DIY Home Improvement, Remodeling & Repair Forum > minimum tubing diameter

02-28-2013, 03:29 AM
thomboz
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Join Date: Feb 2013
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Small Home

My place is only 1100 square feet. The kitchen, bath, & master bath are all next to each other, so 12' is my longest run. That's why I can get away with 3/8. But in a 2 story home with the water heater in the garage, I'm sure it would be a mistake. When i get ready to plumb the whole thing, I'll hook up 50' of PEX, run it out side and see how much flow is lost.

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02-28-2013, 08:04 AM
Fireguy5674
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I looked up the friction loss in 3/8" Type M copper and 3/8" Pex. With copper at 3GPM the friction loss causes a pressure drop of .168 PSI per foot. With 3/8" Pex at 3 GPM you lose .541 PSI per foot. However, with copper you will have fittings that will cause additional loss depending on the actual fittings and the number of fittings. 90 degree vs 45 degree etc. If you use Pex in a home run configuration you eliminate most of the fittings. I suspect the higher friction loss in Pex is more of a result of a smaller inside diameter as opposed to rougher internal finish. So for your shower plumbing in Pex at 12' of run and 2.5 GPM flow you should be looking at a pressure reduction of about 4 PSI. On a fifty foot run at 3 GPM you are looking at a 27 PSI loss.

Pressure loss = Friction Loss per foot @ Flow rate x feet.

Flow pressure = Static pressure - pressure loss

The other thing you need to consider is pressure loss due to elevation. Probably only about 1/2 PSI if you are only going from basement to first floor.

http://huduser.org/portal/publications/pex_design_guide.pdf

This site has all the information and examples for setting up Pex systems. This is where I found the pressure loss chart for Pex plumbing.

I know you are working outside the box but there is a lot of good information there to help you make your decisions.

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Last edited by Fireguy5674; 02-28-2013 at 08:07 AM.

02-28-2013, 08:56 AM
Fireguy5674
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I just found a chart for loss in copper fittings. They don't list 3/8" fittings but in 1/2" every time you put in a 90 degree elbow in it is like adding another foot of pipe to your run. A 45 degree fitting is another .5'. So 3/8" fittings would add up very quickly I am sure.

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02-28-2013, 12:41 PM
JoeD
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Is 3/8 tubing even permitted under the plumbing code?

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02-28-2013, 01:45 PM
nealtw
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This is contractors talking about 3/8 pex home runs, I found the last posting interesting.
http://www.contractortalk.com/f9/why-not-3-8-pex-all-domestic-hw-cw-94143/

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03-01-2013, 07:08 PM
dthornton

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Location: Red Oak, IA, Iowa
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The friction of PEX and copper should be about the same. Steel (commonly referred to as "iron pipe") does attract and hold minerals, which build up and reduce the inside diameter of the pipe. You are right - copper is great at heat transfer (a great advantage for certain purposes), whereas the PEX not so much. Glad you got a good flow from the 3/8". Hey - if it works and you are happy with it, that's what is important. When I think of 3/8" pipe, I always think of toilets (which typically use 3/8") and take "forever" to fill up! The last house I lived in ; if you were in the shower (1/2" pipe) and somebody flushed the toilet, first it would scald you, then it would freeze you before finally balancing back out. NOT a good experience!!! Good luck with the project!

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