Air handlers and furnaces aren't often installed together in San Antonio. If you have a furnace, you most likely shouldn’t need to think about an air handler.
Air handlers are typically used with heat pumps and deliver airflow throughout your residence. Some models also offer secondary heating and cooling parts to help the heat pump.
A furnace heats in a different manner by blowing heated air into your vents, which move it throughout your house. Since furnaces rely on combustion chambers to create heat, they don't use some of the parts you'll see in a standard air handler.