Home Run System with Manifolds

[RockinOnWater]

I'm building a new house and am planning on doing a Pex "Home Run" system with individual water lines serving each fixture in the house. It's a small house with a centrally located water heater, short runs, only a few sweeping turns and very efficient low flow fixtures. It seems like an ideal situation for 3/8" pex branches allowing for quick delivery of hot water with minimal wastage.

I've already calculated the friction losses in each 3/8" pipe run based on the flow rates, length of piping, number of turns, resistance in fittings and head losses and they're quite low. I have no worries about pressure and flow using this size pipe.

I'm trying to figure out exactly how I'm going to terminate the 3/8" pex at each fixture since I'm very limited in my selection for 3/8" parts. After looking through the parts catalogue is see two options (Each option uses a sink as an example):

Option 1 flow path:

• Start at 3/8" crimp fitting branch in the manifold
• Continuous pex run without fittings all the way to stud cavity behind sink
• 3/8" crimp fitting to 1/2" copper stubout
• 1/2" sweat stop valve to 3/8" male thread
• braided flexible supply line to threaded connection on faucet

Option 1 means I can go with a simpler manifold without valves on each individual branch. There's also a rigid copper pipe making the transition through the drywall which I prefer.

I don't like the copper to pex transition inside the wall, especially since a few of these connections will be in an exterior wall (on the warm side of 12" of insulation, however).

Option 2 flow path:

• Start at 3/8" crimp fitting on branch valve on manifold
• Continuous run of 3/8" pex without fittings all the way to stud cavity behind sink
• poke the pex pipe through the drywall and up towards the faucet connection
• use a 3/8" crimp fitting to female thread adapter and install on male threaded faucet connections

Option 2 will have several disadvantages like requiring valves on each manifold branch since there won't be any shutoffs at the fixture end.
It will require fewer transitions and no buried connections but the pex will be exposed and vulnerable outside the drywall running all the way to the faucet.
The piece that adapts the pex to the faucet has female threads that are made of plastic.

What are your thoughts?

 

[frodo]

code requires a valve.

your options are 1 at the manifold or 1 at the fixture.

myself, i prefer 1 valve at the manifold for tubs, and outside hose bibs

and for sinks and toilets one under the fixture.

caution..... you do not want to use a sweat valve on the copper stub out.
the stub out can become very hot and melt the pex. use a compression valve

if you stub out pex. it will be floppy, look like hell

http://www.supplyhouse.com/Crimp-Style-Manifolds-1826000
http://www.supplyhouse.com/Matco-Norca-29-1019LF-5-8-OD-Compression-x-3-8-OD-Compression-Angle-Stop-Lead-Free

http://www.supplyhouse.com/pex/control/search/~SEARCH_STRING=copper%20stub%20outs


i would not use 3/8'' pex, i would use 1/2''
http://screencast.com/t/NVUpUnSrdPzZ

 

[RockinOnWater]

Hi Frodo,

Thanks for your reply.

My plan was to have one valve on each run; either at the valve or at the manifold. What I'm simply trying to avoid is the redundancy of having one of these fancy manifold systems with the individual branch valves when there's already a stop valve at each fixture.

Just my personal opinion, but one main shutoff valve in the entire house is all that's needed for the two times per decade you might have to use it.

I take it you don't have a problem with the pex to copper transition inside the wall cavity?

I can't agree with some people's automatic dismissal of 3/8" pex. The only reasons I've ever heard against 3/8" were anecdotal or "gut feelings". When you actually get out the charts, tables, pens, paper and calculators and actually crunch some real numbers - 3/8" pex is fine for a lot of modern applications.

If you're not concerned about water usage or willing to plan out the plumbing layout and do the calculations beforehand then sure, 1/2" pex is a great "one-size fits all" solution.

 

[frodo]

Hi Frodo,

Thanks for your reply.

My plan was to have one valve on each run; either at the valve or at the manifold. What I'm simply trying to avoid is the redundancy of having one of these fancy manifold systems with the individual branch valves when there's already a stop valve at each fixture.

Just my personal opinion, but one main shutoff valve in the entire house is all that's needed for the two times per decade you might have to use it.

I take it you don't have a problem with the pex to copper transition inside the wall cavity?

I can't agree with some people's automatic dismissal of 3/8" pex. The only reasons I've ever heard against 3/8" were anecdotal or "gut feelings". When you actually get out the charts, tables, pens, paper and calculators and actually crunch some real numbers - 3/8" pex is fine for a lot of modern applications.

If you're not concerned about water usage or willing to plan out the plumbing layout and do the calculations beforehand then sure, 1/2" pex is a great "one-size fits all" solution.

LOL.....
being a plumber, I like high volume of water, if water usage or conservation is a concern, then simply do not open the valve all the way.

If you are fine with 3/8'' and low volume ,then i am thrilled as a brown bear with a honey comb.

I gave you some links that show manifolds without the valves.
I would not buy the manblock for $400.00 when I could use a copper one for $40.00. cause I am cheap.
NO, i do not have a problem with crimp pex in the wall. as long as it is done correctly and is not shark bite or some other BS push on

 

[RockinOnWater]

If you like high flow that much I'd like to see what kind of fixtures and fittings you're using since every stop valve I've ever seen (even ones with a 1/2" inlet) had a hole maybe 1/4" in diameter; or restricted to an area less than half of the I.D. of a 3/8" pex pipe.

 

[RockinOnWater]

I had the night off today and while the family was in bed I decided to do some real world testing of fixtures running off of 3/8" water lines. If you don't want to read through the whole post the short conclusion is that it's not looking good for the myth that 3/8" pex significantly reduces pressure and flow over 1/2". 3/8" pex is fine to use when kept within it's limitations.

Since 3/8" pex tubing doesn't exist in this part of the country I had to settle for the next best thing which is polyethylene tubing with an inside diameter of 0.375". 3/8" pex has an ID of 0.360" and a cross sectional area that is 92% as large as this polyethylene tubing. Even though the PE tubing is not really pex, from now on I'm going to refer to it as 3/8" tubing.

I used an apparatus made from CPVC pipe to allow me to measure the static and working pressure in the water supply line to a sink and shower head. The apparatus also allows me to splice in 50 feet of tightly coiled 3/8" tubing.

Here is the main part of the testing apparatus. It's made from 1/2" CPVC with a bunch of female threaded adapters for connection to various fittings, including a pressure gauge.

I measured the working pressure and flow rate for two sinks and a shower head before and after 50' of coiled 3/8' tubing was spliced in.

This is the testing apparatus installed on the supply line for the sink. The female threaded fitting on the apparatus is installed directly onto the faucet while the braided supply line connects to the bottom. The push-in union is only there for ease of installation onto the faucet since the top piece can be threaded on separately and then the whole thing pushed together.

Here is the test setup with the 50' section of 3/8" tubing spliced in. The red handled ball valve is only there to serve as a coupling between the two coils since I didn't have one on hand I had to cobble it up.

The test condition was having all three valves (stop valve, coupling valve and faucet valve) open all the way. After an equilibrium period of 30 seconds, the working pressure was measured by looking at the gauge and the flow rate was measured by filling a container for 15 seconds and weighing the contents. The kitchen weigh scale was zeroed after the empty bucket was placed on it and then the bucket was weighed again after collecting the sample.

Because 1 kilogram of water equals 1 liter, the weight of the sample in kilograms was multiplied by 4 to get the flow rate in liters per minute. Three trials for each test were conducted with all individual samples agreeing within 15 milliliters.

The first test was for the upstairs bathroom sink you see in the picture but the results were thrown off by a piece of pipe debris that clogged the aerator in between the two tests. As a result the drop in pressure and flow after adding in the 3/8" tubing is artificially high.

Upstairs Bathroom Sink Without 3/8" tubing:
Static Pressure: 31 psi
Working Pressure: 23 psi
Flow Rate: 4.61 lpm = 1.22 gpm

Upstairs Bathroom Sink With 3/8" tubing spliced in:
Static Pressure: 31 psi
Working Pressure: 21 psi
Flow Rate: 3.51 lpm = 0.93 gpm

This represents a 24% drop in flow and 9% drop in pressure. But as I said when the test apparatus was removed and the sink was returned to normal I noticed that the flow rate was very low. I then took apart the aerator to find it clogged with a large plastic burr from the CPVC pipe. That would explain why the drop in pressure is so small in between tests... it allowed pressure to build behind the obstruction in the aerator.

Here is the test setup for the shower head. I couldn't measure static pressure because I didn't have a valve in between the gauge and the fixture this time. This picture shows the 3/8" tubing spliced in the water line. When no 3/8" tubing was used, the back of the test apparatus was connected directly to the braided supply line for the sink.

Upstairs Shower Without 3/8" Tubing:
Working Pressure: 23 psi
Flow Rate: 5.32 lpm = 1.41 gpm

Upstairs Shower With 3/8" Tubing Spliced in:
Working Pressure: 18 psi
Flow Rate: 4.95 lpm = 1.31 gpm

The results are a 7% drop in flow and 22% drop in working pressure after the 3/8" tubing is installed. What's interesting to note is that when the showerhead is installed on the wall it has a flow rate of 1.21 gpm meaning that the mixing valve in the wall provides more resistance to flow than the 50' coil of 3/8" tubing. I would also like to note that with the tubing coil installed it still felt like there was adequate pressure and flow for a good shower, although I didn't actually bathe with it.

So far it's not looking good for the myth that you shouldn't use 3/8" pex - but it gets worse. Let's take a look at another sink test.

Downstairs Bathroom Sink Without 3/8" Tubing:
Static Pressure: 35 psi
Working Pressure: 27 psi
Flow Rate: 5.01 lpm = 1.32 gpm

Downstairs Bathroom Sink With 3/8" Tubing Spliced in:
Static Pressure: 35 psi
Working Pressure: 22 psi
Flow Rate: 4.45 lpm = 1.18 gpm

This test shows an 11% decrease in flow and a 22% decrease in working pressure with the 3/8" tubing installed. Like with the shower head, the qualitative difference before and after is barely noticeable. Since the downstairs sink is about 9' below the upstairs one, we have a few extra psi on account of gravity.

So even though this test shows a significant but manageable drop in flow and pressure after adding the 3/8" tubing, these results are still artificially skewed in favor of the "don't use 3/8 pex" myth because of several reasons:

1. The total tubing run in the 3/8" tubing test setup is ridiculously high. A well designed plumbing system with a central manifold would keep all tubing runs under 20 feet with most under 10 feet.
2. The 3/8" tubing is wound in two tight coils. A properly installed system would have mostly straight runs with very few (and gentle) bends.
3. The 3/8" test setup contains a large number of fittings with abrupt 90 degree turns. A properly installed system would virtually eliminate these fittings.
4. In this test the entire flow is through one tube. In a real world setup where warm water is being used, the demand would be shared between two tubes, sharply decreasing the velocity, flow rate and pressure drop in each tube.

So there you have it. When I get more days off I will probably do an actual scientific side-by-side comparison of two fixtures plumbed in a more "real world" manner and put the final nail in this myth's coffin.

Fortunately, actual scientific data will usually trump anecdotes and gut feelings.

 

[frodo]

(1) What is reasonable?
(2) What does the code require?
Concerning what is reasonable: the national expert on this issue, Gary Klein, suggests that 3/8-inch tubing is only reasonable for a single fixture rated at 1.5 gallon per minute or less. Fixtures with greater flow rates need 1/2-inch or larger tubing.
To learn more about this issue, see Gary's article, Inefficient Hot Water Piping Layouts Waste Hot Water.
No matter what you think is reasonable, your desires may be trumped by your local plumbing code. Before proceeding with your plan, check with your local code official to learn what local regulations require.


depending on your area, most codes require a minimum of 1/2'' for tubs, washing machines and hose bibs
fixtures are assigned a number called a "fixture unit"
that is what is used to determine pipe size, not a bucket and gauge



btw. your ptrap under the sink is plumbed incorrectly, that is a "S" trap. it is illegal

 

[RockinOnWater]

I would consider reasonable (in this context) to be one of two things, either the system performs to the satisfaction of the user or the performance difference between using one system or the other is so small that it's immeasurable. I believe that in a 3/8' pex plumbing system that's installed in a proper manner would satisfy both conditions.

As evidenced by the S-trap that you pointed out, codes aren't enforced here, allowing whoever installed it to do what they want anyways.

So to hell with the codes. Codes only specify what is the worst house you're legally allowed to build. As you pointed out, codes use some arbitrary number called a "fixture unit" to assign pipe sizes rather than taking into account actual tangible data and case-specific specifications that while it's true was collected using a bucket and stopwatch makes it no less real.

The code book knows nothing about the house I'm building. It has no idea what my water usage habits are, where my fixtures are located, how I'm planning on routing the pipes, etc. but I do. The code book could tell me that waiting 30 seconds and wasting a gallon of water in order to take a shower is perfectly fine but dropping those figures by half is a violation. Even if the pressure and flow in both cases is virtually the same. Ridiculous!

And in case you didn't know, I'm well aware that higher flow fixtures are going to require larger pipe sizes. Like I said before, the plumbing system I designed takes into account all the specific details of the house, right down to the number of turns in the pipe and the head pressure difference between the two floors.

A house that's designed using "best practices" and case specific details rather than the "you can't build it any worse than this" code book will perform better than a code-built house, even if the pipe sizes used aren't overkill.

 

[frodo]

I would consider reasonable (in this context) to be one of two things, either the system performs to the satisfaction of the user or the performance difference between using one system or the other is so small that it's immeasurable. I believe that in a 3/8' pex plumbing system that's installed in a proper manner would satisfy both conditions.

As evidenced by the S-trap that you pointed out, codes aren't enforced here, allowing whoever installed it to do what they want anyways.

So to hell with the codes. your a total fool !


Codes only specify what is the worst house you're legally allowed to build. As you pointed out, codes use some arbitrary number called a "fixture unit" to assign pipe sizes rather than taking into account actual tangible data and case-specific specifications that while it's true was collected using a bucket and stopwatch makes it no less real.

The code book knows nothing about the house I'm building. It has no idea what my water usage habits are, where my fixtures are located, how I'm planning on routing the pipes, etc. but I do. The code book could tell me that waiting 30 seconds and wasting a gallon of water in order to take a shower is perfectly fine but dropping those figures by half is a violation. Even if the pressure and flow in both cases is virtually the same. Ridiculous!

And in case you didn't know, I'm well aware that higher flow fixtures are going to require larger pipe sizes. Like I said before, the plumbing system I designed takes into account all the specific details of the house, right down to the number of turns in the pipe and the head pressure difference between the two floors.

A house that's designed using "best practices" and case specific details rather than the "you can't build it any worse than this" code book will perform better than a code-built house, even if the pipe sizes used aren't overkill. as evidenced by the "S" trap,

good luck, good bye

 

[KULTULZ]

As evidenced by the S-trap that you pointed out, codes aren't enforced here, allowing whoever installed it to do what they want anyways.

!!! So to hell with the codes !!!.

I can't figure why some come here. Maybe to seek approval of their unproven schemes?

Why is there code, other than paying fees to the government?

 

[havasu]

Whoa, little fella. I haven't had my morning coffee yet, so I will choose my words carefully. Based on the 7-20% reduction when using the 3/8 tubing, as a laymen I see a huge reduction.

As far as posting a question to the experts here, then not liking what you hear, please understand that these folks are all professionals who give their countless hours to provide a service for nothing more that a thank you, and if you want to install goat bladders for your plumbing, then feel free. Just please don't come back here saying that the professionals said it was OK, because it is not.

 

[frodo]

He is fix'n to screw his house up and does not want to listen.

My opinion on his folly.....Gord head on ! do as you wish!!

another know it all, that knows nothing

 

[CT18]

Why do people come here asking questions. If they dont hear what they want they argue the response given to them by knowledgeable individuals. It is comical with the way some homeowners are nowadays, well i watched flip or flop and they did it.

 

[frodo]

it is the internet. they read crap that is put out buy te seller of a product...who "enhances" the truth of it.
to sell said product.
Then, they come on the forum armed with half truths,,Because "he said" "she said"
some are just plain hardeaded,,,believe what they read.


OF course we all know a plumber knows nothing, has no education, is out to screw you for every dime they can.



let me dispel that myth,
I spent 4 years in a classroom setting crunching the code and 5 years OJT while in school
i am required to attend a continuing education course every year and be tested on the new amendments to the code, to keep my license,,
am bonded for $500.000. undergo a background check every year

plumbers are professional's, are their rip off artists in our profession ? most certainly,
are their those who are dumber than a rock and should not be allowed to reproduce let alone touch plumbing YES

you have that element in every profession
are we thieves and just out for your money.
thieves no,
do i want your money, Hell yes, You pay me for my knowledge to do what I trained to do and you dont know how to do
if you attempt to do your own plumbing with out the correct knowledge. YOU WILL be calling me or one of my brethren to come fix it

usually we say a 2nd year apprentice "knows enough to be dangerous"
that is also what we say about internet/ youtube plumbers

 

[CT18]

That about sums it up right there.

 

[RockinOnWater]

I don't believe everything I read, quite the contrary actually since there's an overwhelming majority that say that 1/2" pex is the smallest viable pipe size for virtually any situation and here I am, the black sheep.

If for example, somebody told me they had a spring that could launch a cannonball 100 feet into the air I wouldn't necessarily believe them. But if I knew the Spring Constant for that spring, how far it was compressed, the mass of the ball and the gravitational constant for the planet we were on I would do a simple calculation to verify the claim. If the numbers worked then I would be more convinced. Better yet, if I had access to all that equipment and it wasn't a public safety risk, I'd set up an experiment and try it myself for real-world verification.

So what I do believe in is math. Like most of you, I also went to university for four years to learn how to do what I do and that involves using applied science to verify numbers. I do it for lots of things including the friction loss calculations for my planned pipe runs with 3/8" pex. So while I didn't mean to insult anybody or imply that anybody is stupid, I have to stand by my observation that there's a huge discrepancy between what you people are saying and what the math is telling me.

For example, the longest pipe run in the entire house is going to be just under 25ft and that's going to a sink in the laundry room. That's the total run, now "as the crow flies". The run has no fittings except for the connections on each end, has four turns with a radius of 6", has a head height of 9' from the manifold and a flow of 1.5 gpm.

Using one pipe to carry the entire load (all hot or all cold) the total pressure loss (friction and head) is 9.46psi for 3/8" and 5.12 psi for 1/2". At 40 psi that would be a 23% pressure drop for 3/8" and a 13% drop for 1/2".

If the load was balanced between the hot and cold (0.75 gpm in each pipe) the pressure drop would be 5.70 psi for 3/8" and 4.296 psi for 1/2". In a balanced flow application, most of the pressure drop comes from the head height, which of course is independent of pipe size.

While most of you are probably going to disagree, I don't see that amount of pressure drop as a deal breaker. Especially when this example is an extreme case. Most of my pipe runs are under 15 feet with a couple about 10 feet and one being 7 feet. I see the benefit of halving the wait time and wastage of hot water as more of a concern.

If anyone would like to see some sample calculations and the friction loss tables that came with the pipe specification sheets I'd be glad to oblige.

As some of you pointed out and I conquer, the tub filler and hose bibs will get 1/2" pipe. So will the toilets. The main line will be 3/4".

So if anybody was offended by what I did or said then you have my most sincere apologies. I think the record should show however that while my tone may have seemed harsh or hot-headed, it was not me who lowered myself to the level of personal insults and name calling.

And even though I'm happy with my calculations I've still taken your comments seriously and decided to test the pipe beforehand. I already set up a crude kind of "pilot experiment" of which I have posted the results. The might not seem flattering for the use of 3/8" pex but keep in mind that the nature of the test probably skewed the results unfairly in favor of using 1/2". I'm now planning on setting up an authentic scientific experiment to test out and compare 3/8" and 1/2" pex in a controlled manner, side by side. If the 3/8" pex performs well I no longer see any reason to not use it in my new house. If it doesn't perform well then I'll go with 1/2".

I might be a "stubborn homeowner" type but I don't think anybody can reasonably say that I'm not going to make an informed decision about what pipe size to use. And if I install a 3/8" pex system in the new house and it works out then I hope you all at least acknowledge that it indeed does have a niche in the residential construction industry, as small and limited as that may be.

 

[frodo]

take into consideration that a system is sized for everything to be running at one time. not the minimum



3/8'' pex gpm minimum flow rate .06 maximum flow rate 1.3
1/2'' pex gpm minimum flow rate 1.2 maximum flow rate 2.3

flow rate for water saving fixtures, 2.2 gpm
new eco friendly faucets flow rate is 1.5 gpm 32% savings
as you can see, the 3/8'' pex does not deliver the minimum gpm for a eco friendly fixture
why is that important ? try to wash dishes with a water stream like a 70 year old man peeing with a bad prostrate
is quite frankly a pita..the veggie sprayer will dribble and be useless

you need to look at the gpm required for a dishwasher and for a clothes washer

i think you will come to realise, the only thing in the house that can be 3/8 is your lav faucets, and ice maker

 

[havasu]

A few years ago, my folks purchased a new mobile home in an upscale area. The main water feed was a 1/2" pex line. For six years until their deaths, all I heard was their complaining about the flow at their kitchen sink, and the amount of time it took to refill the toilets. I told them that they were at the mercy of the mobile home park, and their 1/2" main water line. I sure don't want you to experience this same complaint. What you are proposing is going to be the same. (Coming from a guy who hated mathematics, but still scored an "A" in Calculus).

 

[Matt30]

A few years ago, my folks purchased a new mobile home in an upscale area. The main water feed was a 1/2" pex line. For six years until their deaths, all I heard was their complaining about the flow at their kitchen sink, and the amount of time it took to refill the toilets. I told them that they were at the mercy of the mobile home park, and their 1/2" main water line. I sure don't want you to experience this same complaint. What you are proposing is going to be the same. (Coming from a guy who hated mathematics, but still scored an "A" in Calculus).

There's a few parks around here that started in the 70's. Not really much in the code applied to trailers back then. I've been in a few where the water main was 3/8" copper tubing, and 3/8" copper tubing served every fixture.

I could piss a more steady stream than what came out of the taps.

 

[RockinOnWater]

Maybe we're all just used to low flow and low pressure around here. In my apartment the supply pressure from the main line in the basement is 40psi and that's reduced to 32 psi up on the second floor just due to gravity.

When I did those tests with the sinks and the showerhead hooked up to the 3/8" tubing I thought the pressure and flow felt fine.

My target flow rate for faucets is 1.5 gpm or less and according to specification sheets for the ones I looked at you'd need to reduce the supply pressure a good bit below 40 psi to achieve that anyways, even with the WaterSense ones.

Maybe it's because our low flow and pressure is all we've ever known around here so we're happy with it...