What is the purpose of a thimble in venting systems?

A thimble in venting systems serves primarily to provide a safe passage for flue gases from appliances, such as furnaces or water heaters, to the outside environment. It acts as a protective conduit that allows exhaust gases to exit the structure while preventing the ingress of combustible materials or the escape of harmful gases into the living space. This is crucial for maintaining safety and ensuring proper ventilation.

While the other options mention related tasks, they do not encompass the fundamental purpose of a thimble. Connecting ducts securely is an important feature in HVAC systems, but it is not the specific role of a thimble. Covering openings in walls and improving airflow efficiency are considerations in venting, but they are not the primary function of a thimble, which focuses directly on the safe and effective transport of flue gases. Thus, the correct option accurately captures the essential role of a thimble in venting systems.