My house has a 1/2" underground copper pipe providing the water supply. I am not quite sure how long the run is back to the meter/utility supply, but it's likely ~75-100ft. I actually got the chance to measure the static pressure and the pressure at 2.5GPM flow (the showerhead is the only source of which I know the flow rate). The static pressure is 65psi, which drops to 56psi at 2.5GPM flow.
I will be redoing the plumbing soon, and I'm curious whether it's worth using 3/4" for the interior manifold plumbing. Similarly, downstream of the manifold, if the pressure drop is minimal, I plan on using 1/2" or 3/8".
Does it make sense to run any 3/4" pipes at all? After all, it seems a majority drop would occur in the long run of 1/2" pipe. My plumbing segments will all be less than 40ft total. Similarly, is there a guideline for acceptable pressure drop along a length of pipe?
And lastly, if I may, does the water pressure need to be higher than 0psig at a faucet or fixture?
EDIT: I know now that the underground run is just about 100ft. It actually lines up pretty well with tabulated values for head loss in 1/2" pipe at 60°F.
Best Answer
I would run the line to the manifold in 3/4" and then the branch circuits in 1/2" like you said. The cost increase is minimal and as long as you have the room for the extra 1/4" of pipe width, you won't second guess yourself. If you put in the 1/2" and they upsize you main in a year, you're going to wish you had installed the larger diameter.
Yes, you want more than 0 psi at the point of use. Most faucet manufacturers will say you need ~30 psi minimum for proper operation of the fixture.