How to clear a clogged AC condensate drain line

Your air conditioner pulls a surprising amount of water out of the air, and on a humid day that can be gallons. That water drips off the indoor coil into a pan and runs out a small drain line, usually 3/4-inch PVC. Over a season, algae and sludge grow in that line until it clogs.

Signs the drain is clogged

• The AC stopped cooling, but the thermostat still has power. Many systems have a float switch that shuts the AC off when the pan fills.
• Standing water in the drain pan or around the indoor unit.
• Water stains on the ceiling under an attic air handler.
• No water coming out of the outdoor drain end on a hot, humid day.

1. Find both ends

The line runs from the indoor unit to a floor drain, a condensate pump, or outside near the foundation. Near the indoor unit there's usually a vertical tee with a cap. That's your access port.

2. Vacuum it out from the outside end

Take the filter out of your shop vac, or use its wet setting. Hold the hose over the outside end of the drain line and seal the gap with a rag or tape. Run it for a minute or two. You'll often hear the clog break loose, and what comes out isn't pretty.

3. Flush from the inside

Open the access tee and pour in a cup of distilled white vinegar. Wait 30 minutes, then follow it with a few cups of water and confirm it flows out freely at the other end.

4. Reset and restart

Empty and dry the pan. The float switch should reset once the water is gone. Turn the system back on and check that water drips out the drain after 15 to 20 minutes of cooling.

Keep it from coming back

• Pour a cup of vinegar into the access tee every month or two during cooling season.
• Keep the air filter changed. Dust that gets past the filter feeds the sludge.
• If your line has no access tee, adding one is a cheap PVC project that makes this job five minutes next time.