The Pex out from the hot manifold should run closest to the outer perimeter, rather than the portion of the pex loop that is closest to the return port of the manifold. The tubing should be on the very bottom of the slab, and stapled to the insulation. Visqueen between the gravel and the insulation? You're spot on in placing the rebar on chairs in the center of the slab. 2' OC for the #4's will allow the guys spreading/screeding the concrete to step around the rebar and avoid stepping on it, bending it or smashing it into the Pex.

After the Pex is installed, pressurize the system and leave it hold pressure for as long as possible to ensure that you don't have any pinhole leaks.

Not that you don't, but take very good pics (even with some sort of measuring stick in the photo) to allow you to find those runs under the concrete just in case you ever need to drill into the floor. I was told that if I ran Pex outside for snow melt, it should be 3/4" and 6" OC.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).