What feature characterizes an upflow furnace?

An upflow furnace is characterized by its design, which facilitates the movement of air through the system. In this type of furnace, the return air enters from the bottom and the heated air is distributed at the top. This configuration promotes efficient airflow, as cooler air naturally rises to the top when heated, making the system effective for heating spaces. The design is typically utilized in basements or lower levels of buildings, where the furnace is located below the area being heated.

The other options do not accurately describe the function or design of an upflow furnace. For instance, having the return air at the top would indicate a different airflow pattern, which is not characteristic of upflow systems. Similarly, outdoor installation is not a defining feature of upflow furnaces, as they can be installed indoors. Ductless heating systems represent a different technology altogether, involving different operational principles and setups compared to traditional furnace designs.