Abstract: A package heat exchange system is disclosed having a burner positioned in the central plenum of a substantially cylindrical heat exchanger. A fuel-air mixture is supplied through a blower supplied with fuel from a pressure regulator which requires a somewhat less than atmospheric pressure at the blower inlet to draw gaseous fuel through the pressure regulator. A solenoid-operated, positive pressure valve in the main gasline provides the heat exchanger with an enriched gas mixture on ignition. The enrichment valve automatically closes after 30 seconds of operation. Air for combustion is drawn through a series of orifice plates positioned between the regulator and the blower. The fuel-to-air ratio may be altered by changing the size and/or number of the holes in one of the orifice plates. Further, one of the orifice plates may be deformable so as to form a seal, and to supply a predetermined quantity of air to the burner for combustion.