This term has two meanings: 1. A major part of a furnace that transfers heat created in the fuel combustion area to the surrounding air that is pumped into the rooms. This component can develop rust holes or other leaks and problems. To prevent carbon monoxide gas from entering the living areas and for other reasons, the heat exchanger and heating system should be professionally checked regularly. 2. A condenser or evaporator device in which heat is added or extracted to heat or cool indoor space.