Thread: PEX Piping - Need some Info

We are looking at a home in SC that has PEX installed. I am very familiar with copper piping and standards used in NJ, but as I have never seen PEX and I am puzzled. When I inspected a bathroom under construction I saw one 1/2" PEX pipe running up from the floor and feeding multiple fixtures. The installations that I am familiar with use a 3/4" backbone with 1/2" tapped off to each device. When I researched it on the internet, the recommended installation was a manifold with 1/2" taps. I would have preferred the contractor to install 3/4" PEX looped through each service area and then have individual taps for each device from that 3/4". When I questioned this with the builder, they send me the reply from the plumber:

"...where a bathroom group only had 3-4 fixtures (per Code compliance, one 1/2" waterline can feed up to 4 fixtures). In the event that the bathroom group in question had double lavatories and a garden tub and separate shower, then one 1/2"PEX line would provide water to three of the fixtures (tied in at the3/4" PEX line) and a second 1/2" PEX line would provide water to the other two fixtures in that group (which would also be tied into the 3/4"PEX line). One rule of thumb to keep in mind is that water travels5-times faster through a 1/2" PEX line than a 3/4" PEX line (which is most important in regards to faster hot water)."

Can anyone comment on this reply? I have issues with it, why pull two 1/2" when one 3/4" would suffice. And especially the speed of water in the PLEX.

Thanks,

Rich


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Rich,

Not sure how much help you are going to get from this site. I would expect that most of us do not really know much about sizing and water flow. I can think of nothing wrong with the setup that you describe, but as far as the 5x faster, I could not say. You might want to check the manufacturer's installation instructions. It would be necessary to find out the brand. Around here, Wirsbo AquaPex and Vanguard are the two most common, but it might be different in your area. I was unable to upload the installation instructions into this post because the file is too large. You can download a design guide from PPFA at:

PEX Products - Publications

or installation instructions at:

Pex Tubing by Viega: ViegaPEX, ViegaPEX Ultra, FostaPEX
Uponor - Plumbing Systems

Hope this helps.

Actually that reply sounds reasonable to me but I'm not a plumber nor do I have design experience with pex.

A link I have bookmarked though may answer several of your questions about velocity of water hot and cold ... number of fixtures that can be supplied from 3/4 manifolds to 1/2 inch runs to fixtures etc..

http://www.toolbase.org/PDF/DesignGu...esignguide.pdf

Originally Posted by Rich Goeken

We are looking at a home in SC that has PEX installed. I am very familiar with copper piping and standards used in NJ, but as I have never seen PEX and I am puzzled. When I inspected a bathroom under construction I saw one 1/2" PEX pipe running up from the floor and feeding multiple fixtures. The installations that I am familiar with use a 3/4" backbone with 1/2" tapped off to each device. When I researched it on the internet, the recommended installation was a manifold with 1/2" taps. I would have preferred the contractor to install 3/4" PEX looped through each service area and then have individual taps for each device from that 3/4". When I questioned this with the builder, they send me the reply from the plumber:

"...where a bathroom group only had 3-4 fixtures (per Code compliance, one 1/2" waterline can feed up to 4 fixtures). In the event that the bathroom group in question had double lavatories and a garden tub and separate shower, then one 1/2"PEX line would provide water to three of the fixtures (tied in at the3/4" PEX line) and a second 1/2" PEX line would provide water to the other two fixtures in that group (which would also be tied into the 3/4"PEX line). One rule of thumb to keep in mind is that water travels5-times faster through a 1/2" PEX line than a 3/4" PEX line (which is most important in regards to faster hot water)."

Can anyone comment on this reply? I have issues with it, why pull two 1/2" when one 3/4" would suffice. And especially the speed of water in the PLEX.

Thanks,

Rich

One reason that they like to use the 1/2" so much is that it cost less and it is easier to use. A 1/2" only needs a 5" radius to make a bend or corner and the 3/4" needs 7" .

You also need to see what the supply line to the home is. Keep in mind that a 1" line really around 5/8" inside diameter. You do not want a 3/4" supply line with PEX.

I have never heard the "5 X speed" thing with PEX, yes PEX offers less friction but 5 times and with a smaller pipe? When you reduce the pipe size you reduce the volume of water but at the same time the pressure is increased. An increase in pressure does not equate to an increase in volume.

I would ask the plumber to cite his source for the information he is providing if you do not agree with it.

I have never heard the "5 X speed" thing with PEX, yes PEX offers less friction but 5 times and with a smaller pipe?

Hi Scott,

I think the plumber just means that the cold water will purge out of the line faster when there is a smaller diameter pipe. I'm not sure there's 5x the volume of water standing in a 3/4" pipe, but don't want to look it up or calculate it.

You cannot magically increase pressure by downsizing the pipe. Smaller pipe does not mean an increase in pressure. What it does do is increase the velocity of the water but decreases flow volume. If you have 60 psi at the utility water meter that is your pressure period.You may lose some pressure between the meter and your fixtures but you will never increase it without adding a pump to boost pressure. As a general rule 1/2 pex allows water velocity that is twice that of 3/4 pex. I'm not sure what the flow differences are between the two.

Originally Posted by Brandon Whitmore

Hi Scott,

I think the plumber just means that the cold water will purge out of the line faster when there is a smaller diameter pipe. I'm not sure there's 5x the volume of water standing in a 3/4" pipe, but don't want to look it up or calculate it.

I agree. I'm not sure about calculating the fixture units but you will get much faster hot water at a distant tap using a correctly sized pipe than an over sized pipe. This is one good reason to have "home run" piping from the manifold rather than installing a larger pipe and reducing at each fixture.

Originally Posted by Roger Frazee

You cannot magically increase pressure by downsizing the pipe. Smaller pipe does not mean an increase in pressure. What it does do is increase the velocity of the water but decreases flow volume. If you have 60 psi at the utility water meter that is your pressure period.You may lose some pressure between the meter and your fixtures but you will never increase it without adding a pump to boost pressure. As a general rule 1/2 pex allows water velocity that is twice that of 3/4 pex. I'm not sure what the flow differences are between the two.

Good catch, I should have used velocity in place of pressure. Flow would be reduced with a smaller pipe.

From what I understand PEX can have no direct or indirect sunlight.
Check with manufacture install info.

Best

Ron

Great! Thanks for all the info., especially the installation manual that I was unable to find. I cannot find in the manual any references to the design used in the homes. All the designs appear to be reasonable and use a trunking method or home runs.

Are there any comments on the number of fixtures on a single 1/2" line? I have gone through the recommendations from the Installation manual and I do not see anywhere running a 1/2" pipe to a serving location and then tieing up to 4 fixtures with the same 1/2" pipe. If this is a code issue could someone provide the code reference? The more I get info on this, the more I am not happy with the design/installation.

Also, is it typical to see the PEX just hanging out of the wall? For example, at the water heater (gas) the PEX just hangs out of the sheetrock for about a foot, then has the required metal flexible tubing to the water heater. As that is out in the open, I expect to see that type of installation under the sinks---I will be checking this next week.

Rich Goeken

Originally Posted by Rich Goeken

Great! Thanks for all the info., especially the installation manual that I was unable to find. I cannot find in the manual any references to the design used in the homes. All the designs appear to be reasonable and use a trunking method or home runs.

Are there any comments on the number of fixtures on a single 1/2" line? I have gone through the recommendations from the Installation manual and I do not see anywhere running a 1/2" pipe to a serving location and then tieing up to 4 fixtures with the same 1/2" pipe. If this is a code issue could someone provide the code reference? The more I get info on this, the more I am not happy with the design/installation.

Also, is it typical to see the PEX just hanging out of the wall? For example, at the water heater (gas) the PEX just hangs out of the sheetrock for about a foot, then has the required metal flexible tubing to the water heater. As that is out in the open, I expect to see that type of installation under the sinks---I will be checking this next week.

Rich Goeken

My suggestion for you would be to hire a local inspector to inspect the home as it is being built (phase inspections). Yes, it will cost you some additional money but it will be better to pay it now than later when you have problems. You need a person who is familiar with the local codes and building practices. Most will do this on an hourly basis or a per inspection fee.

It sounds like this home your looking at is using trunk and branch but instead of tapping a 3/4' line with a 1/2" to each fixture. It appears that they are tapping a 3/4" trunk with one 1/2" branch and extending that single 1/2" to multiple fixtures .. as many as 4 fixtures. I wouldn't be comfortable with that design. And it would be worse it they are catching fixtures in multiple bathrooms with that same 1/2" branch. I have plumbed 2 homes with pex using the home run method with manifolds close to the hot water tank. These installations have worked flawlessly over the past 5 years or so.

As for the pex outside the wall where it connects to the hot water.....hopefully this is a 3/4 pex or 1 inch line... the idea with pex is to reduce fittings.. so they may have come out of the wall to connect to the flex tubing using a gradual bend instead of fittings to keep it inside the wall. I much prefer copper piped over to the wall cavity and the connection to pex made there. Other than a sloppy look don't see a problem with what they have done.

Thanks for the replies and suggestions from everyone. I think that I now have a good understanding of the use and application in a PEX installation. I will be going down to SC to look at the models again and inspect some of the homes under construction. I was thinking of getting a local inspector----will do so now.


Rich

Rich,
Which part of SC are you looking in?

We are looking at moving to Sun City, Indian Land, SC, that is near Ft. Mill, SC.

Rich

Originally Posted by Rich Goeken

We are looking at moving to Sun City, Indian Land, SC, that is near Ft. Mill, SC.

Rich

I know exactly where that is, unfortunately it is near Fort Mill and not in Fort Mill, which would be York county and I could get you in touch with an inspector. Sun City is in Lancaster county, I do some work in this county but do not know the inspectors personally.

I would happy to help you any way I can, I am not a home inspector, but I might be able to help you get in touch with the right people.

Hi all,
I have used PEX in my last two personal homes and in a half a dozen "spec" and custom homes. In my area we use a manafold system which is similar to an electrical breaker panel. There is a hot side and a cold side, sometimes with color coded piping. The typical run to a fixture is 1/2". Each fixture has a single home run with no concealed connections. Each W/C has a single supply. Each hot or cold faucet will have single home runs. The pipe itself is installed by pulling from manifold to fixture. The manifold has a seperate valve for each location in the home. The manifold maintains equal pressure throughout the home; a flushed W/C wil not affect the flow of a shower for instance. The single home runs do speed the hot water to the fixture simply due to the decreased volume of water in the pipe. PEX has been used extensively in under floor heating systems, either installed in "Warmboard", stapled below sibfloor, or poured in the concrete. It is very durable and much more resistant to bursting than copper when frozen. As to the 1/2" vs. 3/4", think of the allowable flow in todays plumbing fixtures; it is not a problem. I have seen other builders use concealed connecters and Y fittings, but think this defeats the best reasons to use PEX.
Rick Fifield
Close Look Inspections, inc.

I've never used manifold systems, we just keep branching as needed. Where I'm at (NC) we get away with three fixtures on a half-inch pipe, but we don't always design like that. For instance, we wouldn't put a shower on the same line as a hose bibb, or a washer box. Some older houses we see, especially if they were originally run in copper, will feed the entire house off a half-inch line. They don't seem to have any complaints about pressure, but they're typically not more than 1 or 2 bathroom.