Air handlers and furnaces aren't typically installed together in Boise. If you have a furnace, you likely won’t need to think about an air handler.
Air handlers are usually used with heat pumps and control airflow throughout your residence. Some models also have secondary heating and cooling pieces to assist the heat pump.
A furnace works in a different way by pushing heated air into your vents, which send it across your house. Since furnaces depend on combustion chambers to create warmth, they don't use some of the parts you'll see in a regular air handler.