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Should I Upsize When Replacing Copper with PEX?

December 15, 2020 Viega
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There is a rule of thumb among contractors that when swapping out copper pipe for PEX the safest thing to do is upsize the PEX tubing.

The thinking behind it is that replacing ¾" copper with 1" PEX is a no-brainer way to ensure adequate pressure and performance. And it works. But it’s often unnecessary, wastes water and, on larger jobs, can add significantly to the materials cost.

Rather than automatically upsizing, it’s better to do some simple math to determine if it’s necessary.

For example, let’s look at plumbing a urinal. The International Plumbing Code requires a flow pressure of 12 GPM@25 PSI.

Here are the figures for a copper system:
• 65' 1" copper L pipe
• Five 1" copper 90°elbows = 2.5 feet per elbow
• Total pipe length is 2.5 X 5 = 12.5' + 65' = 77.5' of pipe
• 1" copper pipe @ 15 GPM-0.057 PSI loss per foot
• 77.5' X 0.057 = 4.4 PSI loss

And here are the figures for PEX:
• 65' 1" PEX tubing
• Five 1" PEX 90°elbows = 10 feet per elbow
• Total tubing length is 10 X 5 = 50' + 65' = 115' of tubing
• 1" PEX @ 15 GPM-0.132 PSI loss per foot
• 115' X 0.132= 15.2 PSI loss

So the copper system shows a loss of 4.4 PSI while the PEX system has a 15.2 PSI loss. Isn’t that an argument for upsizing the PEX?

No. Assuming you start with an available pressure of 65 PSI, the copper system would deliver 60.6 PSI at the urinal while PEX would deliver 49.8 PSI. While the PEX flow is less than copper, it’s still well above the required 25 PSI so it will work without upsizing. And using bend supports instead of elbows in the PEX system would lower the pressure loss to 4.2 PSI, virtually identical to copper.

When deciding whether to upsize PEX the key is determining the PSI at the fixture and whether that is sufficient for the job. If so, it doesn’t matter whether the PSI is less than that delivered by the same-sized copper.

Additional considerations for sizing PEX plumbing include:

• Use data provided by the PEX manufacturer for pressure drop and equivalent lengths. This data can vary slightly between manufacturers.
• The methods used to size piping do not change; only the values change.
• Eliminate fittings as much as possible.
• Fewer connections mean less pressure drop and helps eliminate connections in concealed locations.
• PEX tubing with plastic fittings can have a higher maximum velocity but local code may still limit the velocity to 8 FPS on cold & 5 FPS on hot.

Viega’s Design Team is available to help customers with residential and commercial layouts and sizing. Contact design@viega.us

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