The air handler is the part of your central heating system that does all the physical work of keeping your home or business at a comfortable internal temperature. It consists of a blower, and heating and cooling mechanisms housed in a sheet metal frame. There may be other important parts of the cooling process housed outside the air handler. There can be cooling coils or heaters mounted in the ductwork, but most ventilation units use the air handler primarily, and all use it to push air. 

The sheet metal housing is called a air handler cabinet. The metal is usually thick and rust-resistant. Some air handlers are modular and can be reassembled on site, with a different flow direction. It is not recommended to use the heavy metal for structural stability, but the cabinet should hold its weight by itself. When working on air conditioning services in Denver, I noticed that these multi-flow air handlers are very useful because they can be refitted for a new job without replacing the whole unit.

The point at which the duct work meets the air handler is a critical junction in your ventilation system. Use sheet metal ductwork. If the HVAC unit also does heating, nonmetal ducts will not pass building codes. Your HVAC tech should know the exit temperature of the heated gases, and plan the ductwork so that it will not ignite combustible materials. The ducts should be sealed to reduce heat loss and make sure they do not sweat. Unsealed ducts can bead with moisture at temperature variations. Over time this could cause mold or water damage to your building. Moisture also degrades the insulation which will make your heating progressively less efficient. 

The thermostat is where the user programs in how they want the central heating system to work, but the air handler control unit is where all the processes actually start. The control unit coordinates all the inputs from the thermostat and sensors in the HVAC system, and then sends instructions on when to start and stop to the air handler. The transformer has to step the electricity down to a lower voltage in order for it to be used. On older un-fused units, you could blow the air handler if you do not turn off the power to the air handler before working on the system.