On the interior, warm, humid air from your home’s interior is blown through the evaporator coil. The cold coil absorbs heat from the air, cooling it, before the air is circulated back into your home. The humidity in the air condenses on the cool surface of the evaporator coil as liquid water, dripping into a pan below. From the pan, the water flows into a drain tube which is typically routed into a basement floor drain, utility sink, or outdoors.
Over time, algae and mold can build up and potentially plug the drain, so if the drain is either not flowing or flowing very slowly, it will need to be unplugged. A plugged drain can either cause damage by flooding onto the floor or, if the system is equipped with a drain float, cause the system to stop cooling in order to avoid flooding.
First, find the drain line where it leaves the evaporator coil enclosure. The drain is usually a one-inch PVC pipe (white, grey, or black). Follow it to the end where it drains. Often the line drains outside near the condenser unit, but it can also drain into a utility sink or basement floor drain or, in the case of attic units, down an outside wall.
Once located, use a wet/dry vacuum to clear the drain. It’s best to remove the paper filter from the wet/dry vacuum so as not to ruin the filter. Hold the hose of the wet/dry vacuum to the end of the drain line. You can use duct tape or simply hold a rag around the gap. Turn on the vacuum for 2-3 minutes then turn off. This will clear the drain of any growing biological matter.