Abstract: A forced air heating system for homes and other buildings wherein a fireplace with a circulating air heating chamber is interposed between the fan exhaust of a conventional forced air furnace and a network of air outlet registers positioned in the various rooms to be heated. A bypass air duct is positioned in parallel with the fireplace between the fan exhaust and the network of outlet registers to provide an alternate path for the forced air exhausted from the furnace fan. The system has a control circuit sensitive to room temperature, fireplace heating chamber temperature, and other duct temperatures for automatically operating the system either in a primary mode wherein the fireplace is the primary forced air heating source, or in a secondary mode where the furnace heating elements are the primary heat source. Automatically controlled dampers determine through which of the two parallel ducts the forced air from the furnace fan will travel to the outlet registers, dependent on the mode of operation.