Furnace clean and tune. Cycled system. Checked heat exchanger. Found both chambers are rusted and damaged. Burners are worn. Ignition system is worn and damaged as well. Checked and adjusted gas pressures to maximize efficiency. Checked airstream for CO, none found at this time. Average life expectancy for this unit is 14 years. This system is well over that time period. Failure is expected. Parts for this unit will be difficult to find. Second opinion is recommended. Job complete.