https://www.serviceexperts.com/faq/what-is-the-difference-between-an-air-handler-and-furnace

Air handlers and furnaces aren't typically installed in the same home in North America. If you have a furnace, you most likely won’t need to think about an air handler. Air handlers are frequently paired with heat pumps and manage airflow across your residence. Some systems also provide extra heating and cooling pieces to support the heat pump. A furnace heats in a different manner by pushing heated air into your vents, which send it across your residence. Since furnaces depend on combustion chambers to produce heated air, they don't have some of the parts you'll see in a standard air handler.

What is the Difference Between an Air Handler and Furnace?

Air handlers and furnaces aren't often found side by side in Nashville. If you use a furnace, you likely don't need to consider an air handler.

Air handlers are frequently paired with heat pumps and control airflow across your home. Some models also provide backup heating and cooling components to support the heat pump.

A furnace runs in a different manner by sending hot air into your vents, which move it throughout your house. As furnaces have combustion chambers to create hot air, they don't need some of the pieces you'll see in a normal air handler.