So I've been having a leak in my AC handler. A lot of water was dripping into the drain pan which eventually hit my float switch. I recently moved into the house and the AC unit is 10 years old (sears brand). I shop vac the water out and called a plumber.

The plumber came and said that he sees no crack in the coils or primary pan in the handler. But couldn't really figure out why water would drip out of the handler. He thought the drain as clogged and so he blew it out. Days later, still dripping. It was starting to now drip from my ceiling in the attic. Not good. I called another plumber who came out and said that I have a negative pressure system and that my drain line is not vented properly. I think he said I didn't even have a vent installed. So he installs the vent and re-pitches the drain pipe. He said that it would still drip because the interior of the handler was all wet with water and would take time to dry out. That was yesterday morning.

However, by yesterday evening, I was actually getting more water in the pan and coming from my ceiling. I went into the attic and saw that the first plumber removed the "P" trap from the drain. The plumber added a vent pipe but got rid of the p trap. I called another plumber out for an emergency last night and he added the p trap back and that seems to have worked. He suggested running the fan continuously for a while to help dry out the ac insulation inside the handler so that the water stops dripping from the insulation.

However, when I looked at the re-piping he did, he moved the vent pipe before the P trap. Everything I have read states that the vent has to come after the P trap. Is this going to be a problem? I've spent quite a few dollars on these plumber visits and can't spend any more money on this especially where the AC handler is apparently in good shape but just a matter of not having been piped correctly. I called the plumbing company and, of course, they tell me that there is nothing wrong with the piping.

  • Have you checked if the air handler is level? I had an A/C dripping/leaking but all the pipes and drains were clean. Turns out the roof support it was hanging from had twisted over the years and the unit wasn't level anymore so wasn't draining properly.
    – JPhi1618
    Commented Jul 29, 2019 at 18:36
  • The need for a vent pipe depends on how the overall drain pipe system was set up in the first place. But it won't hurt, and I suspect that the location relative to the trap won't matter much. Run the system for a week and see how it works out. By the way, if you got leaks thru the ceiling then your float valve must not be working - or comes on too late. Commented Jul 29, 2019 at 19:48

1 Answer 1


Two long for a comment: Never call a plumber for an hvac issue. Without a trap the air handler may pull a vacuum on the drain line. The line worked at one time so it is probably blocked this is a daily issue for the residential guys, if the coil, pan and drain line were not properly cleaned it’s time to get it clean or the problem will return. Now look at that coil on the air handler if you are cooling your home and there is any humidity guess what, that coil will start producing water we call it condensate. It is how the unit works, now that the trap has been removed make sure not to turn the fan on while in cooling mode or you may have a real flood. Water dripping from the coil is a normal part of how an ac system works, pit a glass of ice water on the counter notice the moisture developing and maybe beads of water running down the glass, now multiply that by a thousand and you have a normal evaporator coil (the one in the air handler). It is normal to see gallons a day on a central air system. The big thing is to clean the “growth and dust” out of the coil, pan and drain. Non acid cleaners are the best, You may want a hvac tech to set up the trap again, depending on the location these can be 2-3 times as tall as a standard trap to keep the water in the trap.

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