A gas furnace is comprised of a heating device as well as an air circulating fan. Depending on which direction the furnace fan blows, as well as how the furnace is installed, describes the unit’s configuration type.
There are three configuration types that furnaces are available in: up-flow, down-flow and horizontal-flow. Depending on how your furnace will be installed (or what configuration type of furnace you currently have if you are replacing one) will determine which configuration type you will choose for your new furnace.
Looking at the pictures below and selecting which is most similar to how your furnace will be installed will help you determine which configuration type of furnace to select.
Up-Flow Split System (often installed in a basement)
Horizontal-Flow Furnace (shown installed in an attic)
Horizontal-Flow Furnace (shown installed from roof or floor joists such as in a crawl-space)
Down-Flow Furnace (often installed in a closet)