Every time your air conditioner runs, it pulls moisture out of the air. On a humid day a typical home system can remove several gallons of water. That water drips into a pan under the indoor coil and leaves through a small pipe called the condensate drain line. When that line clogs, the water has nowhere to go.
What happens when the drain clogs
- Overflowing drain pan. Water spills into the closet, attic or ceiling below the air handler.
- Stained or sagging ceilings under an attic unit.
- Musty smells and damp insulation around the equipment.
- AC shuts off. Many systems have a float (safety) switch that turns the AC off when the pan fills. It protects the house, but you lose cooling until the drain is cleared.
The usual cause is a slimy buildup of algae and debris inside the line. Warm, wet and dark is exactly what it likes.
How to keep it clear
Before you start, turn the system off at the thermostat. If you're unsure about any step, call an HVAC professional.
- Find the access port. Most drain lines have a capped T-shaped fitting near the indoor unit. That's your cleanout.
- Flush it every 1 to 3 months during cooling season. Pour about a cup of warm water through the access port and confirm it drains freely. Many homeowners and technicians also use distilled white vinegar to slow algae growth. Check your equipment manual for what your manufacturer recommends.
- Check the outside end. Find where the line exits the house (often near the outdoor unit or a wall). When the AC is running, it should drip steadily on humid days. No drip on a muggy afternoon is a warning sign.
- Clear a slow line from the outside. A wet/dry shop vacuum held tightly over the outside end for a minute or two can pull out a clog.
- Look in the pan. Standing water in the drain pan means the line is already slowing down.
Add an early warning
A float switch protects the ceiling, but it won't tell you anything if you're not home to notice the AC stopped. A Wi-Fi leak sensor on the floor next to the air handler, or in the secondary pan, sends an alert to your phone the moment water shows up where it shouldn't.
Don't forget the air filter
A dirty air filter can make the indoor coil run too cold and freeze. When that ice melts, it can flood the pan faster than the drain can handle. Changing the filter on schedule protects the drain, too. See how often to change your air filter.
When to call a pro
Call a licensed HVAC technician if the line keeps clogging, the float switch keeps tripping, water is leaking from the unit itself, or you see ice on the refrigerant lines.