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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.

max
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2 Answers2

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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.

JPhi1618
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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

jay613
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