What is an AC condensate line ? This line is an exit route for moisture collected by the evaporator coil as air passes over it. This coil is triangle shaped, with a drip pan at the bottom. It is out of sight and behind the ductwork just above your furnace. Its job is to remove humidity from the air and change it to water. This water collects in the drain pan, and is emptied out the condensate line down and out through the floor drain.