Furnace clean and tune. Cycled system. Checked heat exchanger. Found all chambers rusted and corroded. No visible cracks found with camera. Checked, removed, and cleaned burners and flame sensor. Ok. Igniter tested ok. Checked and tightened electrical connections and clamps. Cleared drain lines. Found ventor motor bearings are worn and noisy. This will cause motor to seize at some point. Preformed combustion test and found normal levels in exhaust. No CO found in airstream. System is worn. Future failure is expected. Job complete.