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Will 3/4” pex with an inside diameter of .68 inches have enough capacity to supply a 3/4” manifold with 1/2” supply lines to a bathtub, washer and toilet and sink with no pressure drop? I plan on running 3/4” pex from water meter to the house. Once inside the house the 3/4” will branch off using a 3/4” Tee. The (1) 3/4 supply line will run to a hot water heater then as hot water leaves the heater from a 3/4 supply to a hot water manifold with dedicated 1/2 supply lines to each fixture. The second 3/4” from the tee joint will deliver cold water will also run to a manifold. Every fixture will have its own dedicated 1/2” supply. The outdoor faucets will have dedicated 1/2” supply lines as well.

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  • Do you have an outside faucet on this line? Is this envisioned 3/4" PEX line all the way from the water meter or would this be from a point under the house through the house. – Jim Stewart Feb 2 '19 at 23:12
  • yes I plan on running 3/4 pex from water meter to the house to 2 manifolds. I amended by post to be more clear. – Frank Underwood Feb 3 '19 at 0:24
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    no pressure drop .... there is always a pressure drop ..... if there was no pressure drop, then the water would not flow – jsotola Feb 3 '19 at 3:49
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    It’s based on “Fixture Units”. Design is based on an average number of fixtures being used at the same time. So, how many “Fixture Units” are in your house? Here’s a chart to help you count fixture units: cleanwaterstore.com/resource/calculators/… – Lee Sam Mar 5 '19 at 4:42
  • FYI, outdoor spigots are typically plumbed with 3/4" pipe. – isherwood Mar 5 '19 at 13:57
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Not a calculated method, but if that home is yours and you're planning to spend a good few years in it, I would put in the 1" main PEX to water heater and as you go to edge water outlets of the house, convert that 1" to 3/4". The cost of 1" vs 3/4" isn't too drastic and having the extra capacity, especially to the water heater will help with pressure issues. 3/4" post water heater is good, unless you have 8+ hot drops.

If that home is a builder home and used for investment or you're trying to minimize the cost, do the calculation on needed flow and capacity. Most of the time 3/4" is good for smaller homes.

Regarding manifolds, all builders that I've seen do not use main manifolds and just use tee's and split off the main pex. It appears, labor wise, its less work and less cost since the manifold adds a few hundred to the project and creates further runs but you lose the ability to shut off each drop.

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    You can use Tees and Valves and STILL be (lots of) money ahead .vs buying a manifold, with the ability to shut off each run. – Ecnerwal Nov 25 '20 at 17:46
  • Good call @Ecnerwal, propex Tees/valves and other components are very expensive in general. I remember paying upwards of $5 for a 1" x 1" x 3/4" tee. Less hardware the better and a good gameplan sketch will help with cost reduction. – Paul M Nov 25 '20 at 19:58
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It depends how many things you intend to use at the same time. Water pressure drops the more things you're using at once. I can run two hoses off a 1" line at once, but I lose about 75% of the water pressure.

If you're running only one thing at a time you should be just fine. If you need two it will be iffy, and I doubt you could do three.

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  • So if I use a sink and washing machine a i would see a drop? I’m trying to get solve my pressure drop problems. I have 40 yr old 3/4 galvanized from meter then once inside everything is on 1/2” water lines . When I use 2 fixtures (toilet and washer ) at once pressure will drop from 80psi down to 12psi – Frank Underwood Feb 3 '19 at 1:09
  • As I said, it would be iffy with two.draws. If you could bump it to 1" I'd say you could run two easily – Machavity Feb 3 '19 at 1:14
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    @FrankUnderwood - it sounds like your proposed setup is exactly the same as your existing setup, except in PEX instead of galvanized. If you're experiencing unacceptable pressure drop now, I don't see how replacing the lines with (effectively) the exact same thing will help. – FreeMan Mar 5 '19 at 18:51
  • 40 year old galvanized is almost certainly rough, corroded, and restricted on the inside, so 3/4 smooth PEX will be a considerable upgrade. The benefit (if any) of going to a larger feed from the meter is directly affected by what's on the other side of the meter, pipe-wise. – Ecnerwal Nov 25 '20 at 17:49
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Home run system is very good option. 3/4 supply is good for most houses. If you have low pressure with taps on 3/4 may be small. If you have big house with many lavatories and people 3/4 may be small. I think the easiest way to check is put a gauge at your meter. Check pressure with system closed. Open many taps. If pressure goes way down 25 to 50 percent bigger pipe will not help. You might need bigger meter before bigger pipe makes sense.

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You'll get water.

I've dealt with an older house where the main feed was dropped to 1/2" and they got water.

Mind you, I replaced the 1/2" with 1" when I got to working on the water system in that house, and water delivery improved. But it's more a matter of what you're willing to live with, and how well it works, not complete failure to work.

My rule of thumb (for a single family house of "normal" scale) is 1 inch to the house, 3/4 inch at the hot/cold split, 1/2 inch to fixtures. As with most rules of thumb it's not based on bothering to work out the flow rates and pressure drops explicitly, but it works well enough for me. In my case I have 1 inch from the well - if you only have 3/4" to the meter, 1 inch from the meter to the house probably won't make much difference .vs. 3/4.

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