I really liked the simplicity of Gary’s drainback system and the large low cost storage tank. I went with a 6’ long x 3.5’ wide x 4’ tall tank as that was the biggest I could reasonably fit in my mechanical room. It holds ~350 gallons. To maximize water space (and save a bit of money) I only used 1” of poly-iso to line the interior of the tank, regular fiberglass batt is between the 2x4 reinforcements on the exterior. Unfortunately I didn’t take any pictures during construction.