Is there a reason that pex should not cross itself in the heat distribution floor loop within a slab floor? All of the research I have come across clearly shows great designing of patterns to not cross but within a 4" slab there should at least be enough depth to physically do so. Fluid loops of HDP for geo heat pumps seem to be OK with crossing.
I like the spiraling pattern of exterior to interior center had keeps hottest fluid along outer heat loss walls but can't see the need of having to chase itself back around out to exterior to get back to manifold without crossing was a loss. Within 4" and midway in slab being the recommended depth of pex there should be plenty of room to run the colder return from center of floor loop back under loop pattern to start point and manifold.
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