What are the four common designs of furnaces?

The identified answer highlights four significant designs of furnaces that are commonly used in heating systems. Among these, upflow, downflow, horizontal, and multipositional designs refer to how air is directed through the unit and how the furnace is positioned within a space.

Upflow furnaces push warm air upward into the ductwork, which is ideal for basements and crawlspaces where heat rises naturally. Downflow furnaces, on the other hand, pull air in from above and direct it downward, making them suited for applications where air needs to be delivered to a lower level. Horizontal designs are often employed in tight spaces, allowing for installation in attics or other areas with low-clearance ceilings, while multipositional designs offer flexibility, as they can be configured for upflow, downflow, or horizontal applications depending on the layout of the home.

In contrast, the other options either include terms that are less commonly associated with furnace designs or combinations of characteristics that do not accurately reflect how traditional furnaces operate in residential or commercial settings.