Which structure is responsible for directing flue gases from the boiler to the outside atmosphere?

The structure responsible for directing flue gases from the boiler to the outside atmosphere is the chimney or breeching. This component plays a crucial role in safely venting the combustion gases produced during the heating process. Proper venting is essential not only for the efficient operation of the boiler but also for ensuring safety by preventing the buildup of toxic gases within the building.

The chimney provides a pathway for the flue gases to exit the boiler room and is typically designed to create sufficient draft, ensuring that gases are expelled effectively and do not leak back into living spaces. Breeching refers to the ductwork that connects the boiler to the chimney, and it often includes features that allow for maintenance and inspection.

Other components listed, such as the heat exchanger, condensate return line, and blowdown tank, have distinct functions within the boiler system. The heat exchanger is responsible for transferring heat from the combustion gases to the water or steam within the boiler. The condensate return line is used to transport condensate water back to the boiler for reuse, while the blowdown tank collects sediment and contaminants removed from the boiler water. These components do not serve the function of venting flue gases, which is why they are not the correct choice in this context.