A self-cleaning heat exchanger in combination with a home furnace system which draws combustion air from outside the home is disclosed. Gases of combustion are drawn up through a plurality of vertical tubes and out a chimney by a fan mounted in the chimney. The lower portions of the tubes are cooled by inside air drawn over them by the return air blower of the furnace, the upper portions of the tubes are cooled by cold outside air drawn across them by the furnace burner unit. Moisture which condenses in the upper portions of the tubes is drawn down through them by gravity. The higher temperature in the lower part of the tubes causes this condensate to revaporize and rise to the upper portion of the tubes, where the process repeats itself. As more water vapor enters the tubes in the flue gas, distilled condensate eventually drops out of the tubes and runs into a drain at the lower end of the heat exchanger. The moisture from condensation of the combustion gases scrubs pollutants from the flue gases passing through the tubes, and is slightly acidic, providing a cleaning function for the interior of the heat exchanger tubes. The heat exchanger is also suitable for use in commercial furnace systems and combustion-type power plants which utilize outside air for combustion.