Build a sheet metal box, put two holes in opposite ends of the box, then put a fan in the box. This is an air handler in basic form. Add a hot water coil, and it becomes a fan-coil for hydro-air application. Add an evaporator coil and it becomes the indoor unit for central air or a heat pump. Put electric heating elements, circuit breakers and controls and it becomes an electric furnace. By today’s tradesman lingo, an air handler usually refers to a unit in use for central air conditioning or for a hydro-air system. Without the risk of fire from combustion or high temperature heating elements. An air handler offers the versatility of safe installation when in contact with combustibles, and multi-position application. Most manufactures build a machine that can be installed for vertical, horizontal, or counterflow (downflow) use.