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The guest house I'm building has a 1" copper water supply line running to the house. It ultimately splits out to 1/2" PEX to all of the fixtures, via a manifold. My question is: between coming into the house and splitting in the manifold, when does the 1" incoming pipe step-down to a smaller diameter?

I was originally planning on running the 1" all the way to the cold-water manifold. However, all water heaters I've looked at expect 3/4" inlet and outlet and so the hot-water manifold will be 3/4" as well. Indeed, quite a few of the manifolds I've seen (even for cold-water) have a 3/4" inlet.

Finally, if I look at my own home, I see that I have 1" incoming supply, but it immediately transitions to 3/4" for a water softener and is never used as 1" inside of the house.

So what is "Best Practice" here? Do I immediately step-down the 1" supply to 3/4" as soon as it enters the house or do I keep it 1" all the way to the cold-water manifold and just accept that the hot-water manifold will be smaller?

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    Think it comes down more to cost. 3/4 cheaper than 1 inch. Use 1 inch if you have it, use 3/4 if you need to buy.
    – crip659
    Mar 7 at 16:43
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    It depends.
    – J...
    Mar 8 at 15:58

2 Answers 2

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Split where you separate the cold feed and the hot feed. So - 1" to house, 3/4" X 1" X 3/4" Tee - one side goes to the water heater, one side goes to the cold manifold. Locate that Tee wherever convenient, it does not need to be just as the line enters the house.

Why? Geometry. The area of a 3/4" pipe is just barely over half the area of a 1" pipe.

Bad examples abound in the wild. I fixed one house that had 1" pipe from the well, necked down to 3/4 and immediately necked down again to 1/2" after the 3/4" shutoff valve, with 1/2" for everything, and the hot water cold feed just being the same 1/2" pipe the rest of the cold water lines were using. So the entire house was being fed via a single 1/2" pipe.

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    1" pipe : 3/4" pipe - ratio of area is 1.8; 3/4" pipe : 1/2" pipe - ratio of area 2.25; 1" pipe: 1/2" pipe - ratio of area 4
    – D Duck
    Mar 8 at 9:35
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There are a few considerations in which oversized pipe can be problematic:

  • it may cost more
  • it may be more difficult to route
  • it takes more time and more water to fill

The last point is relevant in hot water distribution when one has to "wait for the hot water to arrive" or when waiting for an outdoor garden hose or sprinkler pipe to fill.

In your specific case, where the 1" pipe would feed a cold water manifold and you might already have the pipe on hand, I see no reason not to carry on as you had planned. You can use a 3/4" hot manifold and a 1" cold manifold, or reduce the cold supply to 3/4" and use 3/4" manifolds for both hot and cold. The only difference will be materials cost -- the choice won't affect performance of the system in any noticeable way.

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