Pipe size changes 3/4 to 1/2 what’s the effects

[donnielove]

I have a 3/4” copper line that runs about 20 feet starting from the meter. Then it gets reduced to 1/2” for another 20 feet. At this point the plumber installed a T and a shut off for a future water line to the backyard. Well it is time for that faucet but all I have is a 20 foot piece of 3/4” pipe and a freeze protected outside faucet that is also 3/4”. The distance from the faucet to the shut off is 15 feet.

What effect does going from 3/4” to 1/2” and back to 3/4” have on pressure and volume? Should I change the pipe from the reducer and go 3/4” all the way to the T and reduce after the T to pick up the 1/2” line to the kitchen and have the line from the meter to the outside faucet 3/4” all the way.

Just an FYI The 1/2” goes to a kitchen that does not have a basement or crawl space under it. once the 1/2” gets to the end of the current basement it goes up the wall and into the kitchen wall where it routes through the walls to the sink and dishwasher so I will have to keep the 1/2” at some point in the basement because I am not going to open up the kitchen walls to do a total replacement to 3/4”.

Thanks for the help!!

 

[Breplum]

1. Pressure is independent of pipe size. Your branch could be 1/8" and it would still have the same "pressure".
2. Flow is what is affected by the pipe size in conjunction with pressure and distance. Depending what flow you need is the pertinent issue. That 15ft distance is not a source of much friction.
3. Hose bibbs in 1/2" pipe work fine unless you want to put out fires or run serious irrigation.

 

[Sponsor]

 

[Reach4]

What effect does going from 3/4” to 1/2” and back to 3/4” have on pressure and volume?

If you are motivated, http://www.pressure-drop.com/Online-Calculator/ is a nice pressure drop calculator.

Note the "group" drop down, which lets you not only do straight pipe, you can do elbows ("bends") and more.

It's not that that easy to get the units right.

For simplicity, you can enter 0.75 inches ID for 3/4 inch copper, and 0.5 for 1/2 inch. Not exact.

Pressure drops add. Losing 15 psi total is a big deal if the incoming water pressure is 30 psi, but not if the incoming pressure is 75.

Get a garden hose thread pressure gauge. Cheap. Easy. And it will come in handy.

 

[wwhitney]

A simple way to think of the difference is that, for type M copper, each foot of 1/2" pipe is equivalent to 5.6' of 3/4" pipe. For the details, see below.

Cheers, Wayne

Details: the Hazen-Williams approximation for pressure drop as a function of flow, inner diameter, and length includes the diameter in the denominator to the power 4.87. The inner diameter of 1/2" copper type M is 0.569", and of 3/4" is 0.811". The ratio is 1.425, and raised to the 4.87 power that's 5.6.

 

[donnielove]

Thanks to everyone who replied. So in the end it comes down to this;

Should I go all 3/4” as much as I can or just connect to the 1/2” shutoff with the 3/4” pipe and not touch the 20 ft run of 1/2”

 

[Jadnashua]

You didn't respond on what you 'need'.

If you have a 3/4" valve, and you NEED the volume, you won't get the full capacity with that 1/2" section. If you don't 'need' the max volume, then it will be fine as is.

The maximum flow rate for cold water suggested by the Copper Institute is 8fps. You'll get a lot more volume with a 3/4" line than a 1/2" line. A 3/4" line running at 8fps, should be able to flow about 11gpm. A 1/2" line running at the same velocity would be about 5gpm.

 

[donnielove]

Thanks Jadnashua,

Since the pressure at the meter is about 75lbs and I have learned from this site that pressure is not affected by pipe size reduction. I don’t think I need to do anything with pressure it is more than adequate from what I can tell so it would be volume I am most concerned with or as you put it need. I will be connecting the outside faucet to sprinklers for the back yard and would like to run at least 2 at the same time. The flow will be adjusted at each sprinkler via a valve but I guess I am concerned about volume. So 3/4 all the way is how I will go.

 

[Jeff H Young]

if you have 3/4 use it there is no advantage to using 1/2 inch except that if its a hot line you wont have to wait as long to get hot. If your getting adequate flow. It is a bit sloppy but your not going to gain anything going to buy more 1/2 inch pipe. no need to calculate anything you already decided and ran 1/2 inch unless your thinking about ripping everything out for some reason