# minimum tubing diameter

Discussion in 'Plumbing Forum' started by thomboz, Feb 26, 2013.

1. Feb 26, 2013

### thomboz

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I'm completely redoing the water in my place. The cold water heater now is two feet from the kitchen sink, same distance from main bathroom shower, 4 feet from main bathroom sink, and about 12 feet from master bath sink & shower.
I'll be using pex with a home run plan. I want to minimize the waste of hot water and the time it takes to get to fixtures (I know, I'm very close already!). The smaller diameter of the pex, the sooner hot water gets to the fixtures.
My question is, how small can I get till I loose pressure and can't get a good shower? And can I use smaller diameter at the sink than the shower as a sink requires less volume than a shower?
I can buy 3/8" pex locally, but can I go smaller?
Thanks,
ThomBoz

Last edited: Feb 26, 2013
2. Feb 26, 2013

### bud16415

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I’m no expert but I think I would pick a size and use it throughout. The savings on those short runs is going to be very minimal IMO. If you really want to know you can calculate the area of the inside of each tube and subtract to find the difference and then multiply by the length to get the water saved in cubic inches on the run. Then compute how many seconds of shower time you will gain by the smaller pipe size. I’m sure most people waste a few gallons of water just adjusting the temp before they get in the shower.

3. Feb 27, 2013

### thomboz

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OK, I did the math. A foot of 3/4" pipe contains 3oz of water, a foot of 1/2 is 1.3oz. and 3/8 has .75 oz. Or look at it this way, it takes 46 feet of 3/4 pipe to hold a gallon of water, 91 feet of 1/2" and 175 feet of 3/8".
So 3/8" pipe will deliver hot water almost twice as fast as 1/2". That would be very nice when you think how often you use some faucets.
But if it gets there twice as fast, do I get half the flow? And will 12' of 3/8" PEX flow enough for a good shower?
Thanks,
ThomBoz

4. Feb 27, 2013

### nealtw

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Time might be a consideration if you were going 70 ft but? Go with the 1/2 inch so the next guy won't have to redo it.

5. Feb 27, 2013

### JoeD

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Small diameter = smaller volume which could seem like less pressure. I would not go less than 1/2". That is the standard size for most branch lines.

6. Feb 27, 2013

### thomboz

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Look, I post her because I do not want to do it the standard way, I stated that very clearly. I stated my parameters, adequate flow, quicker warm up, saving energy. These are all admirable goals. I understand 'standard', standard is the way to do it that works in most situations for most people. But it is not the best way for many situations. It does allow relatively ignorant people to do an adequate job. I am ignorant right now, but I'm trying to gain knowledge and understanding so I can end up with a superior outcome.
It seems so many people just want everyone else to not think or understand, don't rock the boat, and do it like everyone else. Engineers work hard to develop a system that will work in all situations. But no job is 'all situations'. I'm looking for one of those engineers to give a bit of info. When you tell me to do it the standard way, I know that will work, duh, but it won't get me what I want.
Now perhaps I can't get what I want, perhaps 3/8" on a 12' run will not get me an adequate shower, but I want someone to tell me that, and perhaps why.
Some of us are creative, looking for new and better ways of doing things, finding new and original solutions that are better for ourselves and the planet. Other people just want to be normal. If you want to be normal, then you will not have the answers I'm looking for.
Sincerely,
ThomBoz

7. Feb 28, 2013

### nealtw

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Point well taken: If I can, let's start over. If you look at older feeds for toilets and faucets, they were feed with 1/4" from the angle stop, so your smaller pipe likely won't make any difference. The shower is fed with 1/2 and many people run 5/8 to the head for more valume and then install water savers which kinda defeats that. I think your biggest problem will be to get a decent shower, you may have to modify the shower head.

8. Feb 28, 2013

### dthornton

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I'm no expert either, but I just had my house replumbed with PEX. I went with 3/4" for my shower. I get a very good flow to my shower AND, I don't get either scalded or freeze if somebody flushes the toilet at the same time. My wife can also run the washing machine while someone is in the shower and the water pressure doesn't suffer. Of course, it's your house and you can make it the way you want it, but i really think you'll be disappointed if you use the 3/8" tubing. Remember that even though water is a liquid, there is friction between the water and the walls of the tubing. The "middle" flows better, no matter what size the tubing. Therefore, if the "middle" is larger, the flow will be better. There also may be local codes that dictate what size tubing you must use.

9. Feb 28, 2013

### nealtw

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If he runs continues pipes with no extra fitting like elbows, he may gain on the friction thing. The pex fitting inside the pipe is what will interfere, maybe sharkbite fittings would be less intrusive. Would be easy enough to test at time of rough in. Just time a gallon. Then compare that to another shower with the flow rate set at the same time to see if it would be enough for a shower.

10. Feb 28, 2013

### thomboz

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Thanks for all the chat, it motivated me to do an experiment, so I bought 20 bucks worth of fittings from the hardware store along with 12' of 3/8" PEX. I sweated a 'T' into the hot about a foot or so from the water heater, then connected the PEX, duct taped it to a joist above the shower (most of my walls are being relocated... I bought a total dump of a home and am redoing everything), installed a spare shower head, and took a shower... but with the old shower head still there.
Very nice, plenty of pressure. I took the shower head off and the flow thru the 3/8 alone was amazing. Keep in mind that the new shower head has no cold to it, so it was just barely too hot for me to tolerate... but the old shower with the original copper pipe was not! Apparently the copper gives gives off more heat than the PEX.
While I was hooking everything up, I remembered how much I hate those water saving shower heads, and that I had previously drilled out the orifice on mine, but even so it was so much smaller than 3/8".
Also, I let everything cool down, then put a pot under the PEX line to measure how much water flowed till it got hot... less than two cups, I'm ecstatic.
On a side note, does PEX have less internal friction than copper? I read that one of the advantages of copper over steel water pipe is that it is smoother inside and therefore won't attract mineral buildup.

11. Feb 28, 2013

### thomboz

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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.

12. Feb 28, 2013

### 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.

Last edited: Feb 28, 2013
13. Feb 28, 2013

### 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.

14. Feb 28, 2013

### JoeD

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Is 3/8 tubing even permitted under the plumbing code?

15. Feb 28, 2013

### nealtw

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16. Mar 2, 2013

### dthornton

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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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