Reduction of boiler stack temperatures of between 350 and 450 degrees F to less than 140 degrees F, can save 10 to 15 percent in fuel consumption. In the past, such reductions in flue gas temperatures were unthinkable because moisture in the gases would condense, causing corrosion problems in the heat recovery device and the boiler stack. In the last few years, however, heat recovery devices designed to handle the condensation problem have become commercially available. Two general designs are available, the spray-type condensing heat-recovery system and the heat-exchanger type designed to allow some condensation to occur. This discussion concerns itself primarily with the spray-type heat recovery system. A short discussion of the heat-exchanger type is included to let the reader know that it is available and may have advantages for certain applications.