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I am replacing all the plumbing in my house and to the meter. I will be using PEX. I have manifolds for hot and cold with 3/4" inlets and 1/2" outlets at the valves. Do I need to use 1" PEX tubing from the meter and split to (2) 3/4 to the manifolds? I just want to eliminate pressure drops in the house as much as possible.

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    1 inch will not do anything for you, unless it's coming from the street, and that's too much for a single family home. The cold manifold should have an opening at both of the far ends: one for the supply water and the other to supply the hot water manifold. – Edwin Apr 6 '13 at 6:00
  • There are several auxiliary support products such as whole house water filters and water softeners which use 1" fittings and if you plan to use them, it would be convenient to have a 1" pipe from the meter. – Philip Ngai Apr 6 '13 at 16:21
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    Even with a 3/4" meter, 1" pipe will reduce the pressure drop due to friction. How much is saved depends on the peak flow, it could be significant or inconsequential. The determining factor is not the manifold size, it is the actual simultaneous fixture demand. – bcworkz Apr 6 '13 at 20:41
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There are lots of factors, but the short answer is probably not.

Pressure loss is based upon distance and flow rate. 3/4" pex loses 20 psi per 100 ft at 10 GPM. That's quite a bit of loss, but typical city water pressure is 60-100 psi, and 10 GPM is a high flow rate for typical uses in a house (excluding lawn irrigation). How far is it from the meter to the manifold? From the manifold to fixtures? Are there more than 2 stories in the house? In typical scenarios you'll probably have less than 100 ft from meter to manifold, and maybe 10-40 ft from manifold to fixtures.

Most fixtures have flow limiters, like showers and sinks, or designs that limit flow, like ice makers, dish washers, and toilets. All of those will typically be limited to around 1-4 GPM. The only common appliances that will flow more than that are washing machines and hose bibbs. Even then, the 1/2" tubing running from the manifolds to the individual fixtures will limit flow. 100ft of 1/2" pex has a 40+ psi pressure drop at 6 GPM, limiting wide open flow to around 6-7 GPM. The 100 ft of 3/4" pipe feeding the manifold will only experience an 8 psi pressure drop with that 6 GPM wide open flow on one fixture.

If your manifold is next to your water heater, and there should be less than 10 ft of pipe between the cold manifold and the hot manifold, all 3/4", 8 psi of pressure drop on the cold side will lead to a similar 8-9 psi drop on the hot side, but you'll still have plenty of pressure on both hot and cold sides to feed a shower. So, start with 75 psi city supply, turn on a garden hose, and still have 67 psi cold and 66 psi hot at the manifolds.

Situations that would need larger line from the meter are very long runs, like 500 ft from meter to manifold, very large demand fixtures, like a sprinkler system that doesn't have its own line from the meter, or multiple branches from the main line where friction loss could add up. If you had a large house with 2 water heaters and separate manifolds far apart, a large demand at the first manifold combined with pressure losses getting to the second manifold could justify a larger main line.

There are also online calculators and estimators to size pipe based on fixture units or direct GPM flows if you want to really dive in or you have a unique scenario.

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