Which observation indicates insufficient airflow through an electric heater?

The observation that indicates insufficient airflow through an electric heater is when the heater elements are glowing red or orange. This glowing signifies that the heater is overheating due to a lack of adequate airflow. Electric heaters are designed to operate with a specific airflow to ensure effective heat dissipation. When airflow is insufficient, the heat generated by the elements cannot be effectively carried away, leading to elevated temperatures. As the elements overheat, they can glow brightly, which is a clear indication of potential operational issues, including risks of damage or failure if the problem persists.

Other observations may indicate different issues. A buzzing noise could suggest electrical problems or vibrations but doesn't necessarily relate to airflow. Operating at low volume might refer to noise levels rather than airflow efficiency. Intermittent shut-off could be a protective mechanism resulting from overheating, but it may not directly reflect airflow issues. The glowing of the heater elements stands out as a direct sign of insufficient airflow.