The concept of Air-Rotation was invented and patented by Nelson Johnson, the founder of Johnson Heater Corporation in the 1950s. Prior to this patent, Nelson also experimented with Air-Rotation with large floor furnaces designed for gravity heat distribution in the 1940s. Nelson modified the furnaces to provide a single point of introducing heat into a large open space, and then modified the furnace to circulate the heat output with a ductless fan system. The early systems took the form of a brick mason building a firebrick heat chamber and various forms of indirect-fired metal shell and tube air-to-air heat exchangers. The early experimental data indicated the key to properly conditioning a large open space was the air circulation between a high discharge and a floor level return. Heat could be directed, or ducted into a large building by any number of heat producing devices; but if it was not evenly distributed, there would be hot and cold spots, stratification and wasted heating fuel that would fail to satisfy the thermostat control. Nelson gathered vast amounts of data and monitored the heating fuel used in actual buildings. To monitor fuel usage on a fair basis, he used the then newly developed Degree Day Method, a system pioneered by fuel oil distributors to predict when fuel oil deliveries would be needed by their customers. Nelson was able to show considerable fuel savings while providing other demonstrable benefits. The patent on the Air-Rotation concept was merely the beginning. The application of the Air-Rotation concept has been constantly refined by Nelsons successors at Johnson Heater Corporation.