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pex tubing

Posted by hank3443 (My Page) on
Thu, Jun 9, 11 at 21:47

I plan on using 3/4" pex tubing to plumb a new shower with (6) body jets, The diverter has 3/4 ports and the valve body has 1/2" ports as well as the jets. I'm
a little concerned with the fitting connecting the pex to the copper, it seems it could be a little restrictive. I asking this to see if anyone has used this method and with what kind of outcome. THANKS HANK......

Follow-Up Postings:

RE: pex tubing

The PEX connectors have exactly the same rate of flow as the PEX tubing

RE: pex tubing

How is that lazy pup?

The brass connectors along with many plastic can be as much as 1/4" to 3/8" smaller than the ID of the pex tubing, with the exception of perhaps a few such as Uponor that expands the tubing in a swedging type method over larger and more true size insert fittings???

RE: pex tubing

Contrary to what my esteemed colleague zl700 might want us to believe, per ASTM standards the volume of flow through a fitting must be equal to the volume of flow through the same respective size of line.

In the case of iron pipe, brass pipe or copper pipe the fitting attaches to the outside of the pipe wall and the diameter of the bore through the fitting remains constant but in the case of PEX, PE or PB pipe where the fitting is inserted into the interior of the pipe, by necessity the internal bore of the fitting will be less than the bore of the pipe.

In operation the volume of flow remains constant, but when the internal diameter of the fitting is reduced the "velocity" of flow through the fitting is greater than the velocity through the pipe. While the volume of flow remains constant, as a result of the increased velocity in the fitting the Friction Head Loss is greater in the fitting than it is in the pipe. For that reason the "fitting insertion loss" is expressed as an equivalent length of straight pipe.

By example, the "fitting insertion loss" for a 1/2 PEX 90 degree elbow is 9.4ft.

That means that if you had a 10ft section of 1/2" PEX with a 90degree elbow on the end the TDL (total developed length) of that pipe would be 19.4ft and the associated pressure drop in that pipe would be equal to a straight run of pipe 19.4ft long.

Don't believe it? Then go to the Plastic Pipe & Fitting Associations website and download your own set of pipe installation manuals complete with the fitting insertion loss tables and friction head tables.

its a free download at

RE: pex tubing is planned parenthood. is the association.

RE: pex tubing


Your post is truthful, yet misleading. While fitting pressure drop (friction) can be calculated into equal pipe length, this isnt the best answer, so it depends.

Although some systems rely on crushing the PEX against the barbs via compression rings, others stretch the PEX to receive brass barbed fittings that have the same I.D. as the tubing itself, thereby avoiding additional flow restriction. The reduced barb fittings no doubt cause a pressure drop resulting in reduced, pressure, thus flow or GPM also.

Do the calcs on pressure drop per fitting and there is no argument to be made there. Obviously you cant argue that 1/2" PEX continuous will have a higher pressure, longer discharge flow and higher flow rate than 10 - 10' PEX sticks with 9 restrictive insert couplings????

I think that is where the OP wss going with this

RE: pex tubing

"I think that is where the OP wss going with this"

I think the OP was wondering if the connection was going to be an issue. It won't. Yes, there is a measurable drop, but the drop should be on the same order as running a few extra feet of pipe- ie not much. If he was running through a maze of couplings, it might be different story, but that doesn't sound like the plan here.

RE: pex tubing

At the same pressure a smaller diameter WILL deliver a lower volume.
If you have pressure to waste then velocity may be able to make up for the smaller diameter, but you WILL reduce flow.

RE: pex tubing

The only thing I find missleading is how ppl who profess to be trained plumbers fail to understand a basic concept that is required knowledge of a first year apprentice.

For the record, The volume of flow is determined by the size of the orifice at then end use fixture on the load end.

If the size of the line is decreased we still have the same volume of flow, but the velocity of flow in the line increases and the pressure drop to friction head loss increases dramatically....

Don't believe it, check a "Friction Head Loss Table" for yourself..........

As an example, let us assume we are running a dedicated home run from a manifold to a shower with PEX.

Although most shower heads are limited to 2.5GPM for the purpose of design code requires we size the line for 3GPM.

Now let us examine the Friction Head Loss Table for a flow rate of 3GPM. (The table assumes a source pressure of 40psi for all measurements)

If we use a 3/4" line the pressure loss is 2.3psi/100ft and the velocity of flow is 2.72ft/sec

If we use a 1/2" line we still get the full 3GPM but the friction head pressure loss is now 12.2psi/100ft and the velocity of flow is 5.43fps

If we were to decrease the line to 3/8" we would still get the 3GPM volume but the friction head loss would be 53.9psi per 100ft of pipe and the velocity of flow is now 10fps.

To get a finite answer we begin by computing the physical length of the pipe, then add in the equivalent length of pipe for each fitting as listed on the "Fitting Insertion Loss Table" to determine the TDL (total developed length of pipe) and we use the TDL on the friction head loss table to select the correct size of pipe.

It must also be noted that when selecting pipe from the Friction head loss table we must pay close attention to the velocity of flow because code allows a maximum flow rate of 12fps in plastic pipes and 8fps in metal pipes to minimize pipe wall erosion.

Once you have computed the friction head loss for the line you must also compute all vertical static head loss or gain to determine the resultant pressure at the fixture.

Code further gives us a table of the minimum flow rate at PSI for each fixture, by example under the IRC Table 293.1 a shower must have a volume of 3GPM and a dynamic flow pressure of 8psi.

RE: pex tubing

Again your dancing around with velocity, pressure drop figures not applicable to the discussion only contorts the discussion and answer. Your propensity to be right all the time is not working.

There are numerous pipe flow calculators available on line, I suggest you get one, or just concede, as your alone on this one.

Based on your arguments in essence what you are saying all water mains could be 1/2", all services could be 1/2" all supply piping in the house could be 1/2" don't worry about velocity (not part of the discussion but a pipe sizing concern), pressure drop, etc. your flow will be the same regardless of pipe size.

There is no denying, to achieve similar flows of larger pipes, smaller pipes or pipes with fittings resulting in pressure drop requires higher inlet pressure to match flow rates within certain boundries.

How many 4 bath homes did you do in 1/2"?

RE: pex tubing

For the OP, the answer is that it doesn't matter, since he hasn't mentioned having a large number of bends.
All he wrote was "... it seems it could be a little restrictive..."
For a small number of bends, the answer he seeks is "NO".


I agree that lazypup has given too much information.

The answer is it depends.

So zl2000 is correct.

"While fitting pressure drop (friction) can be calculated into equal pipe length, this isnt the best answer, so it depends."

Also, the point has been made that lazypup never concedes anything.
This too is accurate and precise.
Lazypup is not the best answer-maker.

However, I'm sure I can speak for millions in saying that lazypup is a contributor, and is appreciated too.


RE: pex tubing

"For the record, The volume of flow is determined by the size of the orifice at then end use fixture on the load end. "

It would appear that the land of plumbing has repealed the basic fluid flow equations.

Fluid flow through an orifice or pipe is proportional to pressure and area.

If you have a fixed pressure, and decrease in area reduces flow. This is fluid Mechanics 101.

If you have more than some required minimum pressure to deliver the volume desired then you may be above to give up some to increase flow velocity.

To simplify the flow equations we have all sorts of tricks (like equating bends to an equivalent length of straight pipe friction loss).
As long as the applied pressure remains above some minimum value there is room to play, when the pressure is already low things get a lot more complicated and many of the simplifying assumptions fail.
There is no longer excess pressure to trade against friction losses, from pipe roughness, bends, or anything else disturbing flow uniformity.

In a typical PEX system the reduced diameter is not a significant driver, though if you put enough reduced bore fittings in series you will lose enough pressure under flow that system will no longer perform as desired.

RE: pex tubing

I realize that this thread is long dead, but I just ran across it and thought I might be allowed to add two cents and ask an additional question.

"Posted by lazypup ( on Tue, Jun 14, 11 at 17:51"
.... "Code further gives us a table of the minimum flow rate at PSI for each fixture, by example under the IRC Table 293.1 a shower must have a volume of 3GPM and a dynamic flow pressure of 8psi."

If I'm not mistaken, it's 12 psi.
The reason is, showers require a pressure balance or thermostatic valve which requires of 12 psi to meter properly.

My question concerns the use of PEX tubing. I've noticed that the internal diameter of differentr brands of PEX varies between different manufacturers. Unlike copper, one brand of PEX is considerable smaller than another. So how are we to use a "standard" table of equivalent pressure loss?"

RE: pex tubing

"So how are we to use a "standard" table of equivalent pressure loss?""

Each manufacturer will have their own table.

If the lower limit for operation is 12 PSI, and you have 35 PSI available flow velocity varies.

If you only have 15 PSI then things get a lot more sensitive to restrictions.

The plumbing design tables get away with a pile of simplifying assumptions that are valid for most cases.

Try and feed a house a thru a few hundred feet of line and watch the available flow decrease from just pressure drop with the line wide open.

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