Another gain that the PEX tubing system holds above the metal water line is the rates paid for installations and acquiring the material. The iron water lines cost more than the PEX tubing, and it also will take a lot longer to build an iron plumbing contractor system compared to a PEX tubing system. We have PEX tubing ranges in size from imperial sizes of 1/4-inch to 4-inch. Radiant Heat or PEX Plumbing - The Leader in PEX Tubing & Supplies.