Hot combustion products exchange heat with air in a heat exchanger of a condensing furnace. The hot combustion products flow on one side of the heat exchanger, and the air flows on the opposing side of the heat exchanger. The heat exchanger is electrochemically coated with a coating of copper metal. The copper coating is oxidized with an aqueous oxidizing alkaline solution to form a matte black layer of cupric oxide. As the layer of cupric oxide is black, the layer has a high emissivity and emits more heat, increasing the efficiency of the primary heat exchanger. Alternatively, iron can be electrochemically coated on the primary heat exchanger and oxidized to black magnetite to increase emissivity.