1/2" vs 3/8" PEX Plumbing Design Question

[MadMadScientist]

Hello All,

Here's the situation. I've got a situation that seems to be perfect for a pex home-run manifold.

I've got 2 bathrooms, a small kitchen, and a laundry room all with ~10' (as the crow flies) of the manifold location.

The Plastic Pipe Institute has a Pex Design guide which I have read and which has this very handy table.



From reading the table it seems like just about everything but high use situations would be ideal for 3/8" PEX.

I love the idea of using 3/8" instead of 1/2" because.

1. Its cheaper per foot.
2. Its easier to install-more flexible and smaller min bend radius.
3. Less swiss cheesing of joists and easier to bundle the 3/8" lines together.

I do know something about working with 1/2" PEX having installed over 2000' of it already in my radiant heating system.

I'm all set to work up a parts list and I download the Wirsbro/Uponor catalog....

They don't make ANYTHING in 3/8" basically...no manifolds, no angle stops, just some adaptors to go from 3/8" to 1/2" and the tubing of course....

What gives? Do I just suck it up and use 1/2" for everything paying more for the privilage of a harder install?

Has anyone else come across this issue and how did you solve it?

Does the plastic pipe institute have it wrong? Am I missing something?

I mean, 1/2" to a freaking icemaker? Really?

Any useful advice would be greatly appreciated....

Mads

 

[Gary Swart]

Here's what you'd better be aware of. 1/2" PEX has a smaller inside diameter than 1/2" copper. Usually, you would figure on 3/4" PEX where you would use 1/2" copper. Now, if you do the math and figure the difference in area of a 1/2" circle compared to a 3/8" circle, you will see that there is a huge difference. The 3/8" might be OK for the ice maker, they don't require much flow, but for other fixtures, I seriously doubt that 3/8" will be anywhere near adequate. In fact, I really doubt that 1/2" PEX will carry enough water. I'm not a pro, so you might wait for some of them to respond to your question.

 

[Sponsor]

 

[Jadnashua]

Some things can use a lot of flow: tub, (most) washing machines, outside hose bibs. Keep in mind that 1/2" pex has a smaller ID than copper, so it is already closer to 3/8" copper.

Many fixtures are flow restricted: lavatory and kitchen sink faucets, showerheads (but not tubs where you want to fill it as fast as you can), so larger is often better. Home runs maximize flow since there are minimal elbows, fittings, etc, so that helps, but mostly only on short runs (which you have). Still, you won't get a huge amount of flow.

Also, it's one thing to run to a single fixture, but if it makes more sense to branch it, then you need a larger pipe. An example may be to under the kitchen sink where one line might be used after the shutoffs, to the sink, dishwasher, and frig. Then, maybe add a filter. A 3/8" would be way undersized, and a single 1/2" pex might be pushing it if everything was calling for water at the same time.

 

[MadMadScientist]

Points well taken.
I bet that the plastic pipe guys did do the math and that's how they got the table...
With short runs, a bath lav and a modern low flow water closet why should they need more than3/8"?

I mean, those institute guys didn't pull those numbers outta thin air did they?

 

[hj]

They DO make manifolds in 3/8", but they use "compression" connections rather than crimp, and 3/8" tubing is intended to go from the mainfold, which usually has a valve on each outlet, directly to the faucet, without an additional intermediate valve.

 

[Kimster]

I'm no plumber, but just looking at the old galvanized and copper that I've torn out -- the ID, taking crud buildup into account is at most, 3/8. Some pipes were worse. So, seeing that Pex doesn't build up, I'd say 1/2 would be good for flow to anything, never mind 3/4. 3/8 for everything, on the other hand, might not be a good idea.

 

[Terry]

I've seen some PEX Uponor systems that used 3/8" for some fixtures. They talked to their local inspector first and go the okay beforehand.

 

[MadMadScientist]

I've Found The Answer!!!!

I've Found The Answer!!!!
So it turns out the Uponor has a online forum. I asked in that forum the same question that I asked here...and here is what the Uponor employee said....

dneumansky,
Due to the fact that 3/8" piping is minimally used in the plumbing industry & markets are moving away from 3/8" piping, our demand to manufacture 3/8" fittings & components at a competitve price is not feasible. Uponor does not promote plumbing with 3/8" because of increased head loss & velocity (noise) through piping. Uponor Logic plumbing w/ 1/2" fixture drops greatly reduces hot water delivery times, head loss & install time.
ThermalX
So, there you have it....they couldn't make enough money selling 3/8" stuff so they're pushing the 1/2" as a standard...not that 3/8" isn't up to the task....

super

Mads

 

[MadMadScientist]

In case anybody else wants to read it the Plastic Pipe Institutes PEX design guide can be found here.

http://plasticpipe.org/publications/pex_handbook.html

Seems useful though I wish they'd explain exactly how they got that table!

 

[Doherty Plumbing]

In case anybody else wants to read it the Plastic Pipe Institutes PEX design guide can be found here.

http://plasticpipe.org/publications/pex_handbook.html

Seems useful though I wish they'd explain exactly how they got that table!

Why in the hell would ANYONE rund 3/8th pex to supply water to a regularily used fixture? Honestly I can't fathom running 3/8th pex to anything other than an ice maker. But it's your house not mine.

 

[MadMadScientist]

I agree that it doesn't make instant sense that you can run 3/8" to anything but an icemaker.
But what really suprised me about the table is that they say that 3/8" is fine for a single head shower!!!

That table is for a home run system I should say....
And I too wish I knew how they calculated that table...its counter-intuitive as they say but they MUST of done the calcs themselves right?
Its not like they would just make up the numbers is it?

Your sig says that you're a journeyman plumber-I'm guessing with a ton of experience with copper water piping. Is it possible that PEX is so different than copper that it makes 3/8" a viable option when it wouldn't be in copper?

 

[Jadnashua]

Pex runs have less head or resistance to flow because (if done right) you only have a connection at the start, and another at the end. Copper, or any other assembled supply line, will have various elbows or other fittings along the way. So, unless you are trying to exceed the flow rate of the pipe, you'll get less pressure loss in pex than copper. One of the things you want to consider is the maximum flow rate. You don't want the water flowing faster than about 5-6' per second. So, take the volume in that ID and multiply it by 60-seconds to get the GPM. If your device's normal maximum flow rate is less than that, you probably won't notice a difference with a larger pipe. But, if your flow rate is trying to exceed that fps, you'll notice a drop in pressure, and you'll potentially erode the pipe, causing premature failure (likely take years). Because the volume contains a factor, increasing the ID is not linear with the volume it can contain.

 

[Doherty Plumbing]

Your sig says that you're a journeyman plumber-I'm guessing with a ton of experience with copper water piping. Is it possible that PEX is so different than copper that it makes 3/8" a viable option when it wouldn't be in copper?

Honestly if it were up to me I wouldn't ever use pex at all in a house an only copper. However pex is 900000x cheaper to buy and install so it wins.

The ID of pex isn't THAT much smaller than copper when we're talking about 1/2" so I would say in terms of water flow 1/2" pex will keep up with 1/2" copper. That is to say if you ran a line 20' to a faucet in both pex and copper you wouldn't beable to visually tell which faucet was hooked up with copper and which was hooked up with pex. And this is because you are still limited by the internal water ways inside the fixture itself. If you look at the inside ports on a standard moen shower valve you'll notice that the port feeding water to the shower head is VERY small compared to 1/2" copper or pex. The tub side port IS bigger but still a fair amount smaller then a 1/2" line. And also don't forget you're feeding both hot AND cold supplies to the fixture and then still forcing all that water through those tiny ports.

So would 3/8th work? Yeah it probably would because the combined surface area of the supply lines would be greater then the internal ports on the fixture you're serving. But with high flow you'll get very high velocities and low pressure. Like Jadnashua said in his post.

With water piping you always want a good pressure and slow flow rates. The slower the water flows around the system the longer that system will last.

I don't usually install home run systems because quite frankly I don't like them. However if I did a true home run to every fixture I would certainly run a min of 1/2" pex and would never consider ever using 3/8th pex. 3/8th is VERY small even compared to 1/2" pex.

 

[MadMadScientist]

3239.55 in reply to 3239.20

Here's the reply I got back from the PPI PE...

Hi D

You are welcome – Pls see the discussions below and hope that is helpful for you-


.. in addition to the ice maker, dishwashers are most often supplied with nominal 3/8” tubing. The IPC (see Table 604.5 in the 2009) permits smaller tubing size as part of a manifold system for specific fixtures and includes length an minimum pressure parameters. The Table on page 75 is based on the IPC – Also, per engineered designs, the pressure loss and velocity calculations can easily be performed and shown to fit within the manufacturer’s recommendations as well as limits in some local codes, based on actual fixture flow rates and available pressure.


Also, kitchen faucets, lavatory faucets and showers are all flow limited devices. Showers are the most at 2.5 gpm. Faucets are 1.5 gpm. Refer to Tables 3.2 and 3.3 which give flow velocities and pressure losses. For a very compact plumbing layout with 10-20 ft. of pipe between the manifold and fixture, even at 2.5 gpm, the pressure drop in 3/8” tube is only 8 psi. Since most water systems deliver water at 40-80 psi there is ample pressure available to deliver the water.


So I guess the answer is, yes 3/8" will work but pay a PE to prove it? I would love to learn how to do those calcs just to figure it out for myself...anybody know where I would go to learn something like that?



Mads

 

[Jadnashua]

8psi is a fair amount! And, since you'll want at least 1/2" to things like the tub and hose bibs, I'd just do the whole thing in 1/2" rather than buying two sizes. But, that's me. To get the volume and check things out, you need the ID of the pex (3/8" pex is NOT 3/8", it is smaller). You'll find that 1/2" pex is closer to 3/8" copper, and 3/8" pex is closer to 1/4" copper. You'll have more of a selection of shutoff valves, too.

 

[Nukeman]

There are a couple ways to calculate it on your own. The easiest way is to use a table such as this one to calculate the pressure drop (psi/ft) for a given flow.

http://www.alliedpex.com/LITERATURES/PEX_Pressure_Drop_Table.pdf

At 2.5 gpm, assuming 20 ft line length, that is about 7.7 psi. However, you will note there is a * next to this value for 3/8" PEX. This means that the velocity is 8 ft/s (max velocity under some plumbing codes). Will it still work? Probably.

To get the pressure at the fixture, you take the street pressure, subtact this pressure drop, then subtract pressure drop due to elevation change (figure 0.5 psi/ft as a rough check), then pressure drop due to fittings, etc.

Now some will mention that the great thing about PEX is you can bend and not need many fittings. However, this only helps somewhat in terms of pressure drop. Whenever you change direction, there is a pressure drop no matter if you use fittings or not to do it. Velocity has a magnitude (ft/s) and direction. Even if you don't slow down in the bend, you are still changing direction. This needs an acceleration, which requires a force. This comes from the friction. Tighter turns will have a greater loss since you are changing the direction more quickly.

How well the 3/8" will work depends on your house. If it was a smaller house with the manifold centrally located, then it would be no problem. If you have longer distances (Ranch style house, 2 or 3 storey, etc.), then it might not work so well. Also, think about the future. If the tubing is marginal to support what you have, what if you want to change something in the future? Maybe add a couple body sprays in the shower or something like that? If you started with 1/2" or larger, you could make these changes. With the 3/8", you would have to re-run the line.

In terms of cost, the 3/8" might not really cost much less either. With the 1/2" being more common, there is more competition and more choices. More likely to get a good deal under those conditions both for the tubing and the fittings. Also, since some stuff should be in 1/2" either way, you'll pay for per ft for the 1/2" if you did the rest in 3/8" (since you would be buying smaller rolls). Run the numbers and see what the difference is. It probably isn't that much compared to the total cost of the project.

By the way, look at the table that I posted and compare the 3/8" to 1/2" at 2.5 gpm:

3/8" = 0.385 psi/ft
1/2" = 0.087 psi/ft

Going a tiny bit larger decreases the pressure drop by more than 4.4x. If you are really set on running the 3/8", give me the details on the layout, street pressure, how high the highest fixture is, etc. I'l run the numbers and tell you if it'll work.

 

[Bob Ellenberg]

I am a builder/designer and switched to home run PEX distribution systems. I use the Manibloc which was originally by Vanguad but has been bought up by Viega. The 4th house I put it in was my personal house which has 3.5 baths. We used 3/8" lines to sinks and toilets and showers and 1/2" to tubs and washers (2 laundry rooms). We never had pressure/volume problems and found the time it took to get hot water was short everywhere but our kitchen which was a long way from the water heater (delivery time is less with 3/8" than with 1/2"). No fittings or joints in the walls or under the house--I will never go back to copper or jointed systems and will continue to use the manifolds that have a combination of 3/8 and 1/2 fittings.

 

[jbsteele]

3/8" real world test

I know this is an old thread but I'd like to add a real world test. We have a lav that always took forever to get warm. It's only about 10 lineal feet from water heater but actual pipe distance because of peculiar construction, old chimney, etc. is about 15 feet. I considered an instant hot water at the sink but I decided to try 3/8".

The home is plumbed on trunks, although I've added a few home runs to a mini-manifold to address hot water waits. We also have very low water pressure here in Portland, OR as the system is gravity fed from a municipal tank some number of feet above us. Pressure is between 45-55 psi depending on local use and level in the tank, according to city water rep after I complained about low water pressure.

Switching to a 1" PEX trunk under the house, and an upsized high flow water meter has helped tremendously. Hot water still takes forever however.

So I spliced in a 3/8" home run right next to the hot water heater using a 1" - 1/2" - 1" tee reduced to 3/8" . Ran the 15 feet and connected with another reducer to existing 1/2" pex at lav. (tee is five feet from water heater so 5' at 1" PEX - this "trunk" has to serve a separate apartment, or "ADU" ) This adds up to .23 gallons, most of which is the 1" line. Next trick is to tee right at the hot water heater outlet!

Result: I can't tell the difference in water pressure at lav. The fixture is newer so probably limits water - my rough test using a two liter flask and stopwatch shows 1.4 gal/minute on cold tap and 1.3 gal/min hot. Time to get warm, if Rinnai is recently used, 11 seconds with 3/8. Previous time was 20 seconds. If Rinnai is cold (unused overnight) then times jump to 25 seconds vs 45 seconds on 1/2"

So I'm thinking about the instant thing again - I hate to heat water with electricity, as it costs about double here, but on the other hand, from a cold start, I'm wasting 1/2 gallon of water. in rough numbers it costs a little less than a penny to heat 1/2 of water and then flush it down the drain (gas+water+sewer where sewer is metered indirectly by winter water use)

Probably 10 cold starts a day, that's $30 a year!!!!

 

[Jadnashua]

A hot water recirulation system would give you instant hot water, or very close. The pump is typically something like a 1/25HP thing, and then you'd have hot water at all of the fixtures, if done right. Put it on a timer, or a demand switch if you want to limit how often it runs. With a tankless, a demand is probably the best. Some tankless systems can't handle a recirculation system well, but an on demand control would make it look like an indivudual draw.

 

[haygood]

I understand why plumbers are unwilling to use the smaller piping. In a word, they are wrong.

I've lived for 13 years in a house plumbed with 3/8" PEX. It is a 2800+ sqft ranch, so the piping runs are as long as you'll probably ever see. It uses 1/2" for hose bibs and a tub, with the cold on the tub shared by a toilet. Everything else, including the shower and laundry, are 3/8". It has one of the Viega Manablocs that combine 3/8" and 1/2" outlets with compression fittings. Viega doesn't even make this setup any more, except for crimp fittings, which I would not prefer.

The home is a 3 bath. The entire system is run off of a 3/4" supply line that runs about 50' from the connection in the ground outside the house. The water heater takes water off the top of the manabloc and returns to the manabloc with a total of about 20' of 3/4".

What I'm saying with the above details is this system is running off of a slightly undersized supply line of considerable length. Additionally, all of the water, both hot and cold, run through the manabloc instead of T-ing off before the manabloc. The runs are fairly long. Despite all of those disadvantages, the 3/8" runs have no trouble delivering enough pressure to any fitting. Showers, laundry, and all run just fine. I haven't noticed any noise. The hot water gets delivered more quickly due to the smaller diameter. The system fits where it is partly due to the short bend radii the 3/8" lines allow.

In short, there is no reason at all to use 1/2" lines except for availability. The availability is only suffering because you guys won't use the right stuff.

I am doing a retrofit myself now on a rent house (my home was done by pros right before I bought it). I am pretty annoyed I can't get the manabloc or other fittings I would prefer. For longer runs I am going to adapt the lines down to 3/8". The break-even cost is a run has to be 16' long to pay for the extra crimp rings and adapter. I hate having extra connections in the lines, but the smaller volume in the lines is worth it. However, the lack of fittings to go from 3/8" PEX to 1/2" NPSM faucets is pretty irritating. I will end up using PEX/sweat adapters for most other fittings, so I can use either size PEX for that.

I understand why the user above says he would only use copper, but the reason I have PEX is because the copper corroded underground. That has happened in my house, and both of my rent houses. All run appropriate pressure in slab-on-grade construction. If all you are doing in your area is plumbing crawlspaces and basements, then there isn't much difference, except for build-up problems over time and lack of insulating value in the copper vs. PEX, and the losses at elbows. Oh, nevermind, there are too many disadvantages to copper, too, I guess. In my case, since it is a retrofit, the PEX could be run through the attic. That, and it is far easier. Just to be clear, I don't mind sweating fittings at all.

The compression fittings at the manabloc are nice because they can be redone without damaging the tubing. I have had to reroute a few to make room for changes in the attic, and it takes hardly any time and no supplies to do so. I do have a crimp-ring cutter, but not having to bother with it is even better.

The only disadvantage to PEX was that it left a funny taste in the water for the first year. After that, everything was fine. I know, "who drinks faucet water?"