What does a bright red or orange glow from the resistance elements in a duct heater indicate?

A bright red or orange glow from the resistance elements in a duct heater is indicative of excessive heat, which usually means that the heater is not receiving sufficient airflow. When airflow is insufficient, the resistance elements can overheat, causing them to visibly glow. Proper airflow is essential for duct heaters to operate efficiently; it helps to distribute the generated heat without causing the elements to reach dangerous temperatures.

On the other hand, line voltage being insufficient would affect the performance of the heater but would not typically cause the elements to glow brightly. Oversized supply wires could lead to various electrical issues, but they would not cause the elements to glow in the manner described. Similarly, if duct heaters are operating properly, you would expect them to maintain a consistent temperature without excessive glowing, as airflow would be adequate to manage the heat generated.