____________ are appliances that are constructed and installed so that all air for combustion is derived directly from the outside atmosphere and all flue gases are discharged outdoors.

Direct vent appliances are specifically designed to draw all necessary air for combustion directly from the outside atmosphere. This design means that these appliances do not rely on the indoor air, which is essential for safe and efficient operation. By sourcing combustion air externally, direct vent appliances minimize the risk of exhausting indoor air and improving indoor air quality.

Moreover, direct vent systems have a dual-chamber construction that allows for the intake of fresh air and the exhaust of flue gases, ensuring that all combustion byproducts are expelled outdoors. This creates a sealed system that enhances safety, as it prevents dangerous gases, such as carbon monoxide, from entering the living space.

In contrast, ventless appliances do not require an external venting system and can potentially introduce combustion gases into the home, which can be hazardous. Standard appliances may not have the safety and efficiency characteristics of direct vent systems, and zero-clearance appliances refer to those that can be placed directly against combustible materials, but do not necessarily involve direct venting from the outside atmosphere.