I'm in the planning stages of thermal storage for my Econoburn 150. I'll be using a hand-built rectangular tank holding about 770 gallons in un-pressurized storage with an in-tank heat exchanger. I've looked at the usual array of commercially produced plate heat exchangers and coils. As I was looking at this website http://www.energytransferinc.com/?gclid=CKCe07jNm6QCFQ8E5QodlxKtDg I noticed the picture in the "Custom Heat Exchangers" box. I'm wondering if the same thing could be done with regular fin tube, and how effective it might be. I have quite a bit of spare fin tube in both 3/4 inch size as well as 1 1/4 size. Does anyone know if fin-tube can be used effectively in water as a heat exchanger? Id' appreciate any and all thoughts about heat exchanger options unpressurized storage. Thanks as always. --David H.