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We are installing a shower with a handheld and a fixed showerhead fed by 1/2 inch pipe. Available valve systems are either:

  1. A single function thermostatic valve, rated at 10.0 gpm, with separate volume controls for the showerhead and handheld. (This valve is a 3/4 inch so would have to use an adaptor from our 1/2 inch pipe.)
  2. A multifunction valve (1/2 inch), rated at 8.6 gpm, with one setting for showerhead, one for the handheld, and a shared function.

I don't know enough hydraulics or enough about shower valves to be confident of choosing the best system/optimal configuration.

Assuming the user will run both the showerhead and the handheld simultaneously, will the separate volume valves in system 1 provide better water pressure than the shared setting on the multifunction valve (system 2)?

  • Would the different pipe sizes matter, given that the house piping is 1/2 inch?

  • Does the type of valve matter (integrated valve versus single-function thermostatic with volume controlled separately)?

  • Does the difference in gpm between the two valves matter?

  • Are you worried that running the shower head and the handheld at the same time will hurt your water pressure? – JPhi1618 Mar 9 '18 at 19:24
  • JPhi: thanks for your question. I am trying to ascertain which system will support running both at once. So, if using the showerhead, turning the handshower to "on" should have no impact on the water pressure provided by the showerhead. In reality, both will have an impact, correct??? Which should work best? – Trader Ette Mar 9 '18 at 21:21
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Even a "standard" showerhead outputs only 2.5gpm at household water pressure. Hence, given their specifications, neither of the valves you list are going to be a bottleneck in the waterflow to even two standard showerheads simultaneously.

Water volume and pressure drop through your existing pipes depend on a number of factors that can tricky to calculate, but unless you're willing to replumb back to larger-diameter pipes then those questions are irrelevant: 1/2" pipe is unlikely to be a bottleneck, but you can't be sure without pressure-testing a mix across the final temperatures you want to supply.

  • Very helpful, feetwet, thank you! Are there any criteria you would use to choose between the two configurations other than cost? Would it be an issue to adapt to the 3/4 in valve? Seems we would get better performance w single function valve & 2 volume controls (even tho would requre the adaptor) rather than the multifunction valve? (I am deducing that the holes, literally, are larger in the former! and engineering more straightforward..) I understand your advice re bottleneck, but would there nevertheless be advantage to the higher flow valve? if not, am still interested in recommendations – Trader Ette Mar 10 '18 at 13:30
  • @TraderEtte: 1/2-to-3/4 fittings are plentiful, so it's not a problem to use the 3/4" valve. Since neither valve will reduce the water output, personally I would choose the valve whose functionality I prefer. And personally, where possible, I prefer to install a thermostatic control that is separate from the volume valves. – feetwet Mar 10 '18 at 14:41
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I am surprised that one can get a single shower head rated at 10 gpm. I thought that the limit was 2.5 gpm for a single head, so probably this limit is circumvented by having a shower fixture with multiple heads which in your case would be legally classified as four heads in a grouping to appear and function as one large head.

I doubt that a nominal 1/2" supply line would supply 10 gal/min. The best case would be if the line was 1/2" copper and a short distance to a 3/4" line. If the line is 1/2" PEX then the inside diameter will be significantly less than copper. Our two showers (fed by 1/2" copper lines) have low flow hand-held heads originally with restrictors rated at 1.5 gpm. I had to remove the restrictors because our Bosch Aquastar 125B NG tankless would not reliably stay on when only one shower was being used and there were no other hot water draws. In all showers that I know of it is easy to remove the restrictor.

My guess is that if you are drawing a large flow through the "10 gpm head", you would get very little flow out the low flow hand held shower. You might get some useful flow if you removed the flow restrictor, but you still might not get any useful flow.

  • It's the valves that are rated 10.0 and 8.6 respectively. the showerhead and handheld are 1.75. They are both Brizo (Delta). We tested the showerhead and handheld in-store ( similar ones, same brand) and flow seemed better than expected due to aeration and their "H2Okinetic" engineering. i don't know whether one can access the flow restrictor in these.... – Trader Ette Mar 10 '18 at 12:42
  • OK I get it now. I would think you could get useful flow simultaneously out of two 1.75 gpm shower heads (one fixed and one handheld) on the same 1/2" line. Our set-up has a hand-held head that can fit into a holder and become "fixed". This works fine for us, but we are minimalists. – Jim Stewart Mar 10 '18 at 22:22
  • From the description in the question I would choose option 2. Who needs a valve capable of delivering 10 gpm? – Jim Stewart Mar 10 '18 at 22:36

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