For combustion air provisions, what shape must the ventilation pipe be that is placed in an exterior wall of the furnace enclosure?

The correct answer is that the ventilation pipe must be round. Ventilation pipes are typically designed this way because round shapes allow for consistent airflow and pressure distribution throughout the pipe. The circular cross-section minimizes air resistance and turbulence, which can improve the efficiency of air intake for combustion processes. This efficiency is critical in ensuring that the furnace operates safely and effectively.

Other shapes, such as square, rectangular, or oval, may not provide the same level of airflow efficiency. While these shapes can be used in certain applications, they can lead to more friction and reduced airflow compared to round pipes. Additionally, the structural integrity and ease of installation can also be compromised with non-circular shapes, making round pipes the preferred choice for furnace ventilation in many instances. This is a reason why codes and standards for HVAC systems often specify round pipes for ventilation applications, particularly for combustion air.