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I am in Dallas area.

When the fan of one of my AC units is on, there is stinky smell(like bad vinegar) out from the one drain hole of the vanity in one bathroom. I can here air flow if I put my ears close to the drain hole. This is a double vanity. There is no smell coming from the drain next to it, I can not hear air flow from the other drain neither.

I can turn the fan on/off to get this start/stop.

Update on 10/9/2021: I upload these two photos after reading the comments: one from AC, and one from vanity sink, I have confirmed following: When AC FAN is on, smell air is out from the black rubber connection (marked red). There is also air out from the pipe marked in green at AC in attic, but no odor.

I guess it does not matter if if there is a P trap next to AC for this problem, because the odor is not coming from AC vent. I do not know what is purpose of that black rubber connection and where it connect to, and how it is related to AC air flow.

enter image description hereenter image description here

Update on 10/20/2021: One day after last update, the AC stopped working. Called AC guy. The AC guy told me that the recoil leaked and required a replacement. This unit is only 3 years old. Any way the recoil is replaced. a trap is added at the same time.

Also the back rubber pipe is the drain from AC.

AC guy does not think the small and AC breakdown are related.

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  • 2
    Take a picture of the plumbing under the sink.
    – longneck
    Oct 6, 2021 at 18:07
  • 1
    Sounds like a dry trap. Is this a sink that gets used regularly? Also, pics of the plumbing.
    – FreeMan
    Oct 6, 2021 at 18:16
  • 1
    Is this a new occurrence? How long have you lived there?
    – JPhi1618
    Oct 6, 2021 at 18:55
  • In addition to pics of plumbing under sink, also 1) pics of water drainage from A/C 2) examination of traps for both A/C and vanity to see if they are empty 3) if you can determine whether air flow you are hearing is flowing into or out from drain (hold a piece of toilet paper above it).
    – jay613
    Oct 6, 2021 at 19:57
  • 1
    I appreciate your comments. I uploaded photos to answer the questions.
    – max
    Oct 9, 2021 at 14:58

2 Answers 2

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It's common (in some areas, including Dallas) for the AC condensate drain to be plumbed into a nearby bathroom drain. The drain tube for the AC is connected before the P-trap so sewer gasses can't make their way back into the AC unit. This also means that if air is blowing down the drain pipe, it will come out of the sink and cause an odor.

If this is new, then it could be a clogged AC filter or a collapsed air duct causing a higher-than-normal pressure in the air handler. Check that out, and see if everything is ok.

A permanent fix is to have an air-gap in the drain line next to the air conditioner. This allows air into the pipe for smooth draining, but in case of a positive pressure it would also allow air out instead of it being forced down into your vanity.

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  • I like the thinking that this may be related to pressure in a condensate line, versus just being a vanity trap issue (though I find that more likely). BUT ... why would air flow through the condensate line into the room side of the vanity trap stink? The air is the circulating home air! And if it does stink, wouldn't it stink just as much at the proposed air gap?
    – jay613
    Oct 6, 2021 at 19:28
  • @jay613 the air gap next to the AC would be in the attic, so smell wouldn't matter. A dry trap is a different, distinctly sewer smell. Think of the air handler and blower pushing air through the ducts in the house, and air comes out of the registers. If there is too much pressure in the unit (some units just have a positive pressure) then you can think of the drain line as just another air duct. Also, what I was saying is that the condensate drain is connected to the "room side" of the P trap on the sink. If it was on the other side of the Ptrap, sewer gas could make it up to the AC unit.
    – JPhi1618
    Oct 6, 2021 at 19:36
  • But what is the source of the stink? As you say, think of the condensate line as an air duct. I got that ... it's house air. At both ends ... A/C plenum to room side of trap.
    – jay613
    Oct 6, 2021 at 19:40
  • Also, I think usually the condensate drain is under negative pressure. I learned about this extensively in researching a question I wrote about dehumidifiers. STILL your answer is interesting. If the line has no trap of its own, or it's empty, air intake from the point where it joins the vanity trap may be accelerating the drying of that trap!
    – jay613
    Oct 6, 2021 at 19:42
  • Oh, sorry, the stink would be the buildup in the condensate line. They get various molds and mildew lining them and will clog eventually if not cleaned out. It could even be the smell of the buildup in the vanity drain. That can smell, but it wouldn't be a sewer smell at that point.
    – JPhi1618
    Oct 6, 2021 at 19:43
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Eliminate or cap (1, shown) the unnecessary vent from the condensate line and add a trap (2, shown) to it. Together these will ensure that water, and not air, flow through it.

enter image description here

This is an A/C condensate trap that is transparent for easy inspection and includes ports for easy cleaning.

enter image description here

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  • Thanks for your suggestion
    – max
    Oct 22, 2021 at 21:54
  • Jay, Thanks for suggestion
    – max
    Oct 22, 2021 at 21:55

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