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Thread: upgrading pipe size question

  1. #1

    Default upgrading pipe size question

    Hello, all. Great forum, lots of knowledge around here.

    I have a question I'm hoping someone can answer for me.

    I would like to increase the volume of hot water to the master bedroom. While under my house the other day, I noticed that the master bedroom HW is fed from a 34 foot long run of 1/2" copper that Tees off of a 3/4" HW line. It seems to me the 34 foot run of 1/2" is too small. I'm considering replacing it with 3/4".
    The cold water line is 3/4" the entire run.

    Just curious if there will be any issues with doing this and if I should abandoned the existing 1/2" or leave active (thus having the Master fed with a 1/2" and 3/4"). I'm planning to tie the new line in right before the bathroom fixtures witch are all fed with 1/2" (2 sinks, shower, tub, wc).

    Thanks in advance for any helpful suggestions.

    I probably should add that I'm installing a tankless hw heater at approx 40 ft from the Master Bath fixtures. Current hw tank (that will be abandoned) exist 90 feet from Master Bath (lengths stated are pipe length).

  2. #2
    DIY scratch-pad engineer leejosepho's Avatar
    Join Date
    Nov 2006
    200 miles south of Little Rock


    Assuming you actually do have a decent overall flow but cannot get your shower steamy, you could probably get *hotter* water out to the end of that run by insulating it. A larger pipe, however, might only make matters worse ... and the same can be said for tankless heaters.

  3. #3
    Forum Admin, Expert Plumber Terry's Avatar
    Join Date
    Aug 2004
    Bothell, Washington
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    The 1/2" line is good for two fixtures.
    The reason they ran 3/4 on the cold, was the added toilet.

    A reason to keep the hot at 1/2" would be that it's quicker to unload a 1/2 line than a 3/4.
    Reason to have a 3/4 for the cold, is when toilet is flushed, it doesn't drop the pressure in the shower on the cold side.

  4. #4


    thanks for the replies

    I figured the tankless was a crap shoot. The space savings was critical in my case so I decided to role the dice and see what happens. Just in case, I left plumbing stubbed in a closet wall if I ever decide to put a tank back in or a second tankless.

    Sounds like it is not worth the effort to replace the 1/2" HW line at this time. I suppose the best thing to do is wait and see how it works first. In the past I have had low flow and/or pressure to the Master Shower. That may be related to other issues. I have future plans to remodel this shower. The remodel would most likely include multiple shower heads (seems to be the new thing).

    If this was a new house, would it be standard to plumb the bath fixtures with 3/4 or 1/2"?

  5. #5
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    Terry, good discussion on pipe sizes. I have seen some articles in PM-Engineer mag, and also the IAPMO magazine "Official" that discuss changing codes on hot water pipe fixture unit loading, for exactly the reason you mention.....today, more and more usages are low flow to ultra low flow. Lav faucets are moving to 1.5 and even 0.5; showers may be going to 1.5; even washing macines now, the front loaders, use much less water. SO, the heat loss in larger pipes, and the water waste and time delay for arrival of hot, can all be improved with smaller pipes, with adequate pressure in the small pipe because of lower flow. Look to see some changes in this area over the next few years.


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