Have a look at the drawing below describing a sample situation:

  • a water tank
  • a 1'' diameter pipe bringing the water down, inside the ground, arriving near the house
  • in the house, 2 x 0.5'' diameter pipes distribute the water from a connection T to various faucets

The missing part

I'd like to understand the differences between two possibilities: connecting the incoming pipe to the distribution T using a 1'' diameter pipe (possibility 1) and connecting the distribution the T using a 0.5'' diameter pipe (possibility 2). (As you see on the drawing, the water has to go against gravity about 2m to reach the T.)

My "gut feelings" tell me that the first solution is better to maintain pressure if faucets are used on both ends of the T at the same time whereas the second one is better to fight gravity.


closed as off-topic by tpg2114, Kyle Kanos, ACuriousMind, Qmechanic May 6 '15 at 19:29

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  • 4
    $\begingroup$ This appears to be an engineering question, not a physics one. Perhaps Engineering or Home Improvement might be better suited for this question? $\endgroup$ – Kyle Kanos May 6 '15 at 19:00
  • $\begingroup$ I edited the question and think it must fit the Physics site now... $\endgroup$ – John Doisneau May 6 '15 at 21:24

Azad provides a link to one of the many empirical models that can predict pressure drop, but if you are seriously planning and investing in plumbing a new home - take caution.

A general comment to begin is that tubing or pipe in general will create a resistance to the flow of fluid going through the pipe for any given pressure, so the bigger the pipe, the larger the capacity. The drawback is that as pipe diameter gets bigger, so does the cost of pipe and the fittings that go with it. If your pipe diameter is too small when fluid starts flowing there will be a larger pressure drop, therefore smaller pressure at the faucet.

Depending on your household capacity needs (will you be washing dishes the same time you are doing laundry and showering?) you may actually need something larger than 1" pipe.


I'd say option 1.

Doubling the diameter would reduce the pressure loss in that pipe by 32 times in a constant flow rate scenario. Overall pressure loss will also decrease but not that much.

Surface area increases 4 times then if the flow rate is constant the flow velocity will decrease by 4 times according to continuity equation.

Then we have this Darcy–Weisbach equation which is $\Delta p \propto {L \over D} \rho u^2 $

L is the length (which is const. here)

D is the diameter (which is doubled)

u is the flow velocity (which is a quarter of previous value)

$\rho$ is the density of fluid


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