You say 2 inch styrofoam is not worth it and above gradee it is not enough. Our soil is rather humid. We cannot assume the slab would have been dry. Above grade is only 6 inches.

You say the tank is too small. I guess it is time for me to actually pickup the calculator and actually calculate load. I think the way I should do this is by analysing my electric bill. Will post again on that. The tank I plan to use is in my mind pretty big (4 x 4 x 8 ft)! That would surely take a fair bit of space in my technical room...

Regarding code and antifreeze, from what I understand, canadian code requires double wall heat exchanger unless there is more pressure in the domestic water line than in the antifreeze side. That is why I assumed it was OK.