What does a glowing red or orange light on a resistance element in a duct heater indicate?

A glowing red or orange light on a resistance element in a duct heater typically indicates the element is operating under normal conditions. In this state, the resistance heating element is functioning correctly and generating the heat necessary for the air being heated before it enters the living space. This is a typical operational characteristic for electric resistance heaters, where the elements will glow when they are active and producing heat.

The other choices present scenarios that would suggest a problem. For instance, insufficient airflow would lead to the heating element overheating or even tripping safety mechanisms, but this does not correlate with a glowing element that's functioning smoothly. Likewise, overheating or a failure of temperature control would also not indicate normal operation. Therefore, a glowing red or orange light is a sign that the heater is doing its job effectively, signifying that the system is in a state of normal operation.