Air Flow Configuration
The term air flow configuration describes the physical orientation of a furnace relative to the direction in which heated air leaves the furnace and enters the ductwork.
The most common terms used to describe the air flow configuration are: upflow, downflow, and horizontal.
An upflow furnace may draw cool air through either its top or its base. The identifying feature of this furnace is that it pushes warm out out through the top of the furnace. An upflow configuration is typically selected when the ductwork is located above the furnace, such as in a home's basement.
Upflow furnaces can also be described as a "highboy" or a "lowboy". For the homeowner, the important distinction between the two is that the lowboy is shorter -- typically around 4 feet in height. The lowboy can be installed in low-ceiling basements. A highboy furnace is typically around 6 feet in height.
There is a second distinction between the two: the blower is installed below the heat exchanger in a highboy furnace and behind / next to the heat exchanger in a lowboy furnace.
A downflow furnace takes cool air from the top and discharges warm air from the bottom. A downflow furnace is typically installed when the ductwork is located below the furnace.
Downflow furnaces are sometimes called "counterflow" or "downdraft" furnaces.
A horizontal furnace lies on its side, pulling cool air from one side and pushing warm air out of the other. You will occasionally see a distinction between a "horizontal left side" and a "horizontal right side" furnace. This refers to configuring a furnace to discharge warm air from the right side (in a horizontal right furnace), or the left side (in a horizontal left furnace).
Because they don't require much vertical clearance, horizontal furnaces are used in locations where space is limited, such as in a basement or attic crawl space.
Upflow / Horizontal
An upflow / horizontal furnace can be configured as either an upflow furnace, or a horizontal (left or right) furnace. It cannot be installed as a downflow furnace.
Downflow / Horizontal
A downflow / horizontal furnace can be configured as either a downflow furnace, or a horizontal (left or right) furnace. It cannot be installed as an upflow furnace.
A "multipoise" or "multi-positional" furnace can be configured by the installing contractor in upflow, downflow, horizontal left or horizontal right positions. For this reason, some people refer to it as a "universal" air flow configuration.