Furnace clean and tune. Cycled system. Checked heat exchanger, ok. Checked, removed, and cleaned burners and flame sensor. Checked and adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain lines. Found standing water in burner and blower compartment. Found water was leaking from exhaust venting. Checked glued joints and found several joints have come loose. Re-glued and sealed venting connections. No more leaks found at this time. Preformed combustion test and found normal levels in exhaust. No CO found in airstream. System operating normal at this time. Job complete.