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View Full Version : 3/4 supply.....1/2 section.....3/4 run....

nbahm
02-27-2007, 05:36 AM
Hello all!

Just a quickie. I thought I saw this answer before somewhere, but now I can't seem to find it again.

My question is one of fluid dynamics:

I have a 1/2" (copper) supply coming into my bathroom that changed from 3/4", inside the wall about 4". Impossible to reach without destroying adjacent bath. It was feeding just a shower in this room, but now I want to feed a shower and bath so I need it to be 3/4" for another 15 ft or so....

Can a 3/4 supply line have about a 1 foot section of 1/2" pipe in it without any noticeable drop in pressure or volume?:confused:

Plumber buddy says "No sweat".;) Just put reducer coupling at accessible point and go back to 3/4" for remaining run....."Absolutely NO drop in performance":D

Of course I'm here for a second, third, and fourth opinion because this will be feeding a multiple head shower and I'd hate to regret it later.....:(

Thanks a lot for your time!

Nate

Cass
02-27-2007, 05:41 AM
The simple answer is that no matter how hard you try you can't get the same volume through a 1/2" pipe as a 3/4" pipe if the pressure is equal.

master plumber mark
02-27-2007, 06:03 AM
It depends on the incomming pressure.....

if you have fairley high water pressure you probably
wont notice the reduction....

you will probably be all right and I would not destroy
the other bathroom over it ......

I lived in a house totally run on 1/2 copper
comming in with 3/4 and immediately reduced to 1/2

you never knew the difference....

nbahm
02-27-2007, 07:17 AM
I realize, technically, the volume will suffer......but how much? Will I notice?

Anyone ever had to do this for whatever reason? Results?

I'm thinking it will increase pressure and slightly decrease volume. Correct?

I'm not scared of formulas, if ya gottem'!

Thanks!

Nate

TedL
02-27-2007, 07:28 AM
Other major variables you haven't mentioned:

Static pressure
Well or muni water
Likelihood of simultaneous use of bath & shower

nbahm
02-27-2007, 07:50 AM
~50 psi

muni.

hope to, but don't need to....

jimbo
02-27-2007, 07:52 AM
Distance matters a lot. You are talking a short section of 1/2". I vote with others who say that if you have "normal" pressure available, the drop to the tub will be minimal.

prashster
02-27-2007, 07:52 AM
I can't remember my thermodynamics from college, but it seems like you're creating a nozzle.

I thought the pressure stays constant, but the velocity through the pinched section increases. Don't engines use this kind of arrangement to create thrust?

I have no clue. Just crapping here.

master plumber mark
02-27-2007, 07:53 AM
If what you said is true, you
would have to tear up another bathroom
just to even get to 3/4 pipe.......

just do it and see what happens...

then worry about gutting the other bathroom.

as far as flow charts and graphs goes ......

Las Vegas odds are probably 85% good in your favor...

so what more can you ask...

my 9 year old is now thorwing up and sitting on toilet....

a wonderful day had by all

nbahm
02-27-2007, 08:20 AM
Okay heres the setup:

3/4 supply......~1 ft. of 1/2".......~15 ft. of 3/4"........tee 1/2" to thermo. shower valve and 1/2" to tub filler.

This is just the hot, the cold supply comes further into the room so no issue there.

Oh, BTW, don't think it matters but where the supply reduces inside the wall is actually a tee that also feeds 1/2" to a double sink.

jfkopper
02-27-2007, 10:16 AM
Referring to
http://www.irrigationtutorials.com/copperloss.htm
and assuming worst case that
a) the 1/2" length is Type L copper (type M is thinner and thus larger in ID) and
b) the flow rate is 5 GPM (very hot shower at very high volume),
the pressure loss in 1 foot of 1/2" is less than 0.2 psi.
Since a typical 3/4" metered town water supply presents at least 50 psi at 25GPM to a single family home,
such a loss should not be noticeable.

Since 1 foot of 1/2" type L has a volume of .012 gallons,
http://www.copper.org/applications/plumbing/techref/cth/tables/cth_table2b.htm
the velocity of the water at 5GPM is
(5/.012) = 417 feet per minute or about 7 feet per second, well beyond the 5 feet per second recommended as a working limit and likely quite noisy. Of course at 2.5 GPM, the velocity is 3.5 feet per second, still perhaps noisy but with no sidewall erosion dangers.

Be safe,
/kop

nbahm
02-27-2007, 11:22 AM
So, it'll work relatively the same, but.....it'll be loud as all get out, huh?

If I put split foam insulation around the 1/2" part and pack the stud cavity with fiberglass will the velocity noise still be noticable?

master plumber mark
02-27-2007, 11:49 AM
I really like those charts about the copper

I cant figure them out, but they are very impressive.....

you are not going hear a difference ,

no way , no how.....

especially while you are showering....

nbahm
02-27-2007, 12:06 PM
Did I mention the shower will be running 7.5 GPM (three heads)? Any change in opinion?

geniescience
02-28-2007, 04:38 AM
good to know! Three.

So, you want a lot of water, and as little pipe noise as possible. Very normal. I've never met a person who wanted the opposite. :)

Friction losses are a linear sum of the distance. A short distance of narrower pipe is not a long distance. Everyone keeps posting the same info, worded differently. Your plumber friend is right, within reason. To go deeper, just FYI, it is irrelevant where the narrow piece is in the entire run; it is independent.

So, a short distance doesn't add a lot of resistance to your flow.

It is true that it is more like a nozzle -- and no-one can predict the noises it will produce when "stressed" to its maximum limit, even if one could be there and see all the shapes (bends in of the pipe). Any direct contact to studs or other structure will transmit noise through solid material and then into the air where you can then hear it. So, it you can slide a thin pad between the pipe and its strapping, that will help a lot.

Musical instruments are complex geometry. Even the temperature of your hot water will change the noise the pipe produces. Personally, I think that the shape of the 3/4" pipe right after the 1/2" segment is what will determine a good part of the possible noise. Are you aware of how water produces microscopic vapor bubbles when it is pressurized and flowing fast over a microscopically rough surface like the inside of a copper pipe? This produces a hiss noise or lisping whistle. This noise gets amplified (slightly) at the 3/4" aperture acting like a horn spout does. When these bubbles come out of the 1/2" pipe, they won't get absorbed back into the main stream of water right away, but will at first expand a bit since the pressure has dropped. These two sudden changes will make some form of music or noise too, which then travels inside the water to an elbow or bend and then comes out into the air there. The length of the straight pipe determines at least one of the harmonics in the noise. That's the hypothesis, which you can verify by building a batch of similar setups and measuring noise as you change one variable at a time. If you were so inclined.

If anyone has more to add about pipe noises, please feel free to contradict me; I am eager and willing to learn what ever you know.

Now, Mr. Baum, if you really want to know whether you can fan out to three nozzles running 2.5 GpM each, and have acceptable plumbing performance elsewhere too, here is a way to test before installing. Build a pipe run equivalent (in number of elbows, and in total length, and in 3/4" copper) to the one you intend to install in your walls, and run it into the tub. Time how long it takes to fill the tub to e.g. 60 gallons, while turning on all the water (tub and shower, flushing toilet too). Note if you do not install the shower nozzles in this simulation as you are looking to know the total maximum amount of water you can get, you will not have the ultimate end flow, since a shower nozzle is a restriction, a friction, which reduces flow too. But I wouldn't sweat it. You can repeat this filling experiment with and without all the other variables.

summary:
1. More length = more loss = less water.
2. More bends = more loss = less water.
3. Shape of bends is one factor in creating noise.
4. Padding to prevent direct physical contact is important.

Since noises can start to happen later, it is good to pad even if there is no noise.

hope this helps.

david

Cass
02-28-2007, 04:38 AM
What size line is feeding the house now and what material is it? If you have a PRV I would increase the pressure as high as you can get it. With 3 heads running at the same time you will need as many # pressure as you can get and the small piece of copper may affect it.

What size is your water heater. At 7.5 GPM your hot water won't last long...maybe 10 min. or less depending on time of year and the temp setting of the water heater.

nbahm
02-28-2007, 05:21 AM
I have two fifties in series for my hot water.

I think I will try to change this small section of pipe, just to be safe.

I will try to get to the tee in the wall without disturbing the adjacent bath. I will heat up the existing tee and pull it off the 3/4 supply. I can then either try to solder another tee (not very likely, but I'll try), or I bought a shark-bite tee that may work, but it's kind of bulky and this is a really tight space.

Any other options for a confined space?

TedL
02-28-2007, 06:03 AM
Step back and think if any "outside the box" approaches might work.

Can you get to the fitting from above/below/outside, rather than through the bath?

Could you fish a new pex or copper supply line up to the location?

Etc......

geniescience
02-28-2007, 06:40 AM
i have used Pex-Al-Pex (very malleable, and keeps its shape) to run a new line through a tight space with no access. You can use this to make a new run back to the HW heater or close to it.

Aqua (Kitec) Pex-Al-Pex pipe has a huge __HUGE!__ diameter, far greater than 3/4" pex and also greater than 3/4" copper. I could find its true dimensions somewhere on the company web site and post them here, if this needs to be known precisely.

Since the pipe is a three-layer composite, with aluminum as the middle layer, it keeps its shape when you straighten it or bend it. This is a good feature when you need to send it through a tight space.

Water flows quietly in 1.) a large I.D. pipe, that has 2.) smooth inside walls and 3.) gentle flowing curves. Since the pipe can be bent into curves, you don't have abrupt elbow changes of direction / plane.

A brand new run back to your HW heater is also good for your shower since it supplies water on a dedicated circuit.

david
p.s. if you remove the 1/2" or the Tee you are 99% committed. Hope this alternative gets to you before too late.

jfkopper
02-28-2007, 08:12 PM
Consider bypassing the 1/2" bootleneck with a run a new pipe at 3/4" OR even 1" diameter (@ \$3 a foot in Type L copper nowadays---PEX looks better and better). Pick the exit and reentry points into the existing line to be those easiest to work at. The water flow (7.5 GPM??) will favor the path presenting the lower total psi drop.

Be safe,
/kop

jfkopper
02-28-2007, 08:17 PM
A master plumber saying he's impressed---that makes my day.

/kop

Randyj
02-28-2007, 09:37 PM
If it's difficult or serious demolition to eliminate the 1/2" then as MPM suggested, plumb it the easy way... It will take 15 minutes longer to go back and replace the 1/2 with copper than if you did it in the original install... if it don't work like you want then stick the 12" piece of 3/4 in later.....

nbahm
03-06-2007, 08:26 AM
I finally bit the bullet this weekend. I heated up the old tee and took it out, no problem. Somehow I was able to get enough indirect heat on the new copper fitting (from the extremely small access point I had to work with) to melt some solder (I made sure I used about 6" of solder just to be safe, since I couldn't see the joint and I could barely fit the solder back there).

I did have to (blind) shoot the fire extinguisher a couple times into the wall cavity, though! :eek:

Presure test worked, didn't burn down the house:D , and now since it's all 3/4" I don't have to worry about any restrictions (even if it's not a dramatic change, it always would have been in the back of my mind that it could be BETTER!)

Thanks so much for all your suggestions!