Air handlers and furnaces aren't usually installed side by side in Fort Worth. If you have a furnace, you likely shouldn’t need to think about an air handler.
Air handlers are usually linked with heat pumps and manage airflow across your residence. Some models also provide backup heating and cooling parts to support the heat pump.
A furnace works in a different way by blowing hot air into your vents, which send it around your house. Since furnaces have combustion chambers to make warmth, they don't use some of the components you'll find in a regular air handler.