[Mobile Master Tech]

BBP, thanks for linking to this thread. Nice tank!

First, PEX has low thermal conductivity, especially in 3/4" or larger because of the wall thickness. For not much more, you can get 1' corrugated stainless that has twice the surface area, tons more turbulence and probably quadruple the heat transfer. I got lucky and got a 98ft roll for under $100 shipped on ebay.

If you will be cycling the tank across the PCM's melt temp frequently, PCM makes sense. If only a few times per year, I would just use water and call it a day. Paraffins will be stable at any temp, salt hydrates are sensitive to temperature extremes. Candle wax for pillar and taper candles melts at a higher temp than wax for votives, container candles, etc. The potentially cheap price may offset the range of melt temp and slightly lower enthalpy.

Stratification only happens in the lower reaches of a 5ft or deeper still tank, not much happens in the top few feet or if there is any turbulence whatsoever. If you positioned your HX coils below the level of the wax so the wax doesn't insulate it but still within the top few feet, it will be as effective as if it were at the top. You could also float heatset PET, or even PP bottles full of wax to prevent the insulating effect. Tidier than "free-range" wax. Tie a cord to them if using a PCM heavier than water.

Put heavygage aluminum foil down the center of the bottles and use bottles with 8" or smaller cross section if you use paraffins and you want to get the heat out before the next ice age.