What type of appliances derive all air for combustion directly from the outside atmosphere?

Direct vent appliances are specifically designed to take all the air they need for combustion directly from the outside atmosphere. This design enhances safety and efficiency, minimizing the risks associated with indoor air pollution and negative pressure created by other appliances.

These appliances have a dual venting system, featuring one pipe to bring in outside air for combustion and another to exhaust combustion gases back outside. This setup ensures that the appliance does not use any of the indoor air for combustion, allowing for safer operation in tightly sealed or energy-efficient homes where indoor air quality can be negatively impacted.

In contrast, conventional appliances usually rely on air from the surrounding indoor environment for their combustion process, which can lead to potential safety hazards. Indoor appliances can also draw air from within the space, and portable appliances may not have specific requirements for venting or air supply. Therefore, the unique design of direct vent appliances focused on external air sourcing is what makes them distinct and correct in this context.