Air handlers and furnaces aren't typically installed side by side in Louisville. If you use a furnace, you most likely won’t need to consider an air handler.
Air handlers are usually linked with heat pumps and manage airflow across your home. Some models also provide backup heating and cooling components to help the heat pump.
A furnace works in a different manner by pushing heated air into your vents, which circulate it around your house. As furnaces rely on combustion chambers to make warmth, they don't use some of the pieces you'll find in a regular air handler.