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 usually installed together in Boise. If you have a furnace, you likely shouldn’t need to think about an air handler.

Air handlers are usually used with heat pumps and control airflow throughout your home. Some models also have secondary heating and cooling parts to assist the heat pump.

A furnace works in a different manner by sending warm air into your vents, which move it around your residence. Since furnaces depend on combustion chambers to create warmth, they don't need some of the parts you'll see in a regular air handler.