These are the fan boxes that condition the air that is being delivered to a residential or commercial space. These units come in sizes as small as a toaster oven on up to units that several stories high and bigger than a house. They consist of 3 principal components they are a fan, a filter and a coil all mounted in a box or some type of enclosure. There may be a cooling coil or a heating coil or both. Sometimes the cooling coil is also used as the heating coil too. Each coil might have several layers of copper tubing between the fins. These are called decks. The final design depends on the requirements of the space to be supplied with the air. I have worked on units as big as 88,000 CFM (Cubic Feet of Air Per Minute). These units had squirrel cage fans so large in diameter I could stand inside without my head touching the top of the fan cage. Others I have worked on are mounted in small boxes that are hung from the ceiling. These are very popular for use in small business establishments in split type AC systems. You can see a slide show on how I rebuilt the air handler in the photo above.