Clearing a clogged AC condensate drain line

Your air conditioner yanks more moisture out of the air than you'd think. On a sticky day we're talking gallons. That water drips off the indoor coil into a pan and exits through a small drain line, usually 3/4-inch PVC. Over a season, algae and sludge build up until the thing plugs solid.

Signs the drain is clogged

• The AC stopped cooling but the thermostat still has power. Most systems have a float switch that kills the AC when the pan fills up.
• Standing water in the drain pan or around the indoor unit.
• Water stains on the ceiling below 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 by the foundation. Near the indoor unit you'll usually spot a vertical tee with a cap. That's your access port.

2. Vacuum it out from the outside end

Pull the filter from your shop vac, or switch it to wet mode. 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. What comes out won't look good.

3. Flush from the inside

Open the access tee and pour in a cup of distilled white vinegar. Give it 30 minutes, then chase it with a few cups of water and make sure 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's gone. Turn the system back on and verify 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.
• Stay on top of your air filter changes. Dust that gets past the filter feeds the sludge.
• If your line has no access tee, adding one is a cheap PVC project that turns this into a five-minute job next time.