Abstract: Method and device for maintaining and reinforcing in an accumulation tank a stratified condition of fluid caused by the fluid in the tank when a flowing of the fluid is caused by heating the same. The portion of the fluid in the tank that is to be heated is separated from the fluid in the tank prior to the heating and the heated fluid is permitted to ascend through a liquid column separated from the fluid in the tank and is conducted back into the tank at the fluid-layer that has substantially the same temperature as the heated fluid. The device comprises an accumulation tank in which a tube is arranged substantially vertically from the bottom thereof towards its upper part and the tube receives separated heated fluid which is allowed to ascend through the tube. The tube is provided with openings one after the other along the tube constituting outlets for the heated fluid.