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Choosing Size for Water Supply Line

user-2310254 | Posted in GBA Pro Help on

3/4 inch water supply lines are typical in my area. But I can probably spec a 1 inch supply if I plan ahead?

We won’t have any elaborate multi-head shower enclosures or massive garden tubs. Even so, I imagine it will be easier to maintain consistent water pressure if I go with the larger pipe. Thoughts?

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  1. Expert Member
    BILL WICHERS | | #1

    A 1" line has two major benefits: better flow rate to distant fixtures, and less pressure drop when a second fixture gets opened (think "less hot shower effect when someone flushes the toilet"). On a cold water line, there isn't really any down side. On a hot water line, the downside is it will take longer to get hot water to distant fixtures due to the larger volume of water in the pipe that needs to be "flushed" before hot water gets to you.

    I have 1" supply lines for both hot and cold water in my own house. The "takes forever to get hot" was a big problem on the hot line when I first got the house, but I added a 1/2" return line to make a thermosiphon loop and no more problem (but slightly more gas used by the hot water system due to the losses in the loop). You can take a shower and someone else can flush a toilet or wash their hands with zero effect on the shower, which is nice.

    Note that if you go with a manifold system and home runs to each fixture, that can often be something of a happy medium with similar benefits to a larger main run, but it might cost more than a large main run depending on the layout of your house.


  2. user-2310254 | | #2

    Hi Bill,

    Thanks for the reply. I was thinking of the supply up to the pressure control valve, but it sounds like you ran 1 inch hot and cold lines throughout the house and then tapped off those to service individual fixtures. I like the suggestion of the themosiphon loop as well. On my first house, I resorted to a recirculating pump after the construction was over and discovered how long the lines required to deliver hot water. It worked but was not efficient from an energy standpoint.

    I'm assuming you used copper lines. Would there be any issue opting for PEX?

  3. Expert Member
    BILL WICHERS | | #3

    My house was originally built with copper lines, yes. I stuck with copper for the thermosiphon loop since I was matching existing piping. When I replumbed part of the well supply line and the new filters I added, I did those with PVC. I would not have an issue using PEX, it's good material, and basically a beefier version of polyethylene which has been used for a LONG time. I'm not a fan of the push-on fittings, but if you use the crimp-on fittings that use the metal compression ring you should be fine.

    The thermosiphon loop is awesome -- no pump to buy, no pump to break. It doesn't get any simpler. You don't need electricity for it to run, but you also can't easily put it on a timer. I'm OK for the minimal increase in gas use, but I did insulate ALL of the lines.

    About the only trick is to use a swing-type check valve and not one with a spring and ball since the thermosiphon doesn't have enough force to open the spring-type check valves. I also recommend putting a ball valve on BOTH ends of the thermosiphon loop so that you can isolate the return line in case you need to service that check valve someday.


  4. walta100 | | #4

    I am a big fan of having the larger pipe from the street to the house if that is the question.

    If you will be using PEX pipe I say upsize every pipe as PEX fitting go inside the pipe and that restricts the flow some PEX systems are more affected than others. If you want to skimp modern toilets and sinks will do fine on ½ PEX but not hose bibs, tubs and showers.

    You may want to watch the Gary Klein video on hot water systems.


    1. Expert Member
      BILL WICHERS | | #5

      The restriction of the fitting is usually small compared to the restriction from the length of the run, assuming normal fittings (I've seen some knockoffs with ridiculously small bores). A lot of people think the back pressure of a plumbing system is dominated by whatever the smallest aperature is, but that's usually not the case -- usually the back pressure of a long pipe run is higher than the smaller bore of something like a valve. If you have a bazillion fittings in your PEX run that might be different, but usually PEX minimized fittings because the line can be installed in one piece with swept bends instead of 90 degree fittings at every turn.

      I agree the run from the street should be 1" or larger. It's a lot easier to put in a pressure regulator to deal with excessive water pressure than it is to put in a booster pump if your pressure is too low.


  5. the74impala | | #6

    Pex A does not constrict the flow of water, it's the same diameter through fittings as it is through the pipe.

  6. charlie_sullivan | | #7

    On avoiding pressure drop in PEX, I think the key is to avoid using elbows when they aren't strictly necessary, and instead use the ability of the PEX to make gradual curves. Even if the fitting doesn't restrict the area, a sharp elbow has more flow resistance than a large-radius curve.

    If you avoid unnecessary elbows and keep your runs short, you can use 1/2" lines for hot water and still get plenty of pressure and flow. Then you'll have super-quick warm up times because of the reduced volume.

  7. Expert Member
    Akos | | #8

    There is no point of running more than 3/4 trunk lines inside the house. No issues with 3/4 pipe in fourplex which could have 4 showers running at the same time. This assumes standard shower heads around 2.5GPM.

    1" from the city side is a good idea, costs little extra to install and you never have to worry about pressure loss.

    I like to use these for elbows, same price but much less pressure loss plus no chance of erosion from cavitation:

  8. andy_ | | #9

    Bigger supply from street to house is always nice, around here 1" is minimum.
    As to the size of the PEX in the house, there is a trade off to the larger diameter there which is how long it takes to get hot water to the fixture. IIRC, a 3/4" diameter line holds twice the volume of a 1/2" diameter line. Use a cheap crimp fitting in there and you have all that extra water volume and a reduced flow. Use a 1" trunk and branch system and you may as well put the kettle on.
    I've found the best balance to be 1/2" home runs to a manifold with PEX-A since the diameter of the fittings most closely matches the diameter of the pipe. This gives you good flow at the fixture without waiting twice as long to flush the cold (was once hot) water out of the line. The home run approach means you're just flushing out that one line, not a larger main line and everything it's attached to. You also won't need pumps or other recirc tricks that use up energy.

  9. Jon_R | | #10

    Off topic, but...

    While one can inefficiently address the "time-to-hot-water" issue with circulation, much better to use small, dedicated shower and sink lines all the way to the water heater. Plus a thermostatic shower valve to address the "shower temp drifts upward" issue. But do the calculations and use 3/8" if possible. I shower at 1 GPM and this needs 3/8" lines to achieve plug flow. Bathroom sink flow is even less.

    One can use high flow to reduce the wait, then turn it down - the waste for the former is minimal, but the latter often doesn't happen.

    1. user-2310254 | | #11

      Great input on this topic. Years ago I recall reading an article that advised using a manifold with smaller diameter lines. Like Jon R, the author felt that most fixtures could be supplied with 3/8 inch lines off a manifold. One issue was (at least at the time) that some jurisdictions wouldn't allow anything smaller than 1/2 inch lines. I'll have to check to see if this is an issue with my local inspector.

      On the thermostatic valves, I can't imagine showering without one of those. Well actually, I can. Years ago we visited friends and I got a really unpleasant surprise when someone in another part of the house decided to flush a toilet while I was in the shower. Burr and then ouch!

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