Plumbing - Pipe size question

Here's a general plumbing question for the group:
I have a home built circa 1920 and the water main was recently upgraded to a 3/4" copper pipe. Yet all the copper inside the house is 1/2". I am going to be redoing my kitchen and bathrooms, and am considering replacing all the 1/2" pipe with 3/4". Is this upgrade worth doing? The challenge is that the water pressure coming in off the street is fairly low, and I am concerned that adding a larger pipe will only decrease the water pressure that much more.
Thoughts?
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No benefit unless you upgrade the main trunk in the feed, but after that, 1/2" is plenty as it is reduced further at the fixtures. The downside is that for hot water, you have to drain out more water sitting in the line to get the hot to flow at the fixture.
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Feed your hot water heater with 3/4" pipe and make your main cold runs all 3/4" with 1/2" branches to the fixtures. If your showers do not have temp-pressure compensation you may want to make sure that all the toilets have the 3/4 main trunk come close to the fixtures with unshared 1/2 branches dedicated to each toilet and the same for the shower valve.

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The best thing to do is have a professional size it for you. You are also getting pressure a volume mixed up. You can't change the pressure with a larger pipe or smaller pipe. But you will get more volume out of a larger pipe. Also if you increase the hot water lines ( 1/2 " to 3/4 " ) all you will accomplish is a longer wait for hot water unless you have a recalculating pump. Pipe sizes are calculated on fixture unites each facet toilet hose bib has a fixture unite assigned to it . But one thing the don't account for is low flow fixtures so if your piping is a little under sized it might not be worth changing.
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On Tue, 22 May 2007 21:44:16 -0700, "Sacramento Dave"

A professional ???? How hard is it to use 3/4" as the main trunk and use 1/2" to each fixture. Or, if you want to run 3/4" to everything, do it....
BTW: I recommend running 3/4" to outdoor spigots.
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On 22 May 2007 11:54:20 -0700, " snipped-for-privacy@gmail.com"

It's sort of a rule of thumb for residential plbg. Use 3/4" for all pipes that go to more than one fixture. Use 1/2" for pipes that only furnish one fixture. In other words, 3/4" from the meter across the house, then 1/2" to the toilet, the bathtub, the bathroom sink, etc. Use the same for the hot pipes, and be sure the cold pipe to the tank is 3/4". If you have a riser that feeds both the tub and a sink, then use 3/4 on that.
Larger pipe will give MORE pressure.
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How does it do that? Is there a pressure booster inside of it?
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wrote:

OK. bad wording on my part.... Larger pipe wont increase the pressure beyond what is available from the source, but it will allow the maximum pressure from that source. If you ran a whole house from 1/4" copper tubing the pressure loss would be very noticable. The pressure when the sink is first turned on would be full pressure, but that would drop in a second or two, and if another sink was turned on at the same time, it would really be noticable. I know this for fact from using garden hoses. I got a couple of these cheap 1/2" hoses. If I hook up 2 hoses (100ft), I barely have enough pressure to wash the car. But I also have a few 3/4" hoses and have good pressure. Wanting the max pressure and volume of water from my hose, I not only bought these 3/4" hoses, but I replaced the 1/2" pipe that was feeding my outside spigot with 3/4". I now have satisfactory pressure at the end of the hoses.
Pressure wont increase beyond the source pressure, but smaller pipes and hoses restrict the pressure allowable.
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You can put as big a pipe as you want on it and the pressure will not change. Big pipes don't cause more pressure.
--
Steve Barker





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Your right I'm wrong But I'm just a plumber Pipe Fitter
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I think some of us might be saying the same thing but just talking passed each other.
The city water pressure & flow available at your water meter is what you have to work with.
Installing a larger pipe from curb to house (like removing a 1/2 & putting in a 3/4 OR removing a 3/4 & putting in a 1") won't give more pressure at the curb BUT you'll have less pressure drop (at flow) with the larger pipe.
So when some of the guys in the group say that "larger pipe gives more pressure" they're kinda correct in that the larger pipe has lower pressure drop SO at the far end of the larger pipe the water is delivered at a higher pressure than a smaller pipe would deliver.
No way you're gonna get HIGHER pressure than at the meter (unless you got a booster pump in there somewhere) BUT you will get less pressure drop with larger pipe.
Bigger pipes give less pressure drop so if you're working with low city pressure bigger pipes will preserve more of your limited pressure.
There is one very annoying drawback to larger pipe.......large hot water lines (with no recirculating pump) make the wait for hot water longer. A 3/4" hot water line will more than double the wait time for hot water
If you've got a 10 ft run from water heater to fixture not a problem if it's 40 ft maybe the wait is too long & you'll want a smaller line or a pump.
When Dave suggests a pro size your system...it's not a bad idea IF you're want optimum performance & lower cost.
If you're handy with a calculator & can understand pressure drop tables...you can do it yourself.
I have the pressure drop tables for PEX here's a link to copper tube pressure drops
http://www.copper.org/applications/plumbing/techref/cth/tables/cth_table6.htm
It all depends on the water demand, tube size, run lengths & starting city water pressure.
If you've got a small house or a 2 story with stacked bathrooms & short run to the kitchen / laundry 1/2 copper is proabably fine. If you've got a spread out ranch maybe it's a little small.
I just re-did my 2 story (with stacked bathrooms & medium run to the kitchen / laundry). I ran 1" copper from curb to "water heater / PEX manifold" split to 3/4" there. PEX manifolds feed every fixture with 1/2" PEX BUT I've got two showers on old 1/2" copper that I chose not to replace since it was perfectly good. I also cheated & put the upstairs toilet on the shower cold water line to make the re- pipe easier. The toilet can be flushed when the shower is running with no noticeable effect
According to the copper & PEX pressure drop tables (combined calculation) ..........
I'm losing about 2 or 3 psi (max) from manifold to shower head (both showers running) ....with about a 1 or 2 psi change when the second shower goes on or off. My static water pressure is regulated to 65psi
Long winded post but for the OP.........if your runs are short (less than 50') without doing the calcs I'd guess that your system is a little undersized if ALL of the house is done in 1/2"
The suggestion of 3/4" feed to the water heater is a good one but replacing ALL of your 1/2" with 3/4" is a lot of work & cost for ???? gain.
cheers Bob
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think some of us might be saying the same thing but just talking passed each other.
The city water pressure & flow available at your water meter is what you have to work with.
Installing a larger pipe from curb to house (like removing a 1/2 & putting in a 3/4 OR removing a 3/4 & putting in a 1") won't give more pressure at the curb BUT you'll have less pressure drop (at flow) with the larger pipe.
So when some of the guys in the group say that "larger pipe gives more pressure" they're kinda correct in that the larger pipe has lower pressure drop SO at the far end of the larger pipe the water is delivered at a higher pressure than a smaller pipe would deliver.
No way you're gonna get HIGHER pressure than at the meter (unless you got a booster pump in there somewhere) BUT you will get less pressure drop with larger pipe.
Bigger pipes give less pressure drop so if you're working with low city pressure bigger pipes will preserve more of your limited pressure.
There is one very annoying drawback to larger pipe.......large hot water lines (with no recirculating pump) make the wait for hot water longer. A 3/4" hot water line will more than double the wait time for hot water
If you've got a 10 ft run from water heater to fixture not a problem if it's 40 ft maybe the wait is too long & you'll want a smaller line or a pump.
When Dave suggests a pro size your system...it's not a bad idea IF you're want optimum performance & lower cost.
If you're handy with a calculator & can understand pressure drop tables...you can do it yourself.
I have the pressure drop tables for PEX here's a link to copper tube pressure drops
http://www.copper.org/applications/plumbing/techref/cth/tables/cth_table6.htm
It all depends on the water demand, tube size, run lengths & starting city water pressure.
If you've got a small house or a 2 story with stacked bathrooms & short run to the kitchen / laundry 1/2 copper is proabably fine. If you've got a spread out ranch maybe it's a little small.
I just re-did my 2 story (with stacked bathrooms & medium run to the kitchen / laundry). I ran 1" copper from curb to "water heater / PEX manifold" split to 3/4" there. PEX manifolds feed every fixture with 1/2" PEX BUT I've got two showers on old 1/2" copper that I chose not to replace since it was perfectly good. I also cheated & put the upstairs toilet on the shower cold water line to make the re- pipe easier. The toilet can be flushed when the shower is running with no noticeable effect
According to the copper & PEX pressure drop tables (combined calculation) ..........
I'm losing about 2 or 3 psi (max) from manifold to shower head (both showers running) ....with about a 1 or 2 psi change when the second shower goes on or off. My static water pressure is regulated to 65psi
Long winded post but for the OP.........if your runs are short (less than 50') without doing the calcs I'd guess that your system is a little undersized if ALL of the house is done in 1/2"
The suggestion of 3/4" feed to the water heater is a good one but replacing ALL of your 1/2" with 3/4" is a lot of work & cost for ???? gain.
cheers Bob
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The smaller pipe reduces the effective pressure due to friction loss. Larger pipe will give you far better performance.
Yes, use the larger pipe.
--

Christopher A. Young
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