I’ll be good as long as I keep the tanks on top off the wall. They can carry a tremendous amount of weight. I also have a double fire place in the center of the house and I think there is room for 55 gallon drums on the shoulders of the chimney.
I have an extra electric hot water heater I have plans to install in the attic as well. My first plan was to convert it to solar, circulating water from a solar collector through the tank. But now I think I will use a heat exchanger on top of the furnace to scavenge radiant heat. It requires installing a loop from the furnace to the tank,with a small circulation pump. The loop will use the cold fill and hot outlet on the tank. I’ll use the drain for the domestic side of the system and the pressure release for the fill. Air will escape out the tap like a regular closed system under pressure. The fill can act as an air inlet when water is drawn from the tank under gravity by installing a tee off the fill line with a check valve preventing entry of pressurized water from the pump. The tank can be filled from a rain water catch basin and be tied into the domestic water by installing check valves in both sides of the system.