For most practical applications, two tubing sizes are best. 7/8″ Poly and 1/2″ PEX offer the greatest flexibility. The 7/8″ Poly provides the highest heat output (50 BTU’s per ft.) and can be spaced 16″ on center. But a compromise is made when it comes to bending diameter. However, in a wide open slab environment, bending diameter is not a problem. In floor joists, spacing of 16″ on center or greater is the most practical use for the 7/8″ size. The 1/2″ PEX ( 25 BTU’s per ft.) can also be used in virtually any application, but it should be spaced 8″ on center. It will heat the same as the 7/8″ Poly tubing, but you have to use twice as much of it. It can raise the cost of a radiant system substantially. However, 1/2″ PEX has thicker walls, and hence, a greater pressure and temperature rating. In high temperature applications it’s the best way to go. Its great durability is also ideal for snow melt situations.