High molecular cross-linked polyethylene (PEX-a) with minimum bending radii of 6X the diameter at 68 deg F (20 deg C). Maximum operating temperature: 180 deg F at 100 psi (82 deg C at 690 kPa). Covered by a twenty-five year manufacturers limited warranty. Comes with oxygen diffusion barrier which prevents oxygen from entering the heating system through the tubing wall.