If the combustion air is supplied from the structure, what type of grills used must ensure adequate free area?

The correct choice is stamped metal grills. This type of grill is designed with specific patterns and perforations that enhance airflow while maintaining structural integrity. Stamped metal grills provide a larger free area compared to flat grills, which may restrict airflow due to their solid surface.

Stainless steel grills, while durable and resistant to corrosion, do not necessarily offer better airflow characteristics than stamped metal grills. Their design can be less conducive to natural airflow, depending on the configuration. Plastic grills, on the other hand, are typically not used for combustion air supply due to concerns related to durability and potential melting or warping under high temperatures.

In the context of ensuring adequate combustion air supply, it is critical to select materials that not only allow sufficient airflow but also withstand the conditions typically encountered in HVAC applications. Stamped metal grills achieve this balance effectively, making them the most suitable choice for this scenario.