Tested heating equipment. Measured temp rise and recorded electrical loads. Cleaned flame sensor and tested ignitor. Inspected blower motor. Replaced filters. The system is running too hot due to lack of airflow, the returns are too small but the returns are custom. The flue vent is bolted into the blower cabinet so the heat exchanger was not checked, the most likely areas to fail on this heat exchanger would be all the way on the bottom near the coil. Recommending some high level Co detectors for the home just as a precaution.