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Thread: Diameter of supply line?

  1. #1
    DIY Member hans_idle's Avatar
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    Default Diameter of supply line?

    I'm doing some plumbing work in my house and I have a choice to make. My new tub is within 3 feet of 3/4", 1/2" and 1/4" copper supply lines for hot and cold. To make a long story short, the builder of the house ran the supplies through 2 reducers for some reason and had the 1/4" supply line going to fill the tub.

    I don't want to re-use the 1/4" line because it took too long to fill the tub. But I don't know if I'll experience much difference between the 1/2" and 3/4" lines. In a perfect world with no loss, I figure all 3 would be the same flow rate, just different pressure due to the diameter changes. But in the real world, I don't know how they compare.

    It will be significantly more work for me to hook up to the 3/4" supply line due to location (in a wall but tight against the drywall on the other side). I'm not sure how I'll cut and solder without making a big mess since it is hard to get to. The 1/2" is much more accessible. But if there is a big difference in how quickly the tub will fill, then I'd be inclined to use the 3/4".

    So is there much difference between 1/2" and 3/4"?

    Thanks!

  2. #2
    Plumber Cass's Avatar
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    Are you sure it is 1/4" and not 3/8"?

    1/4" is what is used on ice makers.

    1/4" very bad

    3/8" slow, not good

    1/2" good and very common

    3/4" best way to go if you can

    The pressure is the same reguardless of the pipe dia.

    The flow will greatly increase as you increase pipe size.

    Best is to branch off of 3/4" with 1/2" or 3/4"
    Last edited by Cass; 12-18-2007 at 06:52 AM.

  3. #3
    Sprinkler Guy Wet_Boots's Avatar
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    half-inch is fine for tub filling, unless an extra few seconds to fill the tub are intolerable - I doubt if many/any tub valves are 3/4 inch

  4. #4

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    Quote Originally Posted by hans_idle View Post
    In a perfect world with no loss, I figure all 3 would be the same flow rate, just different pressure due to the diameter changes. But in the real world, I don't know how they compare.
    You have it backwards, Hans, the pressure is the same per diameter, flow rate increases with larger pipe. As pipe diameter increases, there is proportionately less of an increase in flow, because most of the losses in small pipe are friction losses. In small pipe, more of the percentage of water is in contact with the pipe, therefore more of it is affected by friction.

  5. #5
    DIY Member hans_idle's Avatar
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    Quote Originally Posted by Cass View Post
    Are you sure it is 1/4" and not 3/8"?
    Yup, had to triple check it to believe it. It almost looks like whoever installed it made their own mixing valves, with 1/4" line going into the faucet handles and then coming back out into the spout. It took FOREVER for the tub to fill. No wonder. And the 1/4" copper is so flexible and bendy.

    Yes, it look like I mixed up the pressure/flow concept. I'll see if I can tap off of the 3/4" supply, but it means putting a "T" into the 3/4" line in a very tight spot inside the wall, and I won't be able to heat the pipe on both sides due to the wall/cabinet on the other side. I saw somewhere that I can use 2 layers of sheet metal behind the pipe to protect the drywall from burning. I'll have to experiment in the garage a few times.

    If it seems too difficult, I'll just take the 1/2" line. That should be waaaay better than the 1/4" line that was in place.

  6. #6
    DIY Member hans_idle's Avatar
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    Quote Originally Posted by Wet_Boots View Post
    half-inch is fine for tub filling, unless an extra few seconds to fill the tub are intolerable - I doubt if many/any tub valves are 3/4 inch
    Yes, leave it to my wife to pick out the most expense JADO faucet for the tub, and it has a 3/4" hookup. It's actually 3/4" threaded, so I'll have to see if I can find the proper flexible hookup hose for it. So either way, if I use 1/2" or 3/4" for the supply, it will have to connect to the 3/4" faucet connection.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    Many of the fixtures that allow threaded (male) connections, have the interior made so you can stick a piece of pipe in directly and solder it. Having a threaded connection where you can't see it is done, but better to solder it if you can.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8
    DIY Member hans_idle's Avatar
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    I have a follow-up question. I checked the faucet, and it specifically says NOT to solder to the valve body. In fact, 1/2" and 3/4" copper doesn't fit properly into the valve body, so soldering can't be done even if I wanted to.

    The directions show a 3/4" x 3/4" Female to Female NPT fitting being screwed on the end. Then, a threaded pipe being connected to the other end of the fitting. I'm not sure what the best way to connect this is. I can get a 3/4" nipple, maybe 8 inches long, to connect to this setup and then to the the copper line (which will have another fitting on the end to accept the nipple). But I wanted to verify that this is the proper way to thread pipes from copper to a fixture.

    I did some checking and it looks like I could just go from copper to a threaded fitting and then connect to the fixture, in which case I wouldn't need a threaded pipe.

    Any problems with doing it this way?

    By the way, all these pipes will be accessible. I'm making the whole front skirt of the tub removable by having it attach to the tub on cabinet magnets. I have a turbine and outlet underneath that need access, also, so it makes sense.

  9. #9
    Plumber Cass's Avatar
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    For 3/4" FIP X 3/4" FIP just go to Low*s and get a 3/4" IP brass threaded coupling and a copper 3/4" MPI adaptor and away you go.

    They also have 3/4" MIP X 1/2" copper.

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