There are many different grades of PEX pipe. All PEX pipe is manufactured with a print-line listing information about the pipe. Information included in the print-line include the name of the manufacturer, what standards the pipe complies with, and the grade of the pipe. The grade of the pipe is indicated by a series of four digits. The first digit is generally considered to be the most important because the number indicates the resistance of the pipe to degradation by exposure to hot chlorinated water. If the first digit is 1, the pipe has passed a test showing that it will remain ductile and not fail when exposed to hot chlorinated water 25% of the time. If the first number is a 3, the pipe has passed a testing showing that it will remain ductile and not fail when exposed to hot chlorinated water for 50% of the time. If the first number is 5, the pipe has passed a testing showing that it will remain ductile for over 50 years of exposure to hot chlorinate water for 100% of the time. However, there has been controversy over the validity of the chlorine resistance test (ASTM F2023). Therefore, we recommend only using PEX pipe with the highest chlorine resistance rating (5) no matter what your pipe is used for, even cold water.