Air handlers and furnaces aren't often used together in Houston. If you use a furnace, you most likely won’t need to think about an air handler.
Air handlers are typically used with heat pumps and control airflow throughout your home. Some systems also provide backup heating and cooling components to assist the heat pump.
A furnace works in a different manner by sending heated air into your vents, which send it around your house. Because furnaces rely on combustion chambers to create hot air, they don't use some of the parts you'll spot in a standard air handler.