A horizontal furnace, commonly referred to as a horizontal delivery system, is a heating system that pulls cool air in from the left or right side, heats it, then vents it out the opposite side. The main advantage of a horizontal furnace is that it has a low height profile.
This low vertical clearance makes horizontal furnaces well-suited for installation into small residential areas like an attic or basement crawlspace. They are also able to be mounted across roof or floor joists, as the weight of the unit is distributed evenly over a horizontal surface.
Typically, horizontal furnaces are divided into "left side" and "right side" models, depending on which side the heat output is on. Some units are also configurable as either "upflow" or "downflow" models that can also double as a horizontal heat flow system. The most universal models are "multipoise" or multi-purpose furnaces, which can be configured to flow in any straight-flow position, either horizontal or vertical.
"Upflow" furnaces draw cool air in from the bottom of the unit and blow heated air out the top. They are installed in basements to take advantage of the thermal dynamic that causes warm air to rise. These types of furnaces are usually designed as either "lowboy" or "highboy" models, depending on the vertical height of the unit.