What are two reasons flue gases may fail to rise in a chimney?

The choice that identifies two reasons flue gases may fail to rise in a chimney is accurate, as it recognizes critical issues related to airflow and pressure within the chimney system. A blocked chimney can impede the natural draft, preventing flue gases from escaping. This blockage might be due to soot accumulation, debris, or even bird nests, restricting the passage for gas to flow upward.

High negative pressure in the combustion appliance zone further complicates matters. When negative pressure is present, it can draw air from the chimney rather than allowing the hot flue gases to rise. This can happen due to factors like an unbalanced ventilation system or the operation of exhaust fans, which create a lower pressure that disrupts the natural upward flow of gases.

Together, these two factors can significantly obstruct the chimney's ability to ventilate properly, resulting in a failure of flue gases to ascend as intended.