Abstract: In a pulverized fuel-fired furnace (10), the temperature of the mixture of pulverized fuel and primary air leaving the mill (36) is measured and compared to a preselected minimum acceptable temperature. Whenever the measured mixture temperature drops below a preselected minimum, the flow damper (78) disposed in the tempering air duct (44) is closed to minimize the flow of tempering air therethrough and a heating fluid (62) is passed in heat exchange relationship with any ambient temperature air leaking past the flow damper in the tempering air duct thereby preheating the tempering airflow. Preferably, hot water (62) is circulated from the waterwall inlet header (22) of the water-cooled water walls (12) of the furnace (10) through a heat exchanger (60) disposed in the tempering air duct (44) and then returned back to the water-cooling circuit (24) of the water-cooled furnace walls at a location upstream of the circulating pump (26).