There are several different types of tubing used in radiant floor heating systems. They range in size from 3/8"- 7/8" diameters, depending on the heating required and the manufacturer.
Older installations used soft copper or steel tubing embedded in the concrete slab. This could corrode from the moisture inherent in a concrete slab. Most major manufacturers now use cross-linked polyethylene (PEX) tubing or rubber tubing with an oxygen diffusion barrier.
The oxygen diffusion barrier prevents oxygen traveling through the rubber tubing and entering the flexible tubing. The oxygen can corrode metal parts used in the installation of the flexible tubing. These include parts in the pump and fasteners and couplers used to attach the tubing.