I have a half inch main supply coming from the street (house built in 1954). Is there any benefit if I run 3/4 PEX from the water meter to the various feeds and then branch off 1/2" PEX to the faucets? Does it increase the flow or will it reduce it? will it improve pressure?

  • How long would the 3/4" line be? – Joel Keene Apr 25 '15 at 1:39
  • You sure you have a 1/2" main coming into the house? Where in the world is this? – Greg Nickoloff Feb 14 at 21:58

Flow resistance, like electrical resistance, increases with conductor length. If you are going a long way (say, over 50 meters) you will get better flow by running a larger pipe. The volume is a non-issue - (static) pressure won't go down even if you install one of those giant green gas storage containers in the yard.

In no case will you get more flow than what the incoming pipe can provide, larger pipes just mean you will not lose part of this flow to resistance.

In a standard house you don't have enough piping (or demand) to make a difference.

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Running 3/4" piping from the 1/2" feed will slightly decrease the pressure loss, meaning you'll get more pressure at your fixtures, but the difference will probably be very small. Most of the pressure loss will have already occurred in the 1/2" line before it reaches your house.

So, it will help some, but how much will depend on how long the run is compared to how long that 1/2" service connection is.

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Pex and copper have differing inside diameters. Copper has the larger opening. As to 1/2 copper to 3/4 pex I dont know for sure.

This said I would not run my hot lines at the larger size as the pipe holds more water and will waste more water waiting for hot or sitting in the pipes cooling.

If you can insulate both hot and cold pipes... its a minor cost that will pay for it self soon.

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There is no use in ever installing any plumbing that is bigger than your main supply line. At best you will have no issues. Worst case scenario is you will lose some pressure based on the increase in volume.

If you want to increase pressure you need to first increase the main supply line. If there is no chance of that happening then decrease as many things as you can to 1/4" (toilets, faucets).

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  • Why would pressure decrease from an increase in pipe size? Larger pipe will have lower friction losses and reduce the pressure less. What does reducing the fixtures to 1/4" do? You mean just to decrease the flow rate to those fixtures so they aren't robbing others? – Joel Keene Apr 25 '15 at 1:45
  • @JoelKeene - there are a few factors and it is complicated but basically let's use showering as an example. You have a 3/4 going from 10 feet in house to shower on second floor. Your shower head and valve spits out water well... Well the first 5-10 seconds you get a burst. What happens after? Your vertical runs have no throughput until they are full. Horizontally water will flow with less friction. Also you are certainly going to get backwash from other branches as your branch needs water. I have seen plumbing nightmares like this. – DMoore Apr 25 '15 at 5:09
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    A pressurized pipe will never not be full, unless you lose pressure. When you open the shower valve, the pressure at the shower head drops and the water in the pipe flows from the higher pressure to that lower pressure, that's the only reason water ever flows. Because a larger pipe causes less friction, the water is able to flow easier and maintain the pressure at the shower head higher. This is true whether it's vertical or horizontal, the only difference for a vertical run is that there's an addition pressure drop because of the effect of gravity, but that is independent of pipe size. – Joel Keene Apr 25 '15 at 12:27

It is true that 3/4 supplies more water and lowers pressure. When faced with this kind of question--resize to the ridiculous (you'll pass many tests with this trick too).

So if you have a 42 foot diameter pipe coming from the river to your house, what will the pressure be? 1 pound per square foot, perhaps? And almost zero resistance, and volume like crazy. Now pretend it is a natural hot water spring you're feeding from. You'll be waiting hours for all the non-moving water that has cooled in the pipe to move through your showerhead while the natural hot water spring refills the 42 foot pipe with fresh HOT water, eventually making its way to your shower.

The answer in short about your original question is, yes, change your mainline, but not to 3/4, instead to 1 inch PEX, this will greatly help flow, and will stop those annoying pressure drops when someone flushes a toilet while you're in the shower. MAKE SURE your hot water supply to the shower is 1/2" and insulated! There is a really good you tube video about this from one of the U.S. plumbing associations: search plumbing layout in you tube, and then look for a bunch of letters such as AAPPA or some such abbreviation as the uploader.

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    Mind putting a URL for the Youtube video in so we can edit it into your post? – ThreePhaseEel Dec 20 '15 at 1:43

You can have a 2 foot diameter pipe and not lose pressure! As a matter of fact you would be greatly extending your pressure tank capacity greatly. A larger pipe has less surface per volume of water. Going to a larger size would greatly reduce friction and help keep pressure. A half horsepower pump could push water across the United States if not for the friction of the water against the wall of the pipe. Push but not pull

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