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 control airflow throughout your home. Some models also offer secondary heating and cooling pieces to help the heat pump.
A furnace runs in a different manner by blowing warm air into your vents, which move it throughout your residence. Since furnaces rely on combustion chambers to produce hot air, they don't need some of the parts you'll spot in a regular air handler.