What can lead to flue blockage?

Flue blockage can occur due to several factors, but a closed damper or the presence of debris is a direct and common cause. When the damper is closed, it obstructs the chimney's airflow, preventing smoke and gases produced by combustion from escaping. This can lead to a buildup of dangerous gases in the home, including carbon monoxide.

Additionally, debris such as soot, creosote, or even animal nests can accumulate inside the flue over time. This buildup can restrict or completely block the passage, leading to reduced efficiency in the venting system and increasing the risk of hazardous situations, such as chimney fires or carbon monoxide poisoning.

Both the closed damper and debris directly impede the flow of exhaust gases, making this the most pertinent reason for flue blockages compared to the other options provided.