As the air conditioner cools your home, humidity in the air condenses into water on the surfaces of the evaporator coil of your air handler (air conditioner indoor unit). The water drips down into a trough that drains into a drain line to the ground near an exterior wall of your home. Because the drain piping is open at one end to the interior of the air handler that is blowing air into the supply vents around the home, and at the other end to the outside, it will also blow conditioned air out the pipe to the exterior. Actually, depending on where the opening to the drain is located in the flow of air through the air handler, it may suck outdoor air into your home instead of blowing it out. Either way, it’s an energy waster.

   So a U-shaped “trap,” similar to the one under the sinks in your home, except that it is usually square-cornered and made from PVC pipe elbows, is installed near the beginning of the drain at the front of the air handler or outside at the end. The water it holds keeps air from flowing though the pipe, but lets the condensate water drain. In many homes, the U-shape is underground and is created by the pipe running down under the floor slab, then up above ground near the condenser (outside unit), like in the photo above.

    Most manufacturers specify a trap in the condensate drain line as part of their installation instructions. Some systems do not need one, based on their design, and several mini-split manufacturers require that one NOT be installed.

   The photos below illustrate two recurring defects we see with condensate drain lines: termination at or very near the wall of a home, resulting in a moisture problem at the wall, and terminations that blocked by dirt or mulch and will clog easily.

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• Does code require a p-trap in an a/c condensate drain?

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.