When swapping out copper pipe for PEX, upsizing is oftentimes automatic for contractors. But it isn’t always necessary.
Contractors sometimes overdesign or overbuild things just to make sure there won’t be any problems with performance. That approach is not necessarily bad — and is certainly better than doing the opposite — but sometimes it’s unnecessary and can waste money.
For example, take the rule of thumb that when swapping out copper for PEX the safest thing to do is to upsize the PEX tubing. The thinking is that replacing 3/4-inch copper with 1-inchPEX is a simple way to ensure adequate water pressure and performance. And it works.
But it’s also often unnecessary, wastes water and, on larger jobs, adds to the materials cost. In commercial and institutional applications, where the use of PEX is growing, that extra spend can be significant. Rather than automatically upsizing, it’s better to do some simple math to determine if it’s necessary.
As an example, let’s look at plumbing a urinal. The International Plumbing Code requires a flow pressure of 12 gpm at 25 psi.
Here are the figures for a copper system:
- 65 feet of 1-inch copper L pipe
- Five 1-inch copper 90 degree elbows (2.5 feet per elbow)
- Total pipe length is 77.5 feet
- 1-inch copper pipe at 15 gpm and 0.057 psi loss per foot
- At 77.5 feet, that is 4.4 psi loss
And here are the fiqures for PEX:
- 65 feet of 1-inch PEX tubing
- Five 1-inch PEX 90 degree elbows (10 feet per elbow)
- Total tubing length is 115 feet
- 1-inch PEX at 15 gpm and 0.132 psi loss per foot
- At 115 feet, that is 15.2 psi loss
The copper system shows a loss of only 4.4 psi at the fixture while the PEX system has a 15.2 psi loss. Isn’t that an argument for upsizing the PEX to preserve pressure?
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 the urinal will work fine 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 it meets code and is sufficient for the job. If so, it doesn’t matter whether the psi is less than that delivered by the same-sized copper. There’s no reason to upsize.
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 among manufacturers.
- The methods used to size piping do not change; only the values change.
- Eliminate fittings as much as possible. Using bend supports instead of elbows can make a big difference in psi.
- Using fewer connections means 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 feet per second on cold and 5 feet per second on hot.
The lesson here is that it can pay off to question conventional wisdom and do the math yourself.