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All of my radiant projects that are underfloor are done in 3/8" pex. So much easier to run compared to 1/2".

Yes, your loop lengths will be shorter compared to 1/2" but your delta T should still be 10-15 degrees for the comfort factor and maintaining an even surface temp of the floors.

The average loop lengths from the manufacturers are based upon standard, safe circ selection without having to go to crazy high head circs.

But if you do the math, you can go past the recommended lengths, in my own house, I installed 3/8" underfloor (with and without plates) with 400' loops and a standard circ.

I used 1/2" for my slab for the same reasons, I wanted longer runs and less connections to a manifold and less actuators. I wanted to make sure each room I was installing tube in that it had one loop instead of two (easier all around)