Which term describes a system that draws all combustion air from outdoors and vents all flue gases outdoors?

Which term describes a system that draws all combustion air from outdoors and vents all flue gases outdoors?

• A. Direct-vent appliance
• B. Category III appliance
• C. Natural vent appliance
• D. High efficiency furnace

Answer: (A)

Direct-vent appliances pull all combustion air from outside and vent all combustion gases outdoors. This sealed-combustion setup keeps indoor air out of the combustion process and uses a dedicated intake and exhaust, often with coaxial or twin-pipe venting. That exact arrangement is what defines a direct-vent system, making it the correct term for a setup that takes outdoor air for combustion and exhausts everything outside. Natural-vent appliances rely on room air for combustion and vent through a flue by natural draft, so they don’t draw outdoor air for combustion. High-efficiency furnaces are commonly direct-vent, but the term emphasizes efficiency rather than the air-source arrangement. Category III describes a venting category involving mechanical draft, but it doesn’t specify that all combustion air is drawn from outdoors in the same explicit way.

Direct-vent appliances pull all combustion air from outside and vent all combustion gases outdoors. This sealed-combustion setup keeps indoor air out of the combustion process and uses a dedicated intake and exhaust, often with coaxial or twin-pipe venting. That exact arrangement is what defines a direct-vent system, making it the correct term for a setup that takes outdoor air for combustion and exhausts everything outside.

Natural-vent appliances rely on room air for combustion and vent through a flue by natural draft, so they don’t draw outdoor air for combustion. High-efficiency furnaces are commonly direct-vent, but the term emphasizes efficiency rather than the air-source arrangement. Category III describes a venting category involving mechanical draft, but it doesn’t specify that all combustion air is drawn from outdoors in the same explicit way.