The Condensate Trap Trap…trap…trap…

trapCondensate drains are one of the simplest components of an air conditioning system, and one of the most frustrating when there are problems.
The concept is simple enough; water drips from the evaporator coil into a pan where it drains through a pipe to the outdoors. Easy.

For something so simple it sure causes a lot of frustration.

How many times have you gone out on a condensate leak call, blown out the line and cleaned the trap, only to be called back again for the same leak? You arrive on the job and the pan is overflowing with water again. You blow out the line again thinking “I got it this time” only to receive another call back later the same day!
The problem continues until, out of desperation, you run a whole new condensate line. Problem solved.
But what was wrong with the original line? Was there something lodged in the piping? Not likely.
Chances are you ran into a double trap situation.
All condensate drains have a field installed “P” trap installed at the air handler drain connection. The trap forms a liquid seal, preventing air being drawn into the condensate drain that would prevent water from draining.
The outlet of the trap is connected to a drain line that routes the water to wherever the water is being dumped. The trap and the drain line are usually made of PVC pipe, and are typically run a good distance to the final drain point. If the drain line sags or dips, a second trap is created by the low point. The second trap prevents the water from draining.
To make this easier to imagine, picture holding your thumb over the end of a straw filled with water; can the water drain from the straw? No, not until you remove your thumb. Why? Because the blockage (your thumb, or in our case, the dip in the line) caused a vacuum to form. Remove your thumb, and the water drains from the straw; remove the dip in the line, and the condensate drains.
You’ll typically run across this situation in attic installations where the extreme temperatures soften the PVC pipe enough to allow it to sag between the pipe clamps.
There are two possible solutions to the problem. The first is to mount the tubing supports close enough together to eliminate the potential for sagging. The second is to install a vent tee at the outlet of the trap, and for longer pipe runs, at strategic locations along the run. These vents will prevent a vacuum from forming should the drain line sag.
When installing the vents keep the branch of the tee pointing up, and always install a stand pipe that extends a few inches higher than the topmost point of the drain pan lip. The stand pipe will prevent water from overflowing from the vent tees should the drain line become obstructed at the outlet.
A litte easier than running a whole new line?

Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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2 comments on “The Condensate Trap Trap…trap…trap…
  1. Paul Collins says:

    Hi Patrick.

    When draining a condensate line into a standpipe, should the end of the line be submerged below the water line in the pipe or hang above the water. We are getting a musty smell from the pipe when water drains in from the condensate pump.

    • The end of the condensate drain line should be above the standpipe (typically called an ‘Air Gap’). If odor is being pulled into the HVAC unit through the condensate line the trap could be dry, or too shallow.
      Let us know this helps Paul.

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