In approximately 99.2% of the houses that I inspect, there's a 3/4" hole in the air conditioner ductwork that lets air blow out where it shouldn't.
Can you guess what it is? Here's a clue.
Do you see it? It's that clear tube coming off the condensate drain pan on the air conditioner's evaporator coil. As the air conditioner runs, warm air from the house blows over the evaporator coil, which removes heat and moisture from the air in the house. The heat gets carried out of the house via the air conditioner's refrigerant lines, where it gets dissipated at the exterior of the home. The moisture gets removed from the air through condensation, and that clear tube carries the condensate to an appropriate disposal location, such as a floor drain.
The problem with most condensate drains is that not only will they carry condensate away, but they'll also allow air to blow out through the drain, which is ineffecient. When ductwork is properly installed, all of the little gaps and holes in the ductwork get sealed up, including the hole created by the condensate drain.
To fix the hole created by the condensate drain, a trap needs to be installed. This trap will work the same way that any other plumbing trap works; the water sitting in the trap prevents air (or sewer gas) from coming through.
In my opinion, the best kind of condensate trap is a pre-manufactured condensate trap, such as the one pictured below by EZ-Trap. These kits have a clear plastic trap, so you can see when the trap gets filled with gunk. These traps even come with a little cleaning brush so you can easily clean out the trap when needed.
The other type of trap is a field fabricated trap, which takes a little more to assemble and isn't so easy to clean, but is also far cheaper. These can be easily assembled with a few cheap plumbing parts. The photo at the top of this blog was the condensate drain at my own house... dontcha love how the plastic was wrapped with metal tape to keep it from melting where it touched the furnace vent? Ha ha. That wasn't my doing.
The photo below shows my homemade (or if you want to sound professional, "field fabricated") trap, which took me about 15 minutes to put together with about $5 worth of CPVC fittings. I also replaced the entire plastic condensate drain with 3/4" CPVC, which won't kink or get knocked out of place. How easy was that?
At the moment, everything has been dry fit together, yet it still holds water. I suppose I should use some CPVC cement and make it permanent. Whenever I do, I'll just have to remember to not glue the cap at the top. I want to leave that accessible for when I need to clean the trap.