Building a shower: Convert from 3/4" to 1/2"?

[rkb]

I'm building a shower and have 3/4" supply lines which the previous owner dropped down to 1/2" before connecting them to the valve. My sense is that restricting the flow is what led to the bad water pressure in that old shower.

We're going to have a shower head and a hand sprayer, so I was thinking I'd use a 3/4" valve with a paired 3/4" diverter but while I'm not having too hard of a problem finding those valves, I'm having a hard time finding fixtures that use 3/4" input.

So I guess my question(s) are two:

1. Am I right in my assumption that dropping from 3/4" to 1/2" (anywhere in the line) will result in lost pressure?
2. If I do need to stay at 3/4" in order to maintain pressure, does anyone have a suggestion for where I can find nice, modern, fixtures that use a 3/4" input?

Thanks,
Rob

 

[wwhitney]

No, that's not how water flow works. If you think of the water main as a constant pressure water source (approximately true; less true for a well pump), then every component between there and the outlet has a pressure loss, which depends on the flow rate. The final flow rate out of the outlet is the rate at which all the associated pressure losses add up to the source pressure.

So everything along the way contributes, but often one component dominates. For a showerhead or lavatory faucet, it's usually the outlet itself. A 2.0 gpm showerhead will be designed to have, say, 40 psi drop at 1.8 gpm flow, or 80 psi drop at 2.0 gpm flow (to make up some numbers). You're going to get very similar performance from the shower head whether you provide 40 psi or 80 psi to it.

For pipes, the pressure loss is proportional to the length, and goes down with a bigger pipe. So it's not the change in size in and of itself, but the relative lengths of the two pipe sizes. For the same pipe material, there's some equivalency factor between sizes; for example, in PEX, 1 foot of 1/2" pipe = 5.8' of 3/4" pipe. Plus some additional contribution from the changeover fitting itself.

The upshot is that 1' of 1/2" pipe isn't going to be a deal breaker, it will just be a small source of extra pressure loss, which is unlikely to affect overall performance. Also, there's not much point in providing a 3/4" connection to a 2.0 gpm shower head, as a 1/2" connection will do the job fine and the short section of smaller piping to it won't add much pressure loss, compared to the showerhead itself. The shower valve may be a significant source of pressure loss (more for tub filling than a single shower head though), so depending on the details, a 3/4" shower valve may be of value.

Cheers, Wayne

 

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[rkb]

Thanks, Wayne. It sounds like I'm confusing pressure and flow. Appreciate the explanation.

 

[Tuttles Revenge]

The Hansgrohe IBox has 3/4 inputs and lots of options based on the same rough in valve.

If its feeding a shower, its not that big of a deal tho.. if you're filling a tub it matters a lot.