My house runs on well water and the main pipe that leaves the pressure tank is 1-1/4" copper. I am looking at installing a spin down filter and regular water filter for the whole house. My question is if I connect a T fitting which reduces the size of the pipe from 1-1/4" to 3/4" and then after the filters back to the 1-1/4" inch size will it have a negative effect on pressure or flow?

  • 1
    Reducing it to a smaller diameter and then increasing it back up will have an negative effect simply because you're reducing flow. You could notice it with multiple users of water at the same time. Have you looked for filters with 1 1/4" fittings?
    – JACK
    Jul 15, 2020 at 12:41
  • Very similar to (but more extreme size reduction) diy.stackexchange.com/q/196371/18078
    – Ecnerwal
    Jul 15, 2020 at 12:52

1 Answer 1


You are (and may not realize if you did not pay attention in geometry class) talking about a nearly 2/3 reduction in pipe size (area/cross section) when you go from 1-1/4" to 3/4".

Area of a circle = Pi r squared. 1.23 .vs. 0.44 square inches for the pipes under consideration here.

That will definitely have "a negative impact on pressure and flow" as compared to keeping it at 1-1/4" throughout.

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