First let's talk a little about the three types Crosslinking is a chemical reaction that occurs between polyethylene polymer chains. Crosslinking causes the HDPE to become stronger and resistant to cold temperature cracking or brittleness on impact while retaining its flexibility. The three methods of crosslinking HDPE are the Engels method (PEX-a), the Silane Method (PEX-b), and the Radiation method (PEX-c). Several industry participants claim that the PEX-a method yield more flexible tubing than the other methods. All three types of PEX tubing meet the ASTM, NSF and CSA standards.
I think the type a is a little more flexible than type b, but I like the type b with a Mylar coating. The company I have worked for the past 20 years has sold millions of feet of it for radiant floor systems. 300 ft loops are the max because otherwise the fluid will cool down too much by the time it reaches the end of the loop. Layout should be on 1' centers, assuming 1/2" PEX. Because you are only having one zone the layout is pretty easy. Just start from the area where your manifolds will be located, roll it out along the outside wall to the furthest area then back and forth on 1' centers, as you get towards the end of the roll just leave enough to get back to the manifold. You may end up part way down a run needing to head back but that's OK. Just cover that area with next run, in other words your runs will not be just straight lines back and forth but I would rather have all the loops fairly close to the same length. It makes balancing the system a lot easier later on.
