The other day I got a quote for a retail copper coil heat exchanger for my tank, just to put things into perspective as I still struggle to decide what to do. This helped: $3,500. I called my local HVAC supply house, and their price (I assume retail) for 1" soft copper was $5.50 per foot, minimum 60 feet, plus sales tax (9% around here). One-inch pex-al-pex (no minimum) goes for $1.40 per foot, plus shipping. The best estimate for replacing copper with pex-al-pex is 3X as much pex to equal the heat transfer of copper. On paper, I would need about 200 feet of copper coil do do the job, or 600 feet of pex-al, but I think I could get by with less of each. Something to ponder over the winter as I play around with the much cheaper flat plate heat exchanger option.