Furnace clean and tune. Cycled system. Checked heat exchanger. Found both shots are rusted. No visible cracks at this time, but deep rust and etching found. Checked and cleaned flame sensor and burners. Found igniter worn. Checked and adjusted gas pressures to maximize efficiency. Found normal levels of Co And XS air in exhaust. No CO found in airstream. Found ventor motor bearings are worn and grinding during all portions of operation. Large rust spot from water leak found in burner compartment. System is operating safely at this time, but failure is expected. Second opinion is strongly recommended due to age, condition, and cost to repair. Failure is expected. Job complete.