The cross-sectional area of a flue serving a solid-fuel-burning appliance shall be not greater than __________ the cross-sectional area of the appliance flue collar or flue outlet.

The correct answer is that the cross-sectional area of a flue serving a solid-fuel-burning appliance shall not be greater than three times the cross-sectional area of the appliance flue collar or flue outlet. This regulation is in place to ensure that the flue has sufficient size to facilitate proper draft while preventing excessive down-draft conditions that could lead to inefficient operation or even backdrafting of combustion gases into the living space.

Having a flue that is too large can result in insufficient heat and smoke rise, potentially allowing for the buildup of unburned gases, which can be dangerous. The threefold limitation represents a balance that allows for adequate airflow while maintaining a safe and efficient system. It is an important consideration in the design and installation of solid-fuel appliance venting systems, as it helps minimize risks associated with fire and carbon monoxide exposure.

This guideline aligns with standards defined in the mechanical code, ensuring that installations follow best practices for safety and efficiency.