Small diameter PEX to speed flow
stevenplane
| Posted in Energy, Heating & Insulation on
I recently heard a pex rep say that some plumbers use 3/8″ pex on hot water runs of 50′ or less to speed up the flow so users don’t wait on hot water so long.
He said the volume is reduced but not less than the fixture rating.
I understand bernoulli’s principle and know that on it’s face, his claim is correct.
What I’m curious about is 1.) Is there anyone here doing this? 2.) Is there anyone here who thinks it makes a noticable difference?
Seems to me we’d only be talking about a second of difference or less.
Replies
I see problems for very little difference for the end user.
First - that flow will be reduced as the length grows. The assumption would only be true at short run lengths, and at short length, there is already little wait.
Then there is a need for two kinds of PEX and fittings to complicate things.
And too many guys will be thinking, well, if 50' is OK, then sixty feet will do too.
Then seventy....
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try that with a garden hose..
same length hose, two different diameters and fill a 5 gallon bucket......
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Bernouli's principle? What does that have to do with anything? I think you're putting too much thought into this ....
It's only a question of volume. If the system is supplying 5gpm, then 5gpm is what's flowing in the pipe ... whatever the size. When it comes to hot water, all this means is that there are more gallons of cold water that needs to be flushed out of a larger pipe, before the hot water can arrive.
As for the 'short run' issue ... again, it's simply a matter of friction loss. That is, a long skinny pipe will lose a greater percentage of it's pressure than a larger pipe of the same length. Other factors, such as bends, also contribute to this loss.
On balance, the guy's likely correct; with reduced-flow shower heads, you don't need either the volume or pressure you once needed to get a satisfactory spray. Now, at the shower head ... THAT's where you can start looking a Bernouli, Venturi, and a whole slew of other Italians at work!
"If the system is supplying 5gpm, then 5gpm is what's flowing in the pipe ..."Circular reasoning flowing in your pipes there.
The pipe in question is a PART of the system so the system cannot deliver any more flow than the pipe that is most restrictive, just as a chain is only as strong as it's weakest link
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I used circular reasoning because the pipes are round.
Nah - cylindrical most places;)
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Cylindrical reasoning adds a whole 'nother dimension to circular reasoning...
Billy
Is this like the space time continuum thing?
Hi,
The pressure loss for 50 ft of 3/8ths at 2gpm is about 5 psi -- so, not so much -- as long as the house has descent water pressure.
It seems to me that if the 3/8ths lines are combined with the Home Run style plumbing, so you have a dedicated line to each fixture, that it should work fine, and save some water and some waiting time?
I guess if you had something like a bathtub where you would like a lot of flow rate that a bigger line would be needed.
http://www.toolbase.org/Technology-Inventory/Plumbing/plumbing-manifolds
http://www.housingzone.com/proremodeler/article/CA6412958.html
Gary
We changed out the supplies to a kitchen sink for a customer... maybe 30 feet of 1/2" pipe removed and 30' of 3/8" PEX installed. It does make a difference.
According to my calcs, the area of a 3/8" dia. circle is .110" and the area of a 1/2" dia. circle is .196", so there a little more than half as much water to flush out given the same pipe length. Folks here are usually on wells and septic and many are conscious of how much water is wasted.
edit: take a look at the angle stops you are using, chances are the bore is smaller than anything else in the system.
Edited 2/21/2009 6:17 pm by davidmeiland
Approx volume in 1 foot of 1/2" tubing is 40mls.
3/8" tubing has about 20mls.
So half the volume of cold water needs to be displaced by using the 3/8 pex.