Water vapor-condensing secondary heat exchanger

For use in a furnace of the type which burns fuel yielding water vapor as a product of combustion, and including a casing having a substantially vertical path portion for air to which heat is to be exchanged, a water vapor-condensing heat exchanger comprising a hot combustion gas manifold extending substantially horizontally along one side of such vertical path portion, an intermediate manifold extending substantially parallel thereto along the opposite side of such vertical path portion at a lower elevation, a collector manifold extending substantially parallel to and directly underneath said intermediate manifold, a single heat exchange coil made up of a first set of heat exchange tubes having inlet ends connected to the hot gas manifold and extending at a downward slope therefrom across such vertical path portion to outlet ends within said intermediate manifold, a second set of heat exchange tubes extending from inlet ends within said intermediate manifold, reversingly across such vertical path portion and at an upward slope parallel to said first set of tubes, to outlet ends, to which are connected the inlet ends of a third set of heat exchange tubes which extend reversingly across such vertical path portion sloping downward and somewhat below and parallel to said second set, to lowermost outlet ends within said collector manifold, said coil having a set of heat exchange fins common to said first, second and third sets of tubes, means at said collector manifold to apply suction to said lowermost outlet ends of said third set of tubes, and means to drain condensate from combustion interruptedly from both said collector manifold and said intermediate manifold.