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Old 01-23-2012, 03:45 AM   #1
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Use of 3/8 inch rigid copper pipe

In several older homes with plumbing from the 1950's or so, I have seen 3/8 inch rigid copper piping used to supply single fixtures. I can even remember fixing a few of these lines back in the 1970's. Now nobody uses less than a 1/2 inch pipe anywhere. When I ask the inspector why he said 3/8 did not give enough flow. However with today's low-flow faucets it seems that 3/8 would be ideal -- especially on very long runs like for hot water to a seldom used powder room a long way from the water heater. The smaller size would have less volume, and therefor you would have to run the water a shorter time to get it hot. I know there are other solutions such as return water systems, small localized dedicated water heaters, etc. but to me a it would make sense to use 3/8 inch lines again in some applications.

PS -- the "big box" home stores think I am crazy since nobody there has ever seen a 3/8 inch hard copper pipe. The local supply houses mostly say they can't get it any more and only one will order it but it would cost more than 1 inch due to special handling etc. I haven't even looked into fittings or checked with inspectional services to see if they will permit 3/8 any more.


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Old 01-23-2012, 06:15 AM   #2
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Its fine for a short run from a shut off valve to a faucet to help increase pressure,but over a long distance you would have very little volume to feed the faucets.
Which would result in really low water supply at the faucet
Long runs of 3/8 are used on refrigiration systems,but they use a compressor to force the liquid freon through the lines


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Old 01-23-2012, 06:18 AM   #3
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What he said. It is common to see 3/8 after a shutoff valve, but not before it. It is common to use 1/4 and sometimes 3/8 for a single fixture, like an ice maker, etc.
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Old 01-23-2012, 07:33 AM   #4
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Many PEX manifold systems use 3/8 from the manifold to each fixture.
Coastal South Carolina
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Old 01-23-2012, 08:06 AM   #5
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I've never seen a 3/8 pex tube or manifild that small and why would anyone even want one? Does not mean they do not exist, just can not think of any use for them.
3/4 main line, 1/2 to supply shut off. 3/8 from there to fixture is the way to go.
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Old 01-23-2012, 08:38 AM   #6
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This table from the PEX Design Guide (available for free download, just Google it) shows why 3/8" isn't for long runs, even if only supplying a single fixture. For example, a shower @ 2.5 GPM would drop 39 PSI over a 100 ft run with 3/8" but only 9 PSI with 1/2". Granted, 100 ft run is long, but add in all of your other losses from fittings, valves, and turns, 2nd floor rise, etc, and it adds up fast.

The forum software made it really teeny, so it might not really be legible, but it's worth downloading the whole design guide anyway-- it's fully of handy stuff like this.

I've see 3/8" manifolds, but I believe they're for modified home-run arrangements where you run 1/2" or 3/4" trunks and locate the manifold in a bathroom to supply a few outlets with short runs.

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Last edited by gatorheel; 01-23-2012 at 08:58 AM.
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