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Thread: Plumbing 101 - Reduce main supply to 1/2"

  1. #1
    DIY Junior Member millerama's Avatar
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    Question Plumbing 101 - Reduce main supply to 1/2"

    I recently had my plumber replace the main shutoff valve inside my house. Everything seems to be working fine, but I noticed that a section of the repair goes from 3/4" down to 1/2" and then back to 3/4" to the whole house (minus one outdoor faucet).

    My question is, does this have any adverse affects? The water pressure seems ok, but I suspect that it is actually less than before the repair.

    The original pipe was steel and the new section is copper.

    http://www.terrylove.com/watersize.htm
    Last edited by Terry; 03-09-2011 at 11:28 AM.

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    Plumber jimbo's Avatar
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    1/2" is on the small side for a house today, even minus the outdoor bibb. The longer that piece of pipe, the more it will drop some pressure.

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    DIY Junior Member millerama's Avatar
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    The 1/2" section runs about 12 inches or less. I'm not sure what amount is acceptable.

    I guess what the real question is, would it be reasonable or worth the effort to ask the plumber to fix it. My guess is that he ran out of 3/4" pipe or fittings and used 1/2" instead.
    Last edited by millerama; 03-09-2011 at 10:16 AM. Reason: add clarity to the question

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    Forum Admin, Expert Plumber Terry's Avatar
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    A one bath home can get by with a 3/4" main water supply
    A two bath home needs 1" for the main supply.
    http://www.terrylove.com/watersize.htm

    Can you post a picture?
    800 pixels or less.
    I guess you should have called us for the repair.


    Copper looks much smaller than galvanized pipe, are you sure that it's 1/2"?
    Last edited by Terry; 03-09-2011 at 01:02 PM.

  5. #5
    Homeowner geniescience's Avatar
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    Quote Originally Posted by millerama View Post
    ... My guess is that he ran out of 3/4" pipe or fittings and used 1/2" instead.
    I think so too.

    Quote Originally Posted by millerama View Post
    The 1/2" section runs about 12 inches or less. I'm not sure what amount is acceptable. ... reasonable or worth the effort to ask the plumber to fix it...
    I don't know the numbers but I remember that a 12 inch run of a smaller size (anywhere in the line) is equivalent to a longer supply line of the bigger size. Not a big deal. So far you have not posted to say you notice a difference. It works fine.

  6. #6
    DIYer, not in the trades LLigetfa's Avatar
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    It depends on umm... supply and demand. If the supply pressure is great enough and the demand small enough it could be fine. With the 30-50 PSI of my well water, I can average 6 GPM through a hose bib connected via 1/2" copper.

    Old steel pipe despite being 3/4" could be reduced to the size of a pencil over time.

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    Moderator & Master Plumber hj's Avatar
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    Are you sure it is 1/2" copper, and not 3/4" which would still be a lot smaller than a 3/4" iron pipe, and "look" almost the same as a 1/2" iron pipe?

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    DIY Junior Member millerama's Avatar
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    There is definitely 1/2" copper. Most of the repair consists of 3/4" copper, but one section, right before it connects back to the old galvanized, uses 1/2" (3/4 copper to 1/2 copper to 3/4 galvanized). It's definitely possible that there is 1" in there somewhere, I just assumed it was 3/4"

    I'm not posting a photo because it's all behind drywall that was put up the next day.

    From the responses, it sounds like it's not a big deal, I just won't be using that plumber again.

    Thanks.

  9. #9
    Moderator & Master Plumber hj's Avatar
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    If that is indeed the way it was installed, fluid dynamics will cause the flow to increase its velocity through that short piece, so it will have minimal effect on the volume. It is when the length of the pipe increases and friction loss becomes a factor that the flow is compromise. A "reduction" like this is the principle behind "venturi plate" flow meters.

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    Journeyman/Inspector Inspektor Ludwig's Avatar
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    It should be a big deal. You shouldn't have a copper to galv. connection, I hope he installed some brass between the two. I'd get him out there and have him fix it and do it the right way, you're losing volume and increasing the risk of a pinhole leaks down the road with decreasing the size, not to mention the direct connect of the copper to the galv. You need 6" of brass between the two.

  11. #11
    DIY Member RinconVTR's Avatar
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    While its not good practice to install a section of 1/2" pipe at or around the main water line, its also not the neck in the hour glass. The OP said this was near the main shut off, which places it near the water meter.

    Residential water meters are typically 1/2" to 5/8" ID and only flow 15gpm before pressure drops off sharply. Most are rated for 20 or 25, but thats just a rating. Those max ratings include very large pressure drops.

    If you happen to have a very large home with a 35gpm rated meter, then you better not have any reduction in pipe size at or around the meter.

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