Drain pans are installed underneath an air handler or furnace installed in an attic or basement. Its purpose is home safety: in the event of an overflow or leak water due to a malfunction of the equipment, the water will be caught in the pan and drained away through a drain hose. This will keep the water from draining onto the ceiling below. Some cities require by code that this secondary drain pan be installed for all attic air conditioning, air handler or evaporator coil installations. Be on the safe side, and install a drain pan with a safety float switch in your home system.