What is return air? Why does the furnace need it?

Return air is related to forced air type home heating systems. The return air in these systems is circulated from the furnace (heating supply air) in ducts, to individual rooms. In the rooms (except for bathrooms and kitchens), there should be a return duct for the “return air.” This is air that completes the circulation and returns cool air back to the furnace in return ducts. Return ducts are typically under negative pressure, so should be sealed so as not to “suck in” pollutants during the trip back to the furnace to be re-heated. The furnace needs this full closed circulation of air to minimize its “suction” on the rest of the house, which can cause induction of soil gases and radon, and can pressurize the house with supply air, which can increase the air leakage and home heating costs.