The water droplets seen near or on an air conditioner are a normal consequence of the cooling process. An air conditioner functions as a powerful dehumidifier while it cools, actively removing moisture from the air inside a structure. This moisture, naturally present as invisible water vapor, must be collected and routed away. The presence of liquid water shows the system is successfully performing its dual function of temperature control and humidity reduction.
The Science Behind Condensation
The formation of water droplets is governed by the dew point, which is the temperature at which air can no longer hold all of its water vapor and must release the excess as liquid. Warm air has a greater capacity to hold moisture than cold air. As the warm, humid indoor air is drawn into the air conditioning unit, it passes over an extremely cold surface. This surface, known as the evaporator coil, rapidly cools the air streaming across it. When the air temperature drops below its dew point, the water vapor condenses directly onto the cold metal surface, just like moisture forming on the outside of a glass of iced tea on a hot day.
The Internal Water Management System
The air conditioning unit is engineered with a dedicated system to manage the water produced by condensation. The evaporator coil, where the condensation occurs, is positioned directly above a sloped tray called the condensate pan. Water droplets drip down from the cold coil into this pan, which collects the water and channels it into a drain line, which is typically a plastic pipe. This drain line uses gravity to route the collected water safely away from the indoor unit, often leading it outside or into a designated plumbing drain. In some systems, particularly those installed in basements where the drain line must travel upward, a condensate pump actively moves the water out of the pan and into the drain line.
Why Droplets Appear on Exterior Surfaces
While condensation inside the unit is normal, water droplets appearing on exterior surfaces or leaking outside the designated drain area usually indicate a problem. One common cause is a blockage within the condensate drain line, often due to the buildup of algae, sludge, or debris. When the drain is clogged, the water backs up in the pan and eventually overflows, causing leaks near the indoor air handler or furnace.
External condensation can also form on refrigerant lines and air ducts that pass through unconditioned spaces, such as attics or crawl spaces. If the insulation surrounding the cold refrigerant lines or the chilled air ducts is damaged, missing, or inadequate, the surface temperature of the pipe or duct drops below the dew point of the surrounding air. This temperature difference causes moisture from the ambient air to condense on the exterior of the components, resulting in “sweating” and dripping water. Water droplets may also emerge from air supply vents if the unit’s cooling coil freezes due to low refrigerant or poor airflow, causing an excessive volume of water when the ice melts.