What is a condensate drain line?

Start at the indoor unit, the furnace or air handler, and look for a white plastic pipe leaving the drain pan below the coil. A foot or two along it sits a T-shaped fitting with a removable cap: that is the cleanout, and it is where the vinegar goes. Follow the pipe the other direction to its exit, usually a stub near the outdoor condenser or under an eave. Attic air handlers get a second, higher line called the secondary or emergency drain, which dumps somewhere obvious like over a window. Where the air handler sits below the drain, a condensate pump the size of a shoebox pushes water uphill through clear vinyl tubing.

The line has one job: move several gallons a day of condensation out of the building. That water is warm, dark, and constant, which is an excellent recipe for algae and biofilm, and the slime is what plugs the pipe. Once it clogs, the pan fills and overflows, and in an attic or a closet above finished space the water goes straight into the ceiling below. Most systems built in the last 20 years carry a float switch for exactly this reason: a sensor in the pan or line that cuts the thermostat signal and stops the AC before the pan spills. An air conditioner that quietly quits on a 95-degree afternoon with no other symptom is very often a full drain pan, not a dead compressor. Ceiling, insulation, and drywall repairs after an overflow commonly land at $1,500–5,000.

Flush the line every spring before the first hot week, and again in midsummer along the Gulf Coast, in Florida, or anywhere the AC runs six months straight. Pour a cup of distilled white vinegar into the cleanout, let it sit 30 minutes, then chase it with warm water. Skip bleach, which is hard on metal pans and glued joints. If flow is slow, hold a wet/dry vac on the outdoor end for 2 minutes to pull the plug out. Timing lives in how often to clean the condensate drain line, and the seasonal version is on the condensate line checklist.