I have been in and under this tub checking it out. I am pretty sure if I take out the heater loop, everything else is PVC. I like the idea of a heat exchanger. Where in TX are you. I am up in Lubbock. I have considered coiling ABS plastic pipe (black) on the roof of my shop which faces South and using that as a really crude solar water heater. The idea of the TREX pipe sounds good.