What free area is required for openings when air is taken from outdoors for appliances in confined spaces?

The requirement for free area in openings when air is drawn from outdoors for appliances in confined spaces is critical for ensuring proper ventilation and safe operation of combustion appliances. The guidelines designate specific ratios of free area based on the orientation of the ducts and the British Thermal Units (BTUs) of the appliance being supplied.

In the context of the correct answer, vertical ducts require 1 square inch of free area for every 4000 BTU of input. This smaller ratio reflects the vertical configuration's reliance on natural convection and slightly less airflow resistance compared to horizontal ducts. For horizontal ducts, the requirement is set at 1 square inch for every 2000 BTU, which is beneficial as it allows for more efficient air intake due to the effects of gravity and potential blockages that may hinder airflow.

This configuration supports the safe operation of the system by ensuring there is enough supply of outdoor air to mix with the combustion gases, thereby enhancing efficiency and reducing risks such as backdrafting or accumulation of harmful gases.

The other options outline different ratios, which do not align with the established regulations for ventilation and air intake requirements in confined spaces. Thus, while they imply different air intake capacities based on BTUs, they do not provide the proper measurements needed to maintain