The condensate drain system removes water that drips off the evaporator coil during the dehumidification process. As warm humid air passes over the cold coil, moisture condenses (similar to a cold drink sweating on a humid day) and collects in a drain pan beneath the coil.
A typical residential system removes 5-20 gallons of water per day in humid conditions. This water flows through a condensate drain line (typically PVC pipe) to a floor drain, utility sink, or outdoors. High-efficiency furnaces also produce condensate from the secondary heat exchanger.
A clogged condensate drain line is one of the most common HVAC service calls. Algae growth, dirt, and debris block the line, causing water to back up and overflow the drain pan. Consequences include water damage to ceilings and walls, mold growth, and system shutdown (many modern units have a safety float switch that shuts off the system when the pan overflows). Preventive treatment with condensate drain tablets or annual line flushing prevents clogs.