Air handlers and furnaces aren't typically used side by side in Columbia. If you have a furnace, you probably shouldn’t need to consider an air handler.
Air handlers are frequently paired with heat pumps and manage airflow around your house. Some systems also have secondary heating and cooling components to help the heat pump.
A furnace works differently by sending heated air into your vents, which circulate it around your home. As furnaces have combustion chambers to make hot air, they don't have some of the pieces you'll find in a normal air handler.