What is permitted to convey combustion air within dwelling units?

Stud and joint spaces are permitted to convey combustion air within dwelling units because they allow for the movement of air between different areas of the home. These spaces, which are often found within the framing of walls, provide a pathway for air to circulate, enabling combustion appliances to receive the necessary air for efficient operation. Proper combustion air flow is essential for maintaining safe and effective appliance performance, preventing issues such as backdrafting or incomplete combustion.

In contrast, unobstructed doors and windows may provide some level of air movement but are not specifically designed or utilized for combustion air conveyance in the same way as stud and joint spaces. Wall cavities can also facilitate air movement, but they're not typically relied upon as a primary source for combustion air according to code standards, which leads to their exclusion as a suitable option in this context.