Abstract: An air heating device includes two closed conduits, one conduit for the flow of the air supply to a combustion furnace and the other for the flow of exhaust gas from a combustion furnace to a flue. The conduits pass through and around each other multiple times in an interweaving pattern such that the entirety of either conduit is almost completely surrounded by the other conduit, to maximize contact area and thermal energy transfer between the conduits relative to the device size and no use of small tubes or passageways. The shapes and pattern of the two conduits resembling the appearance of a sine wave and cosine wave drawn on the same x axis.