Air handlers and furnaces aren't often used side by side in Philadelphia. If you have a furnace, you probably don't need to spend time thinking about an air handler.
Air handlers are frequently linked with heat pumps and manage airflow around your residence. Some units also have extra heating and cooling components to assist the heat pump.
A furnace works in a different way by pushing heated air into your vents, which send it around your house. Because furnaces depend on combustion chambers to produce hot air, they don't need some of the parts you'll see in a regular air handler.