Unexpectedly low shower flow rate

[Jared Grothe]

I installed a Delta R10000-UNBXHF rough-in valve with T17038 trim and only get 4.5 gpm, despite their manuals and support saying the combination should give me 6.0 gpm. I have flushed the rough-in valve but it didn't make a difference. My water pressure is 60psi, and I'm getting a full 8.5 gpm through each hot/cold input lines individually.

Is this normal, or is there something wrong?

 

[Jadnashua]

The RI valve has two ports...an unrestricted one designed for the tub spout, and a second one that is restricted, designed for the showerhead. If you're only using the restricted one, yes, that's probably normal. To get that 6gpm, you need to be using the unrestricted port. If you install the RI valve normally, the unrestricted port is on the bottom. You MIGHT be able to flip the RI valve 180-degrees, or, worst case, come out the bottom and then run it around up.

If you're measuring the bottom port, would have to see how you connected things and what size and type of piping and fittings were used. Pex, for example, has a fairly significantly smaller ID than the corresponding size copper.