What type of appliance derives all combustion air from the outside atmosphere?

A direct vent appliance is specifically designed to draw all of its combustion air from the outside atmosphere, making it distinct from other types of appliances that may use air from inside the structure. This feature is critical for ensuring safe operation, especially in tightly sealed buildings, where indoor air quality and combustion efficiency can be negatively affected if an appliance tries to source air from the living space.

Direct vent appliances utilize a two-pipe system: one pipe for intake of outside combustion air and another for exhausting combustion gases. This design not only enhances efficiency but also minimizes the risk of backdraft or improper ventilation that can occur if an appliance relies on indoor air.

In contrast, a vented appliance might draw combustion air from inside; non-vented appliances do not vent combustion gases at all, which could lead to dangerous indoor air quality issues, and standard appliances may have varying configurations that don't necessarily guarantee an outside air source for combustion. Thus, the direct vent appliance's reliance solely on outside air makes it the correct answer to the question.