An interesting little part of the condensate drain in a residential air conditioning system is the p-trap.  Note that it is called a p-trap because of its shape and that it is not a “pee-trap”.  That is something completely different. So, NO, that is not what that little vent pipe is for.  You’re just going to have to climb down the ladder and use the restroom like a civilized person.

I don’t know why they don’t call it a “u-trap”.  Yes, it would make more sense.  I had no say in the matter.

The p-trap traps condensate (water) so that air cannot pass through. Because the coil is under positive pressure when the system is running, air would rush right out of the condensate lines.  The p-trap helps prevent this.   Condensate trickles in from the coil side causing an equal amount to trickle out the other side and down to a sewer drain or some other acceptable location.

Code requires a vent-T that allows air to get in behind the escaping water.  Someone must have thought that relatively large amounts of water would be passing through, prompting the need for the t-vent.  Normally a vent like this is required to prevent the drain from gurgling or trapping air, much like vents used in sewer lines.  I’m quite sure that condensate drains would work just fine without the vent-T, but it is required by code.

The sad thing is that even though this is a fairly simple concept to understand, all too may times it is installed with the vent-T on the wrong side of the p-trap, making the p-trap completely irrelevant.  While not a big deal (the leakage out of the t-vent is only a few cfm), it does say a lot about the installer.  I can’t tell you how many times I’ve seen this.  I would guess probably 30% of the time.  If they don’t understand how a p-trap and vent works, how are we supposed to trust them around gas piping and refrigerant lines?

By the way.  I did my BPI field exam in a friend’s house a while back.  They had a brand new furnace in their attic.  The p-traps were wrong.  We also discovered that the gas line leaked where it was attached to the furnace.  It was only finger-tight.  They never used a wrench to tighten it down.  Coincidence?


Here is a little quiz for you.  If the pressure inside the coil is 90 Pascals, how much higher will the water level be on the right side of the p-trap compared to the left side in the diagram above?  Answer below.


If 249 Pascals equals one inch of water column (iwc), then 90 / 249 = 0.36 iwc.  So the water would be displaced by 0.36 inches.  In the lower digram it would not displace much, if any, because the pressure is escaping out of the vent-T.