What is one common cause of an HVAC system overheating?

A common cause of an HVAC system overheating is a clogged air filter or restricted airflow. When an air filter becomes dirty or blocked, it dramatically reduces the flow of air through the system. This restricted airflow leads to inadequate heat exchange within the unit, resulting in the HVAC system working harder to maintain the desired temperature. As the system struggles to pull in sufficient air, it can overheat due to the continual operation without relief, which may eventually trigger safety mechanisms or lead to component failures.

The other options do not directly relate to the immediate cause of overheating. Excess refrigerant levels can lead to performance issues, but it's typically related to cooling efficiency rather than direct overheating of the system itself. Overly large duct sizes could lead to reduced air velocity, impacting efficiency, but not directly causing overheating. Frequent thermostat adjustments can result in inconsistent comfort levels but do not contribute to the overheating of the system itself.