Abstract: An economizer apparatus for a fossil fuel fired vapor generation apparatus which includes a housing having an inlet and an outlet and a wall extending generally between the inlet and the outlet to define first and second flow paths in the housing. The inlet may include means for connection to an associated vapor generation apparatus and means for connection to an associated stack. Heat exchange tubing is disposed in the first flow path having the interior thereof coupled to associated working fluid of the vapor generation apparatus. The apparatus also includes apparatus in the second flow path to control flow in the second flow path. In most embodiments of the invention the apparatus also includes a bypass conduit for selectively directing fluid flow around the heat exchange tubing. Ordinarily this will be desirable when the damper is obstructing fluid flow in the second flow channel.