A question for the plumbers.

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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 increase?
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The Post Quartermaster wrote the following:

If you want pressure, you go from large to small.
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Bill
In Hamptonburgh, NY
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The Post Quartermaster wrote:

Larger will reduce the pressure drop would otherwise have. Neither will increase pressure above supply--it only drops, never goes up.
How long a run?
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40 feet maybe. I just dug a two foot deep trench by hand so it feels like it was a couple of hundred feet. And the thermometer is showing 102 in the shade.
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On Tue, 10 Aug 2010 17:28:07 -0500, "The Post Quartermaster"

But you weren't in the shade all the time, so you should be okay.
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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.
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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 has.
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?
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wrote:

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.
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The Post Quartermaster wrote:

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.
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The Post Quartermaster wrote:

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.
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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.
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On 2010-08-10, The Post Quartermaster

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 more flow.
Cheers, Wayne
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wrote:

Thanks to ya all. I went with the 3/4 all the way because I had enough of it on hand. I appreciate the responses. I learned something and at my age that's always good.
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How so? The 3/4" pipe feeding it will still give the same flow, as will the faucet opening at the tub.
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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.
Cheers, Wayne
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On 8/10/2010 9:16 PM, Wayne Whitney wrote:

WOW! Wayne has WAY WAY over thought this out. LOL!
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Steve Barker
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Actually I just worked it out as an example for the public good. :-)
Wayne
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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.
http://www.freecalc.com/fricfram.htm
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.
cheers Bob
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Wayne Whitney wrote:

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 supply line(s).
I may be wrong, but that's my take on it.
Gil
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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.
Wayne
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