Furnace clean and tune, cycled system. Checked heat exchanger. Found some rusting and corrosion in heat exchanger. Checked, removed, and cleaned burners and flame sensor. Burners are rusted and corroded. Cleaned as best I could. Checked and adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain lines. Found areas of rust in burner compartment. Tightened clamp for drain line. No standing water found. Found major rusting in blower compartment as well. Preformed combustion test and found normal levels. No CO found in airstream. Replaced 201 filter with 213 filter and upgraded kit. System operating normal at this time. Average life of this unit is 14 years.