Furnace

A furnace comprising:

• a housing having a top, a bottom and sides defining a firebox chamber and a heat exchange chamber, a fresh air intake opening adjacent said bottom, a heated fresh air discharge opening adjacent said top, a combustion air inlet opening adjacent said top, and a combustion product discharge opening adjacent said bottom;
• a burner assembly secured to said housing for delivering heated air to said combustion air inlet opening;
• a firebox mounted in said firebox chamber having an opening communicating with said combustion air inlet opening;
• a modular, counterflow heat exchanger mounted in said heat exchange chamber and secured to the underside of said firebox and defining a plurality of alternating combustion product downflow channels and fresh air upflow channels, said channels being defined by a plurality of relatively thin-walled flue members each having:
• an elongated slot-like combustion product inlet opening at one end thereof,
• a combustion product discharge opening in said member adjacent the other end thereof,
• flange means extending laterally from each side of said slot-like intake opening for securing adjacent flue members together in predetermined spaced relation so at to define said upflow channels therebetween,
• spacer means at the other end of said flue members for securing adjacent flue members together in said predetermined spaced relation to permit entrance of the fresh air therebetween into said up flow channels from the fresh air intake opening, the upper ends of said upflow channels communicating with said heated air discharge opening.