Performed routine maintenance inspection on 16 y/o Trane system that operates for whole house. Furnace located in attic. Checked amp draws. Cleaned flame sensor. Cleared pressure switch tubing. Noticed poor combustion. Heat exchanger is in poor condition. Replaced Model 501 media filter. Water protection is poor. Gas line is incorrect as flex line is entering furnace. Drain line was installed incorrectly as system is positive pressure and does not require trap. Flue venting is not properly sized and incorrect. Blower wheel cleaning needed. Evaporator coil has biological growth presence. Recommend UV BlueTube to target and prevent growth. Pressure levels were normal. Condenser electrical components are fair. Coils are fair. Informed customer of all findings.