Which term describes a conduit for removing combustion products to the outside?

The term that describes a conduit for removing combustion products to the outside is "flue." A flue is specifically designed to vent gases from a fireplace, furnace, or other combustion appliances to the outdoors, ensuring that harmful gases such as carbon monoxide do not accumulate indoors. This is crucial for maintaining indoor air quality and safety in buildings.

In contrast, while ducts are used for air movement in HVAC systems, they don't specifically refer to the removal of combustion products. A plenum is a space used to distribute air, typically in ventilation systems, but it does not serve the same function as a flue. Fenestration refers to the design or arrangement of openings in a building, such as windows and doors, and is unrelated to the removal of combustion gases. Therefore, "flue" is the most appropriate term in this context.