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Old 08-18-2015, 12:49 PM   #1
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Pex fittings

I started with 3/4 inch pex feed to 1/2 inch pex to fixtures. I noticed the pex fitting ( elbows , Ts, etc.) have an inside diameter of 1/2 inch for the 3/4 inch fittings and 1/4 inch diameter for 1/2 fittings . Does this reduced diameter in the pex fittings reduce the flow of water ?? I want at least 1/2 inch water flow in my shower . Can anyone help explain this to me ?
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Old 08-18-2015, 01:14 PM   #2
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The flow of water through a pipe depends on the diameter of the pipe, and the pressure loss through the system. Pressure loss occurs for two reasons. The first reasons is that water flowing through a pipe loses pressure due to friction between the water and the pipe. The faster the water flows, the greater the pressure loss. The rougher the pipe, the greater the pressure loss.

The velocity of water in the pipe depends on the square of the diameter of the pipe, so a pipe with an inside diameter of 1/2 inch will require water to flow 4 times as fast as the same gallons per minute through a 1 inch inside diameter pipe. Most of the pressure loss in a pipe is due to pipe losses.

The second type of loss in a pipe system is due to expansion and contraction of the water as it flows through fittings. Expansion losses are larger than contraction losses. For the type of PEX fitting you are using, there will be a loss due to contraction and expansion as the water contracts to flow through the upstream side of the fitting, then expands on the downstream side of the fitting. This type of loss is known as fitting loss. The total pressure loss in your system is the sum of the pipe losses and the fitting losses.

So to answer your question, each fitting adds pressure loss to your system. Wirsbo makes an expansion ring PEX fitting that has the same inside diameter as the pipe, so there are no fitting losses through Wirsbo fittings. The downside to Wirsbo is that the installation tool is expensive, and you need to use Wirsbo products, which are somewhat more expensive than competitors who use crimp rings, but there are no fitting losses.

Are fitting losses important? Probably not, since they typically make up only a minor percentage of total losses in your system. More important is pipe diameter. It is possible to calculate fitting and pipe losses, but this is not a website dedicated to theoretical pipe flow analysis, and the methodology is somewhat complicated.

Conclusion: Make sure your pipe size is correct, try to minimize use of fittings, and don't worry about fitting losses too much. If you can't sleep, buy the Wirsbo system.
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Old 08-18-2015, 10:15 PM   #3
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Are you installing a body shower system with multiple heads? Your specific installation will determine what size piping you'll need. A standard shower valve will easily work with 1/2" pex, especially if you have the option of using a "home run" system, reducing the number of fitting between source and destination. On a standard shower valve, the water ports are not 1/2", they are more like 1/4". A fill valve for a tub/whirlpool are closer to 1/2"
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