What is a significant downside of using a hot surface igniter as a flame sensor?

Using a hot surface igniter as a flame sensor presents a significant downside because these igniters can accumulate a silicon oxide layer, which interferes with their functionality. This build-up acts as an insulator, reducing the sensitivity and effectiveness of the flame sensor. A properly functioning flame sensor needs to detect the presence of a flame reliably, and any insulating layer can prevent the sensor from confirming that the flame is active. Thus, the presence of this insulating layer can compromise safety and efficiency in gas appliance operation.

The other options describe potential issues related to hot surface igniters, but they do not address the unique impact of the silicon oxide layer on operational efficiency and safety in the same way. For instance, while they can indeed burn out quickly, this does not directly affect their role as a flame sensor. Similarly, requiring constant voltage and susceptibility to physical damage are valid considerations, but they do not pose as direct an operational impairment as the insulating layer does regarding flame detection.