An auxiliary duct is supported on the housing of an indirect fired heater of the type having separate heated air and combustion passages. The auxiliary duct extends between an inlet end at the heated air discharge to an outlet end adjacent a burner assembly at the combustion inlet so as to be arranged to direct a portion of the flow of air heated by the heater towards components of the burner assembly including fuel lines, a fuel pump, a fuel filter and a combustion air intake blower for heating the burner assembly and associated components to improve the operation thereof in colder climates.