Furnace clean and tune. Cycled system. Checked filter. Found filter very clogged and dirty. Removed and customer will get new filter. Checked, removed, and cleaned burners and flame sensor. Checked heat exchanger, found all chambers to have some rust and corrosion. No cracks found at this time. Burners were very corroded. Sparker cleaned and checked. Found evidence of water leaking from ventor gasket. Did not find cracks in transition assembly. Ventor motor bearings sound intact and not worn at this time. Cleared drain line. Cleaned and checked blower compartment. System is 5 years old and operating normal at this time. Job complete. Filter is a 20x25x6. A 20x25x4 could be used. Set up for an air bear filter. Office to schedule ac clean and tune in spring.