I am pouring a 12x27 foot slab at my back door. Most of the slab is covered with an overhang from the house. I am going to lay down 2 inches of blue foam board and 1/2 inch Pex over it. It will be covered with 6 inches of concrete. I am going to use a heat Exchanger to isolate the anti freeze from the domestic and heating hot water. If the slab ever gets icy or snow covered, I can turn on the circulation pump and circulate the anti freeze through the Heat Exchanger and Pex, and melt the ice and snow. What type of anti freeze should I use. The hot water is pressurized to 55 pounds ant the anti freeze loop will be un-pressurized. Also any suggestions on the spacing of the Pex.