Air Handlers – An air handler is usually located in the garage, a closet, or in the attic (unless you have a package unit then the air handler is incorporated in the system). It is essential to any AC unit and heating forced air system. All AC units need an air handler or way to move air. It can be a part of your furnace and houses the evaporator coils, the blower motor, and some HVAC controls. It can be an up flow, down flow, or horizontal flow AHU (air handling unit). To determine which one you have follow the return duct. The return duct should originate where you put the HVAC filter and/or is the biggest register grill in the house. If the return ends in the bottom of the unit it is an up flow. If the return duct ends in the top of the unit it is a down flow. If the unit looks as if it is lying on its side with the return duct coming in one side the supply ducts going out the other it is a horizontal flow.