How often should you clean the AC drain line?

Short answer

Clean the AC condensate drain line every 1 to 3 months during cooling season. In a humid climate where the air conditioner runs most of the year, make it every month. The line is the thin pipe that carries water away from the indoor unit, and it clogs with slime that grows in the dark, wet pipe. A cup of vinegar poured in takes 10 to 15 minutes and costs $0 to $10.

Why it matters

An air conditioner cools the air by running it across a cold coil, and moisture in the air condenses on that coil the way it does on a cold glass. On a humid day that adds up to gallons of water, all of which drips into a pan under the coil and runs out through the drain line.

The inside of that line is dark, damp and warm, which is exactly what mold and algae like. They grow into a slimy plug. Water backs up into the pan, and when the pan overflows it goes wherever the unit is: onto a closet floor, through a ceiling from the attic, or down a wall. Many systems have a float switch that shuts the AC off when the pan fills, which saves the ceiling but leaves you with no cooling on a hot day. A regular flush keeps the slime from ever getting thick enough to plug the pipe.

When to do it more often

The slime grows whenever the line is wet, so the schedule follows how much the air conditioner runs:

A humid climate where the system runs most of the year: every month.
A unit in the attic, where an overflow does the most damage: stay at the monthly end even in a milder climate.
A line that has clogged before: it will clog again, so flush it monthly until a full season passes cleanly.

In a dry climate, or a home where the AC runs for a few weeks a year, the longer end of the range is enough.

What you need

A wet/dry vacuum, to pull a clog out from the outside end of the line
A funnel, to pour into the access opening without spilling
A flashlight, to find the access cap and check the drain pan
Distilled white vinegar

Steps at a glance

Turn the AC off at the thermostat.
Find the drain line at the indoor unit: a plastic pipe with a capped tee or a small vertical opening near where it leaves the unit.
Take off the cap and look into the pan with the flashlight for standing water.
Pour a cup of vinegar slowly into the opening and let it sit for a while to dissolve the slime.
If water doesn't drain, go outside to where the line exits the house, seal the vacuum hose to it, and pull the clog out.
Flush with water, replace the cap, and turn the AC back on.

You're done when water poured into the opening runs out the end of the line outside within a few seconds.

Signs it's overdue

Water around the indoor unit, on the floor beneath it or in a stain on the ceiling under an attic unit.
A musty smell when the AC runs, from the slime in the line and pan.
The AC shuts itself off and won't restart, which is the float switch doing its job.
Standing water in the drain pan when you look in.

DIY or pro

This is one of the easiest jobs in the house. There's nothing to take apart, and the vinegar does the work. The only skill is finding the access opening the first time, which the flashlight and a little patience solve.

Call a pro when the line is fully clogged and the vacuum won't clear it, when water has already come through a ceiling, or when the unit sits in an attic you can't safely reach. Clearing a clogged line costs $75 to $250. Once it's clear, a monthly cup of vinegar is what keeps you from paying that again.

FAQ

Can I use bleach instead of vinegar in the AC drain line?

Vinegar is the safer choice. Bleach fumes are unpleasant in a closed closet or attic, and bleach can corrode metal parts of the drain pan over time.

How do I know if my AC drain line is clogged?

Look in the drain pan for standing water, and pour a cup of water into the access opening. If it doesn't appear at the outside end within a few seconds, the line is blocked.

Where does the AC drain line go?

Usually to a pipe that exits the house near the outdoor unit, or into a floor drain or utility sink. Follow the pipe from the indoor unit to find its end.

Why does my AC keep shutting off on hot days?

A full drain pan trips the float switch and stops the system. Clear the line and the AC should run again.