I am converting a seperate shower and tub to a large walk-in spa shower. The cold water at the existing shower is 3/4" (and branched off to the tub,shower & toilet in 1/2"). The new shower valve system is 3/4" so I am good there with just a bit of re-plumbing. The problem is with the hot plumbing. The plumbing is all under slab and the 3/4" part of it is non accessible. The tub and shower hot branch off a 3/4" to 1/2" under the slab somewhere. The access between the two 1/2" hot water lines is configured in a way as I could bring them together at the new shower wall. If I were to bring them together with a T that fed them to a 3/4" line to the new valve would I get my flow back from the 1/2" drops? I am installing 6 body sprayers and need the flow. I did get sprayers with the lowest rate possible (only 1.5 gpm per head). So basically, if I have X amount of volume in a 3/4" pipe and reduce it into two 1/2" pipes then bring both 1/2" pipes back to 3/4" will I retrieve my volume? It would work kind of like a loop would it not?
Plumbing – Will returning two 1/2“ lines to 3/4” get a volume return
plumbing
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Best Answer
It depends on your flow rate, and the length of your run. The short answer is you won't have a noticeable pressure drop for a relatively short run.
Using this table, the nominal inside diameters of the pipes actually give you a slightly greater cross sectional area from the two 1/2" pipes (0.608 sq. in. vs 0.533 sq. in.).
However, the head loss is still greater for the two 1/2" pipes. For the 9 GPM you'll need for the body jets, you'll have a loss of about 15.8 ft/100 ft pipe on the 3/4". For the 4.5 GPM you'll need through each 1/2" pipe, you'll have a loss of 16.9 ft/100 ft pipe.*
This comes out to a loss of 6.85 psi per 100 feet of pipe for the 3/4" pipe, or 7.33 psi per 100 feet of pipe for the two 1/2" pipes. You'll have to add another 15.6' of equivalent footage for the two extra T and elbow fittings as well.
*Note: I linearly interpolated these values, but the head loss curve is NOT linear. So not an EXACT answer, but probably close enough.