)
Here is a link to a calculator that allows you to determine pressure
drop as a function of pipe size and flow.
bottom line.....if the flow is low (like a few gpm), the pressure drop
will be low, pretty much independent of pipe size.
if the flow is high (10 gpm +) ....pipe size is more important.
Hate to be picky, Wayne. But.... I agree with everything you say up
until the last sentence. If the 3/4' and 1" pipes were connected to the
same source, and are flowing at the same time, then yes, you'd get more
flow from the 1" pipe.
But, as in the OP's case, he mentioned feeding 1" pipe from 3/4" pipe
and you will not get any more flow than the 3/4" pipe is providing.
although the friction loss in the 1" pipe is less that that in the 3/4"
pipe the gpm flow through the 1" pipe is not going to be any greater
than that received from the 3/4" pipe.
That's why fire departments use large size lines to feed pumpers from
hydrants and static sources, so they may feed a number of smaller size
lines whose combined discharge flows can not exceed the flow from the
I may be wrong, but that's my take on it.
Sorry, that's not how it works. For a constant pressure source, less
friction loss gives more flow.
It's like resistors in series (V = pressure, I = flow) connected to a
constant voltage source, except for each "resistor" the V-I relation
is roughly V = I^2 R.
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