Why A/C Units Need a proper P-Trap, and how to build one.

Recently, my wife and I returned from a week long vacation.  When we arrived home we immediatly turned the A/C back down (we had set it high since we weren’t going to be home).  Later on that night, we were eating dinner, when I hear water pouring in one of our bathrooms, I go look, and discover that water is POURING in through the roof.   The only thing above the roof that had water was the A/C.  As I thought about this, I realized what the problem was… We didn’t have a P-Trap on our A/C unit!!!!!

What is a P-Trap you say?

A P-Trap serves a very important purpose on an A/C unit.  See, when moist air hits something cold, the water condenses out of the air and collects on the cold surface.  Air Conditioning units pull A LOT of water out of the air.  That is why in the summer, your car drips water. All air conditioners have a special drain for this condensation.

Some A/C units (like ours)  have a negative air pressure inside them when running.  What this means, is that the air is being sucked through the air conditioner instead of blown through, and so the inside of the air conditioner is at a lower pressure than the outside air.  This presents a problem for the condensation drain…  Since the Air Conditioner is at a negative pressure relative to the outside air, the outside air will rush into the air conditioner through any openings (the condensation drain, is an opening).  On our A/C the air rushing in through the condensation drain was preventing the water from exiting through the drain.  So water was building up in our A/C unit and when it would turn off “Woosh…”  All that water would pour out of any hole it could find!  Since we hadn’t run the A/C in a week when we got home, it ran for an hour or two straight.  During which time it collected A LOT of water inside.  When it shut off….  Whooosh…  Water came pouring out of places it never had before causing it to flow out of the roof in our bathroom.

So…  What is a P-Trap.  A P-Trap is NOTHING more than a properly shaped piece of pipe…  No moving parts, just the proper shape so gravity does the work for us.   See below for an image of a sink P-Trap (which works the same way).

See, water won’t flow up-hill…  So the 180 degree turn in the drain causes water to get “trapped” in the bottom of the P-Trap.  Water can still flow, because when water goes down the drain, it pushes the water on the other side out.  On a Sink the purpose of the P-Trap is to keep nasty sewer smells from coming up the drain. (you know what an outhouse / latrine smells like?  Well this little 180 degree bend is the only thing that keeps your kitchen from smelling that way.).

The same thing applies to our air conditioner, but in our case, the purpose of the P-Trap is to prevent outside air from being drawn through the A/C unit.  The weight of the water in the trap is enough to prevent this, while still allowing condensation to flow freely.

Another key component of a P-Trap is an air vent…  The Air vent prevents the drain that comes after the P-Trap from siphoning the water in the trap out.


Here is our finished product.  The important dimensions for a P-Trap are…  That the exit point of the trap is 4 to 6 inches lower than the entrance point.  The overall height of the trap should be 8 to 12 inches, and the distance between the two sides of the trap should be 4 to 6 inches.  But, honestly, these things are fool proof, the only really important thing is that the exit is at least 3″ below the entrance, and the 180 degree turn at the bottom sits at least 3″ below the exit as well.  This simple piece of plumbing fixed our problem perfectly, and condensation now runs freely when the A/C is running instead of Ga-Whooooshing….  When it turns off.

Special Thanks to Bill McMain, my good friend who is conveniently a HVAC engineer, and enjoys answering my questions about A/C magic…