What structures are designed to carry away combustion gases from fuel-burning appliances?

Chimneys and vents are critical structures designed specifically to carry away combustion gases that are produced by fuel-burning appliances. These gases can include products like carbon monoxide, nitrogen oxides, and water vapor, which can be harmful if not properly vented outside.

Chimneys typically serve to safely route these gases from the appliance to the outdoors, and they often provide a draft that helps to pull air into the combustion process, promoting efficient fuel burning. Vents are similar but can be more specific to certain appliances, such as direct-vent water heaters or gas fireplaces. Both components prevent the buildup of dangerous gases inside a building, ensuring that the indoor air quality remains safe for occupants.

Ventilation ducts, while related to air movement, are primarily used for distributing conditioned air (heated or cooled) throughout a space and do not serve the primary function of expelling combustion gases. Fuel gas lines are responsible for delivering gas to the appliances, and heat exchangers are components that transfer heat but do not vent gases. Thus, the specific function of carrying away combustion gases makes chimneys and vents the correct choice for this question.