Air conditioners dehumidify the moisture-filled air inside a room in addition to cooling it. Because of this, condensation collects inside the air conditioning unit and has to drain out. Both central air conditioners and room units have drain pans that catch this condensation and allow it to flow outside.
Central Air Conditioner
A central air conditioner is normally mounted in a chamber of the furnace above the furnace blower and combustion chamber. The unit's evaporator coils are located here, and directly underneath those coils is the drain pan. The moisture that the unit removes from the humid interior air condenses on the evaporator coils and drips into the pan. A pipe normally runs from this pan, leading downward to a floor drain. The condensation drains through the floor and eventually flows to the outside.
Room Air Conditioner
Unlike a central air conditioner, a room air conditioner, such as a unit installed within a window, has both an evaporator pan and a condenser pan for the moisture removed from the air. A room air conditioner contains both the evaporator and the condenser in its shell, unlike a central unit where the condenser is outside. The evaporator coils are normally located directly behind the room air conditioner's front grille. The water collects in the pan underneath those coils, and then drains through a tube toward the back of the unit into the condenser pan. From there, the water drains out the back of the air conditioner through a hole or a tube.
How the Water Forms
The moisture floating around within the humid room air is, of course, invisible. Once this moisture encounters the evaporator coils inside a central or room air conditioner, it condenses and transforms into liquid form. This liquid collects on the evaporator coils and eventually drips off, falling into the pan. This process allows the cooler, less humid air to flow out of the air conditioner and into the room.
Drain Pan Maintenance
Knowing where the unit's drain pans are located allows you to perform the necessary maintenance that is needed to keep the air conditioner operating properly. Because an air conditioner also removes dust from the air, the pans in both a central and a room unit collect this dust, which can clog the system that allows the water to drain out of the unit. For both central and room air conditioners, vacuum the drain pans and the evaporator coils regularly using a wet/dry vacuum. Vacuuming helps protect against the clogged water draining into spaces it shouldn't, such as when a window unit drains onto the carpet instead of draining outside.