How to Clear Your Air Conditioner Condensate Line

Why this matters

When your air conditioner cools the air, it also pulls water out of it. That water drips off the indoor coil into a pan and runs away through a small plastic pipe, the condensate line, to the outside, a floor drain or a small pump. On a humid summer day a typical home system can make several gallons of it. The line is narrow, the water is warm and the inside of the pipe is dark, so slime and algae grow in it, and sooner or later it clogs. Then one of two things happens: a safety switch shuts the air conditioner off, or, on a system without one, the pan overflows onto the furnace, into the ceiling or across the floor. Clearing a slow or blocked line is usually a ten-minute homeowner job, as long as you know which situations are not.

Before anything else: if water has reached the furnace's wiring or controls, is dripping from a ceiling under an attic unit, or is pooled around the equipment on the floor, turn the system off at the thermostat, keep clear of the wet area, and call your heating and cooling contractor rather than working the drain. If the floor around the unit is wet and the unit's power switch is in the wet area, leave it and turn the system off at the breaker instead. See related: How to Shut Off Power at Your Electrical Panel.

The parts you are working with

 SIDE VIEW: AIR CONDITIONER COIL ABOVE A FURNACE

 +--------------------------------+
 |  INDOOR COIL: cools the air,   |
 |  water drips off it            |
 |  =========DRAIN PAN==========  |
 +---------------------------+----+
                             | pan outlet
                             |
                  [CAP] -----+  CLEANOUT TEE: cap
                             |  comes off, cleaner
                             |  is poured in here
       [FLOAT SWITCH]--------+
       turns the AC off if   |
       water backs up        U  TRAP: a bend that
                             |  holds water as a seal
                             |
                             +-----> DRAIN LINE out to
                                     an outside wall, a
                                     floor drain or a
                                     condensate pump

The order of the tee, the float and the trap varies from one installation to the next, and some systems have the float switch in the drain pan instead of the line. On an attic or closet unit there is often a second, shallow pan under the whole unit with its own float switch or its own drain line ending somewhere you can see, often over a window. That second pan should always be dry.

The float switch is why your air conditioner may have quit. It is a small switch with a float inside. When the drain backs up and the water rises, the float lifts and the switch cuts power to the cooling. The thermostat may still show it calling for cool. Nothing is broken: the switch is doing its job, and once the water drains, it drops back and lets the system run.

Step 1: Turn the system off, twice

Set the thermostat to off. Then turn off the service switch at the indoor unit, which looks like an ordinary light switch on or near the furnace or air handler.

Why both: the thermostat stops the cooling call, and the service switch makes sure nothing restarts while the cap is off. With the blower running, an open cleanout tee can pull air and splash water, and you are working with water a short distance from the unit's wiring.

Step 2: Look before you open anything

Check the drain pan if you can see it through an access panel or from the side, the secondary pan if you have one, and the floor.

Stop and call your contractor if:

• The pan is full to the brim or overflowing. Opening the tee at that point can let water run where it should not, and a pan that full often means more is wrong than a clog.
• The pan is rusted through, has holes, or is leaking at its seams.
• The secondary pan has water in it. That means the primary drain already failed and water climbed past it.
• There is a ceiling stain under an attic unit. See related: What a Ceiling Stain Can and Cannot Tell You.

What breaks if you skip it: a rusted-through pan or water in the secondary pan is a repair, and pouring cleaner into a line above a failed pan only adds to the water going where it should not.

Step 3: Open the cleanout and look at the water level

Take the cap off the cleanout tee by hand. Look in with a flashlight. Standing water near the top of the tee means the line is blocked downstream. A tee you can see down into, with water only at the bottom, means the line is probably flowing and the trouble is elsewhere.

If the cap is glued on, or there is no cleanout at all, do not cut into the pipe. Note it and ask your contractor to add one.

Step 4: Pour the cleaner your contractor recommends, slowly

Pour slowly into the tee. Many shops recommend plain white vinegar, often about a cup; some recommend a diluted bleach solution instead. Use the one your contractor names for your system, and use only one.

Never mix cleaners in the line. Bleach mixed with vinegar, or with ammonia-based cleaners, releases a toxic gas. If you are switching from one to the other, flush the line with plain water first, and if you do not know what was poured in last, ask your contractor before adding anything. Pour in a ventilated space and keep the cleaner off the coil, the furnace and your skin.

Give it time to work, commonly about 30 minutes, then pour a cup or two of plain water to flush it through.

Step 5: Check that the outside end runs

Go to where the line ends: a small pipe sticking out of the wall near the outdoor unit, a pipe over a floor drain, or a condensate pump, a small box with a reservoir. You should see the water you poured come out.

• Nothing comes out: the clog is still there. Many contractors suggest a wet/dry vacuum held tight over the outside end for a minute or so to pull the plug out. If that does not clear it, stop and call. Do not push wire or blow compressed air or gas into the line; pressure can blow apart the joints at the pan.
• It ends at a condensate pump: water should drain into its reservoir, and the pump should switch on when the reservoir fills. A pump with a full reservoir that does not run, or that smells hot, is a call for your contractor. Do not open the pump.

Step 6: Cap on, power back, and watch it work

Put the cap back on firmly. Check that the area around the unit is dry and nothing is dripping near the wiring. Then turn the service switch on, and set the thermostat back to cool.

Wait 15 to 30 minutes with the air conditioner running and check the outside end again. On a humid day you should see it dripping steadily. If the float switch had shut the system off, it should now be running and cooling.

If it shuts off again within the same day, stop clearing and call. The line is draining slowly for a reason a cup of vinegar cannot reach, such as a sagging section, a poorly built trap or a line that does not slope downhill.

Worked example: the July afternoon the cooling quit

The house: a basement furnace with the air conditioner coil on top, a cleanout tee with a cap on the drain line, a float switch just below it, and the line running out through the foundation wall near the outdoor unit.

3 p.m., 91 degrees outside. The house is warming up. The thermostat shows it calling for cool, but the outdoor unit is silent and no air comes out of the vents. The breakers are all on.

Step 1. Thermostat to off, then the service switch on the side of the furnace to off.

Step 2. Through the coil's access opening, the drain pan has about half an inch of water in it: not full, not rusted, no water on the floor. There is no secondary pan in a basement system like this one. Nothing in the stop list applies, so the work continues.

Step 3. The cap comes off by hand. Water is standing about an inch below the top of the tee. The line is blocked downstream, and the float, sitting in that same backed-up water, has lifted and shut the cooling off.

Step 4. Their contractor recommended vinegar at the last tune-up, so a cup goes in, slowly. The level drops very slowly. After 30 minutes it has dropped only a little.

Step 5. Outside, the end of the pipe is dry, with green slime at the opening. The homeowner holds a wet/dry vacuum hose tightly over it for about a minute. A plug of slime comes out, followed by a run of water. Back inside, the tee has drained down. He pours two cups of plain water and sees it come out outside.

Step 6. Cap on, floor dry, service switch on, thermostat to cool. The outdoor unit starts. Twenty minutes later the outside end is dripping steadily and cold air is coming from the vents. He writes the date and what he did in the house records and puts a reminder to pour a cup of vinegar each month through the cooling season, which is what the contractor had suggested.

What would have changed it:

• If the pan had been full to the brim, Step 2 would have stopped him, and the contractor would have been the first call that afternoon.
• If this had been an attic unit with water in the secondary pan, the primary drain would already have failed far enough for water to climb past it, and that is also a stop at Step 2, with a look at the ceiling below.
• If the float switch had shut it off again the same evening, a second clog in a single day points to a line with a slope or trap problem, and that is the call in Step 6.

Keeping it from happening

• Change the filter on time. A clogged filter can let the coil freeze, and when the ice melts it can send more water to the pan at once than the line can carry. See related: How to Change Your Furnace Filter.
• Pour your contractor's cleaner on a schedule through the cooling season, if they recommend one.
• Know where the outside end is and glance at it on hot days: a steady drip while the air conditioner runs is what healthy looks like.

References

• See related: HVAC Condensate Management Reference
• See related: How to Change Your Furnace Filter
• See related: Normal and Not Normal Sounds From Your Heating and Cooling
• See related: When to Stop and Call a Licensed Pro Around the House