Pipe sizing for valve with 1/2" internals

Discussion in 'Shower & Bathtub Forum & Blog' started by JimKoz, Feb 18, 2020.

1. JimKozNew Member

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I'm installing a shower valve with a 4-way diverter. The manufacturer says the insides are 1/2" chambers. There will only be 2 devices active at one time (wall, rain, body jet, hand). Each device is 2 GPM so the total flow rate will never be greater than ~4 GPM. Is there any benefit to running 3/4" to this valve? Using PEX.
Seems like a waste ...

Thanks
Jim

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You don't mention distance. Distance is a factor in friction loss calculations.

4. jadnashuaRetired Defense Industry Engineer xxx

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If I was going with PEX for this, I'd run it with 3/4". 1/2" PEX's ID is significantly smaller than nominal 1/2" copper, especially if you're using a crimp connection versus an expansion one where the fittings add, potentially significantly, restrictions.

5. JimKozNew Member

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I don't see how distance is a relevant factor here. Pressure in the system is set by the regulator, so I'm thinking it's only a matter of volume. Unless I'm missing something ...

6. JimKozNew Member

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So maybe the question is moot ... the valve uses 1/2" NPT fittings but the actual opening inside the valve is about 5/16". I guess they wanted to increase pressure while sacrificing volume. So, there seems to be no reason to use 3/4" pipe, instead of 1/2", to supply a 5/16" hole.

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You lack an understanding of fluid dynamics. Google for online friction loss calculators and run a few numbers. As for the constriction, water just speeds up through it and is not the same as having the entire run be that size.

8. Reach4Well-Known Member

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In traffic flow, the cars per minute is limited by the slowest section.

In pipes, the pressure drops (resistances) add like electrical resistors in series, if that means something to you.

I think 1/2 inch PEX would be enough, and would cause less delay for hot reaching your showers than 3/4 would. It is not a matter of waste, since 3/4 inch pex is not that much more expensive on an absolute basis than 1/2.

9. jadnashuaRetired Defense Industry Engineer xxx

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Pressure would be the same in a static (no-flow) situation whether you had a soda straw or a fire hose. WHen you then start to draw water, the friction along the way will cause the dynamic pressure to drop. Smaller pipe causes more restrictions/friction, and at the same volumes, will have lower pressure. now, without knowing what pipe and how it is run, cannot give you a pressure drop calculation. The more pipe it has to go through, the more friction, the less pressure and therefore volume available at the end.

The Bernoulli principle effectively says, a short restriction causes the fluid to speed up, then it will slow down again after that restriction. It does cause a bit of friction, but depending on how long that restriction is, it may not cause much measurable volume/pressure change outside of a laboratory. It's there, you may not notice. Make the restriction longer (say a 1/2" pipe between 3/4" ones), and you certainly will notice the flow reduction. FWIW, at the same pressure, a 3/4" pipe will carry twice the volume of water as a 1/2" one. You may not need it, but if you do, a larger pipe is called for. Running water faster than design creates its own problems of pressure loss, noise, and pipe erosion.

10. SylvanStill learning

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Decreasing or increasing a pipe will not create more pressure

If your supply is 40 PSI on a 4" diameter main and you decrease the pipe to 2" your pressure will still be 40 PSI BUT the Velocity will increase with a major loss of volume

A lot of people confuse pressure with velocity . If you ever used a garden hose and used a variable nozzle you will seethe results

11. jadnashuaRetired Defense Industry Engineer xxx

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You don't 'create' pressure with a larger pipe, but the lower friction allows it to dynamically maintain that pressure at reasonable velocities.

The potential disadvantage to a larger pipe is that it will take more water to purge any cooled off water there prior to receiving hot. That problem goes away if you use a hot water recirculation system and insulate the pipes. Potentially saving thousands of gallons of water a year.

My first one recently died after about 15-years. The new one uses all of about 14W when it's running, which isn't all that often.

12. Reach4Well-Known Member

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The pump energy is going to be a very small amount of the energy cost to run a recirculation system that is not on-demand only.

13. jadnashuaRetired Defense Industry Engineer xxx

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Installing one without insulating the pipes will be more expensive to operate. But, if you also limit it by using a timer for when it's needed, or, turn it on when you need hot water, the cost is minimal.

One brand uses a 1/2 Hp motor, which overspeeds the water flow, but achieves the hot arrival quickly. That one is designed as an on-demand unit - it only runs when you activate it then shuts off once hot arrives. The unit I have and had, runs intermittently to bring warm water in. A 1/2-Hp ~ 378W, huge jump from 14W.

Especially if you're paying for water and sewerage, not throwing what amounts of unutilized water to get hot means you're paying extra for the water (even if you pump your own...you're using energy to get it into your house), and potentially sewerage fees.

Keep in mind that most systems don't run until you achieve full WH temperature at the sensing point. The new one I have shuts off when the water reaches 95F and turns back on when it drops to 85F. Warm enough to usefully wash your hands, and full hot within seconds. Plus, everything closer to the WH along the way will have hotter water. Shutting off prior to arrival of full hot also means much less hot/warm water gets pushed into the cold line, if you don't have a dedicated return. So, as the heat transfer changes radically at the delta-T rises, because it's not that much warmer than the ambient, it lessens any energy losses.

So, pick a system that's engineered for efficiency, and the convenience is not a major cost.