Air conditioning and heat pump systems need drain pans to capture water created by condensation dripping off of evaporator coils. This water then flows through a drain line to remove it from inside the building. Drain pans are located in the air handler unit underneath the evaporator coils. Some HVAC units come with a secondary pan, known as the “auxiliary drain pan,” to provide another layer of protection from potential water damage. In fact, some building codes require secondary drain pans for HVAC air handler units located in the attic.