I have a 400 gallon unpressurized solar storage tank for space heating. I'd like to preheat Domestic Hot Water by running a copper loop through this tank. Can I use refrigeration tubing or is K copper a better option?

What's the temperature of the water in the tank? You want to preheat the DW supply by convection with a loop? Is there a problem with running a line down from the tank to the DW? Back to you, Tom