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I have a ½" water supply feeding my shower currently. We're currently doing a remodel and would like to install a shower system (shower head and three body sprayers).

Can I use the current supply to feed it? Or would I need a larger supply for branching to two different sets of controls? As a reference, we're considering the following system.

For a little more detail, this shower is on the second floor with a 3/4 inch to ½ inch conversion in the crawlspace right before it goes up the wall to the shower. And this ½ supply is solely dedicated to the shower.

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That depends on your pressure to start with a big pipe doesn't increase the pressure it just stops frictional pressure loss and increases flow rates –  UNECS Nov 24 '12 at 21:09
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What do the instructions say that the shower system's demand (or flow rate) is? –  The Evil Greebo Nov 24 '12 at 21:46
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The linked system flows 4.5 gpm at 35psi. Without knowing the available pressure and flow and the pipe size and lengths and elevation change from the point of measurement to the shower, we still couldn't say for sure. My gut feeling is it's workable but marginal. And if you take out the flow restricters so the heads work properly, and water is used elsewhere, forget about it. –  bcworkz Nov 24 '12 at 23:31

3 Answers 3

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For copper pipe, hot (~50 C) water should only flow at approx 5 ft/second. Flow of less than one or two ft/sec can also be a problem. Based on the diameter of the pipe (1/2 in type L), 0.72 gpm flows at 1.0 ft/second, so this would suggest a maximum of 3.6 gpm for the pipe (assuming 5 ft/second). Also, you must worry about the pressure drop. Use table 6 of The copper Handbook, at 5 gpm, the pressure loss is 0.161 psi/ft. Assuming your supply is 50 psi, and you need 35 psi to be delivered to the shower head, you can have a maximum pipe length of (50-35)/.161=93 ft. Figure 7 shows how fittings should be counted due to their larger pressure drop. A 90 degree fitting counts as one foot. Additionally, water pressure will decrease with elevation by approximately 0.433 psi/foot.

As long as you meet these two restrictions, you're fine. Since you need 4.5 gpm, and your pipe is likely less than 93 feet, things should work fine.

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1/2" supply should be fine. You should however have at least a 3/4" main line supplying your branches

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Can you expand on this answer a bit? What is a main line, and what are branches? –  Tester101 Dec 31 '12 at 18:22

Residential pressures typically range from 40-70 psi (pounds per square inch). At 45 psi the flow rate of a clean straight 1/2" pipe is just over 30 gpm (gallons per minute) and in 1992 the US limited shower faucet to 2.2 gpm so most worldwide are the manufactured with that limit in mind. So a clean straight 1/2" pipe should supply over 13 shower faucets continually. The idea behind have a larger supply/main is that you may be showering but that isn't necessarily the only water running in the house. So before the lines branches off for the shower; using a larger supply line for both hot and cold will limit the loss of flow you experience (and the flashes of hot and cold water when someone flushes or the washer cycles)

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What do you mean by "clean straight"? –  auujay Jan 25 '13 at 20:57
    
Running 1/2" pipe at 30 gpm will certainly cause the walls of the pipe to be worn away. See wikipedia. The pressure drop would be around 4.5 psi/ft (if the Hazen-Williams formula still holds at such a high velocity). So, given a 10 foot pipe, the final pressure would be near-zero at 30 gpm. –  Pigrew Jan 25 '13 at 21:52
    
Clean = new/not gunked up; Straight = no elbows and such. The numbers are calculated for a 1/2" cross section pipe at 45 psi. Even the ugliest configurations would get at least 75-80% of the flow through. I guess I should say at 45 psi you can achieve at least 22 gpm with 1/2" piping. –  Jason Jan 28 '13 at 22:06
    
Accordingly to said Wikipedia article (after some conversions); if you are using 1/2" copper piping limit the flow to 16gpm to avoid erosion corrosion. Which would still be 7 high-flow shower heads. –  Jason Jan 28 '13 at 22:20

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