Which of the following will cause excessive temperature rise on a forced-air heating system?

A dirty air filter is significant in causing excessive temperature rise in a forced-air heating system because it restricts airflow. When the filter becomes clogged with dust and debris, the system struggles to draw in air. As the blower motor continues to run, it increases the temperature of the air that is being circulated because of insufficient air moving through the heat exchanger. This can lead to overheating and can potentially cause damage to the furnace if the issue is not addressed, as the safety mechanisms may fail to engage properly.

In contrast, while an oversized furnace can lead to short cycling (the furnace turns on and off too frequently), it does not directly cause the same kind of excessive temperature rise due to airflow restrictions as a dirty filter would. Improper ductwork can restrict or impede airflow, but its impact varies based on the severity of the issues involved (like leaks or sizing problems). A low refrigerant level is related to cooling systems rather than heating systems and would not apply in this context.