I used a 1500 litre concrete tank that I rescued off a hill side. I dug it into the ground and poured urethane foam in arround it (4" min), spacing it off the bottom so that it was insulated from underneath as well. I built a deck with a hatch over the top. I used heat exchangers to heat it with solar (closed loop) and pumped the water direct through a wetback. There was also a heat exchanger to heat the domestic water (via a califont for backup) and water for the floor heating was pumped direct. Worked very well