We're installing a new tub/shower and are planning to use a Grohe valve - and thus a a Grohe Rapido Smartbox for the rough in. The ports on the Smartbox are all 1/2" and Grohe does not advise against using PEX for the tub filler (as some vendors do, I think to avoid resistance in the tub filler that could cause water to leak from the shower head).

So, my question is, would there be any advantage to running 3/4" PEX to the tub filler -- especially in terms of the fill rate?

  • Bigger pipe usually equals more water flow. This depends if the whole line/water system is 3/4. If plumbing is 1/2, then one line of 3/4 will not help much, except to lighten the wallet.
    – crip659
    Commented May 11 at 23:47
  • It's 1/2" from the manifold -- about 4' away as the pipe runs, but 3/4" to the manifold. So, once you restrict the flow through some 1/2" there isn't a benefit to opening it up again?
    – dlu
    Commented May 12 at 1:20
  • It get complicated. 1/2 inch reduces flow rate/increases pressure. If enough so you notice, depends on other factors, like if other faucets are open, pressure of the whole system.
    – crip659
    Commented May 12 at 20:04

1 Answer 1


Increasing the outlet size will not provide more volume of water as the volume is determined by the piping supplying the valve and the valve itself. So if the volume is set, and you increase the area, you reduce the pressure.

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